Application Note 2009 AN1298.2 Introduction Instrumentation Ampli

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Application Note 2009 AN1298.2 Introduction Instrumentation Ampli

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Instrumentation Amplifier Application Note
Application Note 2009 AN1298.2
Introduction Instrumentation Amplifier. Review Standard Instrumentation Amplifier Design Techniques Monolithic Instrumentation Amplifier Architecture Introduction Instrumentation Amplifier Product Family. Instrumentation Amplifier Specifications Instrumentation Amplifier Product Family Theory Operation. Features Instrumentation Amplifier Product Family Care Feeding Instrumentation Amplifiers Application Circuits. Pressure Sensor Interface Circuit Thermocouple Input with Converter Output Thermocouple Input with 20mA Output Current Input with Converter Output Voltage High Side Current Sense. Multiplexed Voltage Current Sense Bi-Directional Current Sense.
CAUTION: These devices sensitive electrostatic discharge; follow proper Handling Procedures. 1-888-INTERSIL 1-888-468-3774 Intersil (and design) registered trademark Intersil Americas Inc. Copyright Intersil Americas Inc. 2007, 2009. Rights Reserved other trademarks mentioned property their respective owners.
Application Note 1298 Introduction Instrumentation Amplifier
This Application Note describes Intersil bipolar input (see Table Instrumentation Amplifiers, theory operation, advantages, typical application circuits. These devices micropower Instrumentation Amplifiers which deliver rail-to-rail input amplification rail-to-rail output swing single 2.4V supply. These Instrumentation Amplifiers deliver excellent specifications while consuming only 60µA typical supply current. Because they provide independent pair feedback terminals gain adjust output level, these Instrumentation Amplifiers achieve high common-mode rejection ratios regardless tolerance gain setting resistors. ISL28271 ISL28272 have ENABLE reduce power consumption, typically less than 5.0µA, while Instrumentation Amplifier disabled.
TABLE MINIMUM CLOSED INPUT STAGE AMPLIFIERS LOOP GAIN (kHz) ENABLE? Bipolar PMOS PMOS Bipolar
+VCC
Instrumentation Amplifier confused animal confused cousin, amp. symbol looks like (see Figure many same basic properties specifications Offset Voltage, Input Bias Current, CMRR, PSRR, etc. make Instrumentation Amplifier from simple circuit. behavior Instrumentation Amplifier profoundly different than amp! very difficult make precision Instrumentation Amplifier from simple circuit many have tried, most have failed. Instrumentation Amplifier provides voltage subtraction block followed fixed gain block; i.e.
Gain (EQ.
Often, there optional output reference input which allows output voltage shifted fixed voltage:
Gain (EQ.
PART EL8170 EL8171 EL8172 EL8173
VOUT GAIN
VREF
ISL28270 Bipolar ISL28271 PMOS ISL28272 PMOS ISL28273 Bipolar ISL28470 Bipolar
+VCC
FIGURE
contrast, definition only provides extremely high gain with provisions apply negative feedback establish fixed gain unique transfer function, H(s), such integrator filter.
Review Standard Instrumentation Amplifier Design Techniques
VOUT
Difference Amplifier
most basic topology, Instrumentation Amplifier configured from single four resistors shown Figure this often referred Difference Amplifier.
VOUT
-VCC
-VCC INSTRUMENTATION AMPLIFIER
FIGURE
VOUT (IN+ IN-) R2/R1)
FIGURE INSTRUMENTATION AMPLIFIER
AN1298.2 2009
Application Note 1298
ININ+
VOUT
common voltage 10V, inputs will sitting voltage 9.9V. This circuit would possible operated with since input's voltage would exceed supply voltage.
100k
VOUT
VREF
FIGURE
this configuration, gain resistors
Gain Gain (EQ.
100k VREF
(EQ.
FIGURE
ability reject voltage that appears both INand (i.e., common mode voltage), resistor values must match such that common mode rejection ratio (CMRR) matching ratio R1:R3 R2:R4. High common mode rejection ratio requires very high degree ratio matching. shown that CMRR
CMRR Where (EQ. (EQ.
Amplifier Instrumentation Amplifier
provide high input impedance, amplifier Instrumentation Amplifier used Figure
VOUT
Worse case CMRR occurs when tolerance their maximum, their minimum value. following table shows relationship between resistor tolerance CMRR gains 100.
TABLE RESISTOR TOLERANCE ±0.1% ±0.01% GAIN -20.4dB -34.1dB -54.0dB -74.0dB CMRR GAIN -15.6dB -28.9dB -48.8dB -68.8dB GAIN =100 -14.8dB -28.1dB -48.0dB -68.0dB
FIGURE AMPLIFIER INSTRUMENTATION AMPLIFIER
this configuration, gain resistors
Gain Gain (EQ.
(EQ.
ability reject voltage that appears both (i.e., common mode voltage), depends matched resistor values such that, common mode rejection ratio (CMRR) matching ratio R1:R3 R2:R4, and, high CMRR requires very high degree ratio matching. example, with common mode voltage, resistor tolerance's must least ±0.01% achieve 12-bit accuracy (72dB).
Difference Amplifier advantage simplicity ability operate with high common mode voltage inputs, IN-. However, input resistance resistor values does provide high input resistance common most Instrumentation Amplifier circuits. Additionally, input must driven very source impedance since CMRR will degraded source resistance that contributes value causes increased mismatch between Also note that common mode voltage will bias internal nodes voltage that ratio gain circuit. example, Figure gain
Classic Three Amplifier Instrumentation Amplifier
adding third amp, "Classic Three Amplifier Instrumentation Amplifier" configured shown Figure
AN1298.2 2009
Application Note 1298
Introduction Instrumentation Amplifier Product Family
VOUT
INVOUT VOUT
VREF
FIGURE CLASSIC THREE AMPLIFIER INSTRUMENTATION AMPLIFIER
Usually, resistors through equal value resistors gain:
Gain gain Gain (EQ. (EQ. FIGURE AMPLIFIER INSTRUMENTATION AMPLIFIER
With this circuit, Gain with single resistor, RGAIN input impedance very high. However, common mode rejection ratio, CMRR, just like Difference Amplifier topology, still resistor matching between Extremely tolerance resistors precision resistor trimming required achieve high CMRR. equations Table shown Difference Amplifier apply directly Classic Three Amplifier Instrumentation Amplifier configuration.
This Application Note describes Intersil Instrumentation Amplifier Product Family, which includes following features: Bipolar transistor inputs voltage noise PMOS transistor inputs input bias current Micropower operation requiring only supply current Rail-to-rail inputs rail-to-rail output swing Single supply operation from 2.4V supply independent pair feedback terminals gain adjust output level allow these Instrumentation Amplifier achieve high CMRR (>104dB) regardless tolerance gain setting resistors. Internal loop compensation provide optimum bandwidth trade-off shown Table ISL28271 ISL28272 have ENABLE reduce supply current typical less than tri-state output stage high impedance state.
Monolithic Instrumentation Amplifier Architecture
Each three basic Instrumentation Amplifier architectures that have been already discussed have been implemented standard integrated circuit packages. achieve high CMRR, extensive resistor trimming required with lasers other suitable techniques. While each these devices provide adequate specifications precision Instrumentation Amplifier, each device compromise based operating voltage range, supply current, common mode operating range, input impedance, etc. These instrumentation amplifiers external resistor gain; while this seem advantage, there considerations which make single resistor configuration undesirable from design viewpoint. temperature coefficient (TC) external resistor will direct gain drift. Also, external filter applied feedback network because internal device.
Instrumentation Amplifier Specifications
Many Instrumentation Amplifier specifications very similar standard specifications operational amplifiers. However, unique architecture Intersil Instrumentation Amplifiers make some these specifications differ slightly. Table summarizes Specifications Features Instrumentation Amplifier Product Family.
AN1298.2 2009
Application Note 1298
TABLE PARAMETERS Input Stage Minimum Gain Gain Supply Current: Enabled Channel Supply Current: Shutdown Minimum Maximum Input Offset Voltage Offset Drift Input Bias Current, Maximum Input Offset Current, Maximum Input Bias Current Cancellation Bandwidth (-3dB) Bandwidth (-3dB) Slew Rate (Typ) Rail-to-Rail Input Rail-to-Rail Output Output Current Limit, Output Shutdown Mode Gain Accuracy CMRR (Typ) PSRR (Typ) 1kHz 0.1Hz 10Hz Input Protection Diodes Rails Input Protection Diodes across Inputs Input Diode Current Package Operating Temp. Range RoHS Compliant ±0.35 0.55 0.24 3000 EL8170 ISL28270 ISL28470 Bipolar 2000 2000 ±0.5 ±0.5 ±0.1 ±0.12 0.55 ±0.15 0.55 0.08 ±0.2 2500 1000 2000 2000 EL8173 ISL28273 EL8171 ISL28271 EL8172 ISL28272 UNITS
Bipolar 2500 1500
PMOS 0.55 0.14
PMOS -0.19 nv/Hz µVP-P V/µs µV/°C
AN1298.2 2009
Application Note 1298
+Ven
INI1
FBV5 VOUT
GAIN
FIGURE SIMPLIFIED SCHEMATIC
Instrumentation Amplifier Product Family Theory Operation
Each features specifications Intersil Instrumentation Amplifier Product Family will discussed more detail future section this Application Note, first, let's study internal operation this unique Instrumentation Amplifier Product Family. simplified schematic shown Figure
since -(EQ.
(EQ. (EQ. (EQ.
where gain output stage Assume Ry/Re (i.e., equal value).
(EQ. (EQ.
(EQ. (EQ.
Assuming high transistors:
(EQ.
Since very large:
(EQ. (EQ. (EQ. (EQ. (EQ.
Similarly
FB+, (EQ. (EQ.
Summing currents:
(EQ. (EQ. (EQ. (EQ.
(EQ.
from Equation negative feedback applied around amplifier that voltage applied feedback terminals (FB+ FB-) must equal voltage applied input terminals (IN+ IN-).
AN1298.2 2009
Application Note 1298
standard data sheet connection:
INVOUT VOUT
input terminals (IN+ IN-) feedback terminals (FB+ FB-) single differential pair devices aided Input Range Enhancement Circuit increase headroom operation common-mode input voltage. result, input common-mode voltage range these Instrumentation Amplifiers rail-to-rail. parts able handle input voltages that slightly beyond supply ground making these in-amps well suited single 3.3V voltage supply systems. There need then move common-mode input voltage these Instrumentation Amplifiers achieve symmetrical input voltage. bipolar transistor input stage MOSFET input stage allows user choose bias current, high input resistance. Rail-to-rail operation both inputs outputs important unique feature. rail-to-rail inputs allow input voltages slightly below rail (typically Ground) slightly above rail. conventional technique achieve rail-to-rail input stage separate input stages, shown Figure input stage provides common mode input range rail (VS+), other input stage provides common mode input range bottom rail.
FIGURE AMPLIFIER INSTRUMENTATION AMPLIFIER (EQ.
Features Instrumentation Amplifier Product Family
simplified schematic block diagram shown Figure illustrate rail-to-rail operation both input stage output stage. same schematic applies PMOS input devices when transistors replaced with P-Channel MOSFETs ultra-low input bias current.
INPUT RANGE ENHANCEMENT CIRCUIT
INQ1
P-Channel N-Channel
VSIBC INPUT BIAS CURRENT CANCELLATION
FIGURE SIMPLIFIED SCHEMATIC
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INPUT OFFSET VOLTAGE (µV) -100 -150 -200 -250 -0.5 COMMON-MODE INPUT VOLTAGE +85°C +25°C
5.5V -40°C
TRANSISTION CIRCUIT
OUTPUT STAGE
FIGURE TYPICAL RAIL-TO-RAIL INPUT AMPLIFIER
FIGURE AMPLIFIER INSTRUMENTATION AMPLIFIER
Unless input stages transistors exactly matched, changes offset voltage input bias current will result common mode input range transitions between input stages. contrast, Product Family uses single input stage inputs single input stage inputs. Input Range Enhancement Circuit (IREC) provides bias voltage that approximately above rail which used bias current sources shown Block Diagram. Since there single input stage, there input stage transition point create shifts offset voltage bias current input common mode voltage changes. effectiveness Single Input Stage IREC circuit technique evident shown following Figures offset voltage EL8170 (Figure typical rail-torail input amplifier (Figure 14).
INPUT OFFSET VOLTAGE (µV)
addition shifts offset voltage input common mode voltage changes, input bias current will change dramatically input stages transition from transistor input stage transistor input stage. following graphs compare input bias current over common mode input range EL8170 (Figure typical rail-to-rail input amplifier (Figure 16).
AVERAGE INPUT BIAS CURRENT (pA) 1500
1000 3.3V 5.0V
2.9V
-500 -0.5
COMMON-MODE INPUT VOLTAGE
FIGURE EL8170
+85°C INPUT BIAS CURRENT COMMON-MODE INPUT VOLTAGE INPUT BIAS CURRENT (nA) +25°C -0.5 +25°C
-45°C -0.5
COMMON-MODE INPUT VOLTAGE
FIGURE EL8170
COMMON-MODE INPUT VOLTAGE
FIGURE TYPICAL RAIL-TO-RAIL INPUT AMPLIFIER
AN1298.2 2009
Application Note 1298
input stage transistors biased with adequate amount current speed, consequently, their base current increases. order keep input bias current low, Input Bias Current Cancellation Circuit used apply equal opposite compensation current inputs. This compensation current subtracts from base currents, resulting input bias current reduced typically around 500pA. This shown Figure INinputs, where identical proper matching between stages. compensation current, (Icomp) derived from circuit equal base current
operating voltage range Instrumentation Amplifier product family from 2.4V 5.5V making ideally suited operation 3.3V power supplies. Also, will operate with single lithium battery. Additionally, they well suited battery operation since supply current only 66µA maximum. Another unique feature built into ISL28271 ISL28272 ability tri-state output stage high impedance state when part disabled ENABLE pin. This allows several outputs wired together multiplexer function. This feature will shown Applications section. Because Instrumentation Amplifier product family provides independent pair feedback terminals gain adjust output level, these Instrumentation Amplifiers achieve high CMRR regardless tolerance gain setting resistors. used terminal center adjust output voltage. Because high impedance input, economical resistor divider used voltage terminal without degrading affecting CMRR performance. voltage applied terminal will shift output voltage VREF times closed loop gain, which resistors Since feedback terminals differential inputs, they used applications such current sources true Kelvin sense feedback voltage. addition, complex network placed feedback path frequency shaping filter circuits. basic Instrumentation Amplifier configuration shown Figure
INIcomp
Icomp
INPUT BIAS CURRENT CANCELLATION
FIGURE INPUT BIAS CURRENT CANCELLATION CIRCUIT
Input Bias Current Cancellation Circuit typically active from 10mV above negative rail (VS-) positive rail (VS+). only does Input Bias Current Cancellation compensation circuit keep input bias current very small, also maintains very small input bias current variation over wide operating range shown Figure +25°C +85°C.
AVERAGE INPUT BIAS CURRENT (pA) 1500 3.3V 1000
INVOUT VOUT
+85°C +25°C
-500
FIGURE TYPICAL RAIL-TO-RAIL INSTRUMENTATION AMPLIFIER
-1000 -0.5
COMMON-MODE INPUT VOLTAGE
gain this circuit ratio such that:
(EQ.
FIGURE
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this configuration, adjustable gain possible with external resistors gains from unity 10,000. external gain setting resistors used minimize temperature coefficient (TC) mismatch common with single gain setting resistor. Notice that resistor value mismatches only effect gain, CMRR degraded resistor mismatches case with other basic Instrumentation Amplifier configurations discussed previously. feedback terminals used apply reference voltage shift input voltage. These high impedance reference input that affected gain. basic circuit shown Figure
INVOUT VREF VOUT
this case:
(EQ.
feedback terminals also used apply reference voltage shift output voltage shown Figure with connected VREF instead ground.
INVOUT VREF VOUT
FIGURE
FBRf
(EQ.
FIGURE BASIC CIRCUIT
back equations derived previously:
(EQ.
(EQ. (EQ.
Since high input impedance, simple resistor divider could used VREF voltage shown Figure
INVOUT VREF VOUT
Since current must flow into VREF, driving point impedance VREF will effect accuracy this configuration. Therefore, VREF should impedance from amp, voltage regulator, voltage reference. Alternately, resistor divider used obtain VREF, Thevenin resistance divider network must much lower than values Thevenin resistance must included value VREF. However, CMRR affected reference voltage source resistance.
Care Feeding Instrumentation Amplifiers
voltage, high accuracy measurement system, extreme care must taken with Instrumentation Amplifiers with respect grounding scheme, Kelvin sense connections, guarding shielding, interface digital world. connections made incorrectly, most perfect measurement circuit still have errors resulting from poor grounding considerations understanding impact Ohm's Law. analog mixed signal must have well thought-out grounding scheme with multiple ground planes traces. There must heavy current current analog ground planes that connect system measurement points.
FIGURE
AN1298.2 2009
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point measurement system must established prevent high currents from interfering with basic measurement. This shown Figure interfacing thermocouple Converter. "High quality measurement Ground" must only connect critical ground points analog front-end; this ground must make single point connection converter Analog Ground (AGND). There must other connections such digital grounds power supply returns "High quality measurement Ground" except single connection Converter pins (AGND DGND).
INPUT FILTER J-TYPE THERMOCOUPLE (51.7µV/C)
sure there digital noise introduced into Analog Ground, grounds tied together only point Converter. Furthermore, resistor used connect grounds; this ensures separate each ground layout software layout person does arbitrarily connect grounds. resistor cheap insurance against noisy inaccurate analog system! This Thermocouple Circuit will discussed more detail Applications section.
IBIAS RETURN
OPEN BIAS (Vtc) (Vcjc)
CONVERTER VOUT (Vtc Vcjc) Rf/Rg)
191k,
ISL6007DIB825 VOUT (2.5V) VREF
LM35DM (10m/C)
191k,
GAIN Rf/Rg GAIN 191k/1k GAIN
HIGH QUALITY MEASUREMENT GROUND
AGND CONNECT AGND DGND POINT DGND
FIGURE
1.2V DC/DC CONVERTER OUTPUT
0.005
PROCESSOR LOAD 10A,
0.1µF
EL8171, EL8173, ISL28273, ISL28271
INVOUT 48.7k, 0.1% VOUT 2.5V
GAIN
0.1%
FIGURE
AN1298.2 2009
Application Note 1298
Instrumentation Amplifiers used high accuracy current sense applications shown circuit Figure Notice Kelvin connection shown current sense resistor indicated slanted connections resistor. avoid errors caused drops, connections must made directly leads 0.005 current sense resistor. Just contact resistance trace resistance will cause error current reading. Guarding driven guards layout technique reduce errors caused leakage currents improve high frequency CMRR. This done surrounding high impedance input leads with traces that driven source impedance voltage that equal common mode voltage. point circuit where dissimilar metals come contact small thermocouple voltage developed. Fortunately, copper lead frame surface mount device same copper material etch, thermocouple effect minimized. However, there many other places where thermocouples generated; example, across connector finger, across relay contacts, even across resistor! Yes, poorly constructed resistor show many µV/°C thermocouple voltage. been found that external components (resistors, contacts, sockets, etc.) create thermocouple voltages that exceed 10µV/°C. must recognized that thermocouple voltages developed difference temperature between ends dissimilar metal junctions, absolute ambient temperature. both ends metal junctions isothermal (i.e., same temperature) there thermocouple voltage developed. Therefore, first rule avoid thermocouple effects eliminate spots (e.g., linear voltage regulators). spots cannot avoided, then ends metal junctions must oriented they isothermal lines PCB. second rule minimize thermocouple effects balance number junctions loop that error voltages cancelled become common mode voltage that reduced CMRR amps signal chain. number junctions balanced, then necessary create junction adding series resistor that effect circuit operation balances number junctions. Unknown most design engineers danger internal clipping when operating instrumentation amplifier single supply. Unfortunately, internal nodes invisible user impossible measure; manufacturer's data sheets often ignore issue, they have obscure "typical characteristics" graphs misleading paragraphs that attempt explain phenomena. Since Instrumentation Amplifiers operate current summing mode explained "Instrumentation Amplifier Product Family Theory Operation" page there possibility internal clipping. review classic three amplifier instrumentation amplifier configuration shown Figure effect internal clipping clearly shown.
INA1
VOUT
VREF
FIGURE
Simple circuit analysis shows that internal voltage are:
(EQ.
(EQ.
clipping conditions will occur effects considered: cannot exceed maximum output voltage which supply voltage (VCC) saturation voltage A1's output stage.
(EQ.
cannot below Ground saturation voltage A2's output stage.
(EQ.
reality, this places such severe restriction single supply operation that makes this circuit almost impossible general purpose single supply instrumentation amplifier. example, output stage saturation voltage prevents even common mode voltage! overcome this issue, modern monolithic instrumentation amplifiers level shift transistors raise input voltages Ground shown circuit Figure
AN1298.2 2009
Application Note 1298
there possibility internal clipping. long total common mode voltage plus input signal between supply voltage there will internal clipping. output voltage will swing within railto-rail output specification 10mV either rail 100k load. There restriction differential input voltage common mode voltage provided output voltage does exceed full scale range input voltage level, gain, CMRR, VREF level. input feedback terminals Instrumentation Amplifiers have internal protection diodes both positive (VS+) negative supply (VS-) rails, limiting input voltage within diode drop beyond supply rails. EL8170, EL8172, ISL28270 ISL28470 have additional back-to-back diodes across input terminals also across feedback terminals. overdriving inputs necessary, external input current must never exceed 5mA. other hand, EL8171, EL8172, ISL28271, ISL28272 ISL28273 have diode clamps limit differential voltage input terminals allowing higher differential input voltages lower gain applications. recommended however, that input terminals these devices overdriven beyond avoid offset drift. external series resistor used external protection limit excessive external voltage current from damaging inputs. resistor used protect inputs against 100V transients inputs. overvoltage condition continuous, resistor must rated adequate power dissipation.
FIGURE
internal voltage now:
0.7V 0.7V (EQ.
(EQ.
Now, danger internal clipping situation been improved made worse since additional 0.7V been added VO1. example, maximum common mode voltage only 1.5V this instrumentation amplifier operating supply voltage with gain 10mV input signal! doubt validity these statements, check vendor data sheets analog devices that exhibit these characteristics. Since Instrumentation Amplifiers operate current summing mode explained "Instrumentation Amplifier Product Family Theory Operation" page
VSEL8170 ONLY
VSEL8170 ONLY
INPUT BIAS CURRENT CANCELLATION MAXIMUM INTO PROTECTION DIODE!
INPUT BIAS CURRENT CANCELLATION
FIGURE INPUT PROTECTION DIODES
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Application Note 1298
TRANSFORMER COUPLED SOURCE VOUT VOUT
INVOUT
VOUT
FIGURE
FIGURE
Input bias current from inputs Instrumentation Amplifiers must find path their home (i.e., Ground). While seems obvious casual user, this often ignored principle when designing with Instrumentation Amplifier, results many telephone calls Applications Engineer. Many voltage sources provide path Ground such thermocouples, microphones, transformer coupled circuits, coupled circuits. Without return path, input bias current will accumulate stray capacitance inputs until they clamped rails protection diodes. output Instrumentation Amplifier will slowly increase decrease until saturates into rail. return path shown following circuits must supplied provide return path input bias current.
COUPLED SOURCE
INVOUT VOUT
FIGURE
INVOUT
VOUT
error budget calculated summing factors which contribute output voltage error. Most error sources referred input multiplied Gain output voltage error term shown following. Offset voltage: Normally instrumentation amplifiers have offset voltage specifications input offset voltage (VOSI) output offset voltage (VOSO) specification such that input offset voltage multiplied gain, output offset voltage exhibits unity gain output voltage. Therefore, output voltage error from offset voltage
Gain (EQ.
FIGURE
unique architecture Instrumentation Amplifiers, there only offset voltage specification required. input offset voltage (VOSI) amount voltage applied inputs terminals such that voltage across zero, input offset voltage will difference between terminals terminals:
(EQ.
AN1298.2 2009
Application Note 1298
Gain (EQ.
Offset bias current: Similar circuit, input resistance creates error source that modeled same offset voltage such that:
Gain (EQ. (EQ.
0.1Hz 10Hz Noise: error introduced voltage modeled same input offset voltage, noise required wider bandwidth than 0.1Hz 10Hz, noise calculated evaluating Input Noise Voltage Density (en) over desired bandwidth. Multiplying noise will give good approximation peak-to-peak noise.
Gain (EQ.
Common Mode Rejection Ratio: error introduced common mode voltage modeled same input offset voltage, VCMR.
CMRR (EQ. (EQ. (EQ.
Gain Error: Gain error results from factors. first basic gain deviation from ideal gain equation, Gain Rf/Rg); EL8173 this error (E.g.) typically ±0.2%. Second tolerance (ERf ERg) resistors which Gain.
(EQ.
CMRR
CMRR Gain
Temperature Drift: effect operating over expected temperature range must included these calculations based data sheet specifications.
1.2V DC/DC CONVERTER OUTPUT
0.005
PROCESSOR LOAD 10A,
0.1µF
EL8171, EL8173, ISL28273, ISL28271
INVOUT 48.7k, 0.1% VOUT 2.5V
GAIN
0.1%
FIGURE TABLE ERROR BUDGET CALCULATION ERROR SOURCE Offset voltage Input Offset Current CMRR 0.1Hz 10Hz Noise Gain Error Tolerance SPECIFIED VALUE 400µV 0.5nA 104dB 10µV 0.2% 0.1% Total Error REFEREED OUTPUT 20mV 0.25mV 0.24mV 0.5mV ERROR 0.8% 0.01% 0.01% 0.02% 0.2% 0.2% 1.24%
AN1298.2 2009
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Example Error Budget Calculation
Consider circuit shown Figure core voltage current monitor circuit operating +25°C. importance Error Budget shown Table that shows overall accuracy which expected which factors determining overall accuracy circuit. this circuit, Offset voltage factor which driving Total Error; tighter accuracy required application, offset term could removed hardware calibration with digital potentiometer software calibration. Total Error could reduced 0.5% just decreasing offset voltage term factor Because independent pair feedback terminals provided Intersil's Instrumentation Amplifiers, CMRR degraded resistor mismatches. Hence, unlike three especially instrumentation amplifier, Intersil solution will reduce cost external components allowing more tolerance resistors without sacrificing CMRR performance. CMRR will greater than 100dB regardless tolerance resistors used. effects loading rail-to-rail output stage must also considered since output stage exhibits "ON" state resistance. pair complementary MOSFET devices with approximately resistance drives output VOUT
±2.5V 10pF RL/R 9.08 178k 19.6k 100k FREQUENCY (Hz) MAGNITUDE (dB)
within millivolts supply rails. 100k load, PMOS sources current pulls output below positive supply, while NMOS sinks current pulls output down above negative supply, ground case single supply operation. load current increased, maximum output voltage will decrease result voltage drop caused sourced load current times MOSFET resistance. Likewise, minimum output voltage will increase result voltage drop caused sink load current times resistance. current sinking sourcing capability internally limited about 26mA with supply. Care must taken with excessive load capacitance, CLOAD. shown following graphs, excessive load capacitance will cause excessive peaking frequency response. result will ringing output voltage under transient conditions, potentially oscillations resulting from unstable operation. Instrumentation Amplifiers used applications where there large load capacitance (cable driving, filters, gates, etc.), suitable buffer should used output Instrumentation Amplifier. Noise calculations Instrumentation Amplifiers very similar those circuit. noise model shown following where Input Noise Voltage Input Noise Current noise sources lumped into terminal.
47pF 27pF RF/RG 9.08 178k 19.6k 100k FREQUENCY (Hz)
MAGNITUDE (dB)
FIGURE EL8171 FREQUENCY RESPONSE SUPPLY VOLTAGE
MAGNITUDE (dB) 47pF 27pF RF/RG 9.08 178k 19.6k 100k FREQUENCY (Hz)
FIGURE EL8171, EL8172 FREQUENCY RESPONSE CLOAD
MAGNITUDE (dB) 820pF RF/RG 99.02 221k 2.23k
390pF
100k FREQUENCY (Hz)
FIGURE EL8171 FREQUENCY RESPONSE CLOAD
FIGURE EL8172 FREQUENCY RESPONSE CLOAD
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en(V) en(I1) en(I2) en(Rs) en(Rfg)
INVOUT VOUT
NOISE MODEL
FIGURE
Where: en(V) Input Noise Voltage over desired bandwidth en(I1) voltage noise generated Input Noise Current over desired bandwidth source resistance (Rs):
(EQ.
1.57 term equations noise equivalent bandwidth representing order roll-off equivalent there brick wall filter 1.57*Fh. have brick wall filter that cuts right (infinitely steep) then this term order 1.57 order 1.11 order 1.05 order 1.025 determine total output noise from sources, summation taken multiplied gain.
(EQ.
en(I2) voltage noise generated Input Noise Current over desired bandwidth feedback gain resistors Rg):
(EQ.
en(Rs) thermal noise over desired bandwidth en(Rfg) thermal noise over desired bandwidth source resistance gain setting resistors calculate noise, over desired bandwidth:
1.57 (EQ.
peak-to-peak output noise typically times value (rule thumb).
(EQ.
where: specified noise density nV/Hz corner frequency upper frequency interest lower frequency interest calculate resistor thermal noise over desired bandwidth:
4kTR 1.57 (EQ.
eno(pp)
where: resistor value Boltzman's Constant, 1.39*10-23 temperature Kelvins upper frequency interest lower frequency interest
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7.5k 7.5k
INVOUT
VOUT 2.5V
100k
100k
FIGURE
Example Noise Calculation:
Consider circuit shown Figure bridge amplifier operating 0.5Hz 100Hz bandwidth with full scale output voltage 2.V.
50nv/Hz 100Hz 0.1pA/Hz 50Hz From EL8170 data sheet specifications From EL8170 data sheet performance curves From EL8170 data sheet specifications From EL8170 data sheet performance curves Balanced bridge Thevenin resistance 7.5k resistor 90.9k
50nV 100Hz 100Hz 0.5Hz 1.57 100Hz 0.5Hz 0.81 (EQ.
0.1pA 50Hz 50Hz 0.5Hz 1.57 100Hz 0.5Hz 0.12 (EQ.
0.1pAnV 50Hz 50Hz 0.5Hz 1.57 100Hz 0.5Hz 0.0014 300°K 100Hz 0.5Hz (EQ. (EQ.
0.12 300°K 100Hz 0.5Hz
0.04
(EQ.
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90.9k (0.81V +0.012V +0.0014V +0.12V +0.04V 0.82V Vrms (EQ.
determine total output noise from sources, summation taken multiplied gain. Note that total output noise dominated basic Input Noise Voltage higher source resistance could used without degrading overall error resulting from noise.
(EQ.
VOUT
VOUT
100k
0.1µF
GAIN BELOW 14Hz GAIN BELOW 140Hz
This represents error 0.02% 2.5V full scale output. very unique feature Intersil Instrumentation Amplifiers ability filter circuit feedback network shape frequency response amplifier. This ability possible with other monolithic Instrumentation Amplifiers because they single resistor input stage gain. Adding filter circuits feedback network Instrumentation Amplifier implemented with discrete components amps resistors) very difficult because capacitor mismatch will result very poor high frequency CMRR. complex impedance network added shown following pass function. frequency gain using standard equation:
Gain (EQ.
GAIN
GAIN
14Hz
140Hz
FIGURE
this circuit, shown that frequency pole higher frequency zero are:
0.1F 100k 14Hz (EQ. Pole Frequency
Instrumentation Amplifiers unity gain stable; i.e., they require gains greater than depending device. Therefore, they must never allowed unity gain even high frequencies! included this circuit, would dominate high frequencies, Instrumentation Amplifier would unstable oscillate. tests have shown that >33pF enough cause oscillation. Adding series with creates zero transfer function that higher frequencies parallels that 10.7k, gain high frequency 11.7 which stable condition. tests have shown that value used with oscillations.
0.1F 140Hz
Zero Frequency
(EQ.
AN1298.2 2009
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VIN+ CLOSED CLOSED CLOSED CLOSED ISL43640 WP-L 48.7k MEASURE VIN+ VINMEASURE CMRR ZERO MEASURE ZERO MEASURE 25mV REFERENCE
EL8173 INVOUT FBVS- VOUT 150k, WP-L
VIN-
WRITE PROTECT
ISL43640
ISL95810
(VREF) 100k 9900 10µF
WRITE PROTECT
VOUT
66.5k
0.01µF
ISL6007DIB825
OFFSET CORRECTION (±25mV)
1.37k,
ISL95810
CALIBRATION REFERENCE (25mV)
PROGRAMMABLE GAIN,
FIGURE ANALOG FRONT-END CIRCUIT
Application Circuits
Instrumentation Amplifier With Auto Zero Auto Gain Calibration
circuit shown Figure shows analog front-end circuit with Auto Zero Auto Gain Calibration eliminate offset voltage gain errors EL8173. intended part overall data acquisition system with Converter microprocessor perform auto zero/gain software routine. Figure does include Converter processor hardware/software.
TABLE SWITCH CLOSED MODE Measure input voltage VIN+ VINCalibrate with external common mode voltage applied Calibrate offset voltage zero Calibrate gain with 25mV reference voltage applied
Since offset calibration voltage operating very close zero voltages (±25mV), driving point impedance kept very (1k, avoid variations caused increasing bias current. configuration carefully selected that D-Pot never operated negative voltage. ±25mV offset calibration source obtained programming with appropriate digital code Equation
Vcal
(EQ.
+25mV reference voltage obtained with ISL6007's 2.5V output voltage divided down factor with accuracy gain calibration determined accuracy ISL6007 tolerance resistors Therefore, recommended very tolerance resistors precision resistor divider network. gain EL8173 programmed D-Pot, according Equation
Gain -Code 150000 1370 Gain -Code 1370 151.7k Gain -1370 Code (EQ.
During calibration mode, analog switches connect inputs EL8173 calibration source voltages zero volts, external common mode voltage, 25mV reference voltage. Digital potentiometer (D-Pot) applies programmable offset voltage ±25mV EL8173 adjust EL8173 output zero voltages. Digital potentiometer (D-Pot) programs gain EL8173 from 2.5V output with +25mV reference voltage applied inputs.
AN1298.2 2009
Application Note 1298
Other nominal gains gain adjustment range made changing values R10. Another complication SPPT application large temperature dependence both total bridge resistance peizosensitivity (the ratio bridge output excitation voltage times pressure). Bridge resistance increases with temperature while peizosensitivity decreases. Some SPPT designs (e.g. Nova Sensor NPC-410 series) carefully equalize these opposite-sign tempcos. payoff comes when such SPPTs excited with constant current because increase with temperature bridge resistance (and therefore bridge excitation voltage) then cancels simultaneous decrease peizosensitivity. 10mV/psi pressure-proportional strain gauge signal outputted differentially pins sensor; this signal superimposed common mode voltage 1.2V from bridge excitation voltage. level differential output voltage amplified EL8173 with nominal gain high common mode rejection capability EL8173 eliminates common mode output voltage bridge. bridge biased from constant current source (Q2) digitally controlled potentiometers provide zero (DPOT1) full scale (gain) adjustments (DPOT2). detailed circuit shown Figure circuit provides precision offset adjustment, DPOT1, transducer initial null offset error.
Pressure Sensor Interface Circuit
Programmable Pressure Transducer Circuit
silicon piezoresistive-bridge pressure transducer (SPPT) dominant technology automotive, industrial, medical, environmental pressure sensor applications. SPPTs share similar architecture which thin (5µm 200µm) micro machined silicon diaphragm incorporates implanted piezoresistive Wheatstone-bridge strain-gauge. Applied pressure bends diaphragm, imbalances strain gauge, thereby produces differential output signal proportional product pressure times bridge excitation voltage. SPPTs must supported appropriate signal conditioning calibration circuits. Finite elasticity limits SPPT diaphragm relatively small deflections which generate only modulation bridge resistance elements signal output levels, creating need high gain, low-noise, temperature-stable amplification. signal conditioning circuit must also include stable, high resolution, preferably non-interactive, zero span trims. automation calibration sensor circuit enormous benefit production environment.
CURRENT MIRROR HEADROOM FROM SUPPLY DMMT3906 BRIDGE EXCITATION, 600µA CURRENT SOURCE 1000pF ISL28276 2N3904 Zout (2.5k EL8173 OUT+ OUT- NPC-410 0PSI 5PSI (±25mV Vbridge 3.6V) (±10mV Vbridge 1.5V) OFFSET CALIBRATION CIRCUIT INVOUT FBR8 115k VOUT 0.5V/PSI
ISL60002-11 (1.200V)
Vbridge, 600µA*6k 3.6V ISL28276 DPOT VOLTAGE ALWAYS POSITIVE BUFFERED +Vbridge 39.2k
GAIN DPOT2 50k, TAPS X95820 1.96k
DPOT1 X95820 50k, TAPS EL8176 54.9k -Vbridge
GAIN CALIBRATION CIRCUIT
FIGURE
AN1298.2 2009
Application Note 1298
accomplish this, bridge excitation voltage programmably attenuated DPOT1 applied EL8173. range zero adjustment voltage from +25mV -25mV. resolution 200µV proportional bridge excitation voltage, thus improving temperature stability zero adjustment. 10mV/psi bridge output signal amplified convenient 0.5V/psi output level with EL8173 feedback calibration network consisting R10, DPOT2. gain varied from with resolution 0.10. Bridge bias provided constant current circuit (U1, U2a, which sets current V/2k 600µA. current mirror (Q2, reflects output current source 600µA into grounded bridge (PS1). result combination transducer EL8173 circuitry signal conditioned precision pressure sensor that compatible (thanks DPOT1 DPOT2) with full automation calibration process, very total power draw (<2mA), most which goes transducer excitation current mirror circuit. Thermocouples present several unique challenges when interfacing them real world measurement system. Thermocouples generate very output voltage that must amplified with high gain amplifier. Each thermocouple type requires different gain when interfacing Converter with fixed full scale voltage, VFS/VoMAX. Thermocouples generate absolute voltage that proportional temperature. Instead, they generate voltage that relative voltage that proportional temperature difference between "hot" "cold" end. thermocouple tables showing output voltage temperature "cold" placed bath 0°C. Since very impractical place bath PCB, electronic cold junction compensation used. Each thermocouple type requires cold junction compensation rate, dVO/dT. output voltage thermocouple non-linear, dependant type thermocouple. Linearization most often done with diode break-point techniques microprocessor software, covered this Application Note. circuit shown Figure uses unique features Intersil EL8173 Instrumentation Amplifier simplify Thermocouple interface high resolution Converter (U5). programmable gain digital (U3) programmable temperature sensor (U2) allows digital selection four most popular thermocouple types:
Thermocouple Input with Converter Output
Thermocouples industry standard temperature sensor measuring wide range temperatures from -250°C 2300°C. four most popular thermocouple types shown Table however, time dissimilar metals placed contact, thermocouple created Seebeck Effect.
TABLE POPULAR THERMOCOUPLE TYPES TEMPERATURE RANGE TYPE MINIMUM -200°C -328°F +32°F -200°C -328°F -250°C -328°F MAXIMUM +900°C +1652°F +750°C +1382°F +1250°C +2282°F +350°C +662°F TMIN (mV) -8.83 0.00 -5.89 -5.60 TMAX (mV) 68.79 42.30 50.64 17.82 dVO/dT +50°C (µV/°C) 61.00 51.70 40.50 40.70
AN1298.2 2009
Application Note 1298
EL8173 TYPE THERMOCOUPLE INPUT FILTER 10µF 357k VOUT 10µF ISL21400 1.6Hz 4.53k (VTC) INVOUT (VOUT) 150.0k HI7190 OSC1 10Mhz OSC2 CLOCK DATA DATA SYNC DATA READY RESET
AVDD DVDD AVSS VINHI VINLO AGND SCLK SDIO SYNC DRDY MODE
(VCJC)
SLAVE ADDRESS (0101000x)
1.05k
WP-L
VRLO DGND VRHI WRITE PROTECT
10.7k
VOUT
10µF
COLD JUNCTION COMPENSATION
ISL95810
ISL21009-25
PROGRAMMABLE GAIN GAIN
0.01µF
FIGURE
programmable gain amplifier (U1, provide gain from that programmed with digital each thermocouple types shown Table
TABLE THERMOCOUPLE TYPES TYPE VOUT 68.97mV 42.30mV 50.64mV 17.82mV GAIN 36.34 59.10 49.37 140.3 D-POT CODE10
pass filters (R1, provide noise filtering with cut-off frequency. used return current path EL8173 input bias current. additional pass filter (R4, attenuates ISL21400's output noise voltage with cut-off frequency. high resolution (24-bit) Sigma-Delta Converter, HI7190, converts output instrumentation amplifier, EL8173, with full scale input voltage 2.5V ISL21009-2.5 voltage reference.
Thermocouple Input with 20mA Output Current
Another output option thermocouple input circuit industry standard 20mA current transmitter. theory operation 20mA current transmitter circuit described Intersil Application Note AN177 with Figures this theory operation applies thermocouple circuit shown Figure therefore, will repeated.
Cold junction compensation provided programmable reference/temperature sensor (U2) resistor divider network according following table with register
TABLE COLD JUNCTION COMPENSATION TYPE VCJC (µV) 61.0 51.7 40.5 40.7 REGISTER
AN1298.2 2009
Application Note 1298
J-TYPE THERMOCOUPLE (51.7µV/C) EL8173 INPUT FILTER BAT54C PROG. 10µF VOUT (VCJC) FBGAIN 58.6 1.6Hz 10µF CURRENT TRANSMITTER (VTC) INU5 ISL60002BIH325Z-TK LM2936M-5.0 VOUT VOUT 0.001µF 10µF 4.7µF 499k EL8176 IRLL014N 2.5V) 127k 80.6k 100k 57.6k, (INTERNAL "GROUND") B140
+Vloop 7VDC 30VDC
ISL21400
SLAVE ADDRESS (010100x) PROG. COLD JUNCTION COMPENSATION ISL21400 REGISTER VALUES J-TYPE VCJC 51.7µV/C
VOUT (51.7µV/C) 10.7k,
357k
LOOP RESISTOR
FIGURE
circuit uses unique features Intersil EL8173 Instrumentation Amplifier simplify Thermocouple interface 20mA Current Transmitter circuit. Since this circuit shown single J-type thermocouple, fixed gain 58.6 used that output voltage EL8173 +2.5V maximum thermocouple temperature. ISL21400 programmable voltage reference/temperature sensor used cold junction compensation. Since ISL21400 non-volatile storage register values, programmed either prior assembly programmed shown this schematic. must cautioned that programming "ground" same potential "Internal Ground" loop supply ground; therefore, when programming loop supply power supply associated grounds must connected, programming system must floating ground. pass filters (R1, provide noise filtering with cut-off frequency. used return current path EL8173 input bias current. additional pass filter (R4, attenuates ISL21400's output noise voltage with 1.6Hz cut-off frequency. Since 20mA loop voltage high 24VDC, high voltage linear voltage regulator (U5) used generate internal supply.
Input with Converter Output
Another popular industry standard temperature sensor whose resistance varies with temperature, typically specified with nominal resistance +25°C. example, PT100 resistance 0°C. shape resistance temperature curve described Equation second order equation, with unique alpha value defined 60751. PT100 with alpha 0.385%/
(EQ.
Where: 3.9083 E-3, -5.775 E-7, -4.183 E-12 below zero above 0°C. RTDs typically biased with minimize selfheating effects; this operating current generates very voltage levels shown Table
TABLE TYPICALLY BIAS RTD's TEMPERATURE (°C) +100 +200 84.3 100.0 138.5 175.8 VRTD (mV) 84.3 100.0 138.5 175.8
AN1298.2 2009
Application Note 1298
Since often operated great distance from receiving electronics, differential voltage sensing used reduce errors generated high mode voltage induced noise. circuit Figure shows interface high resolution Converter using EL8173 Instrumentation Amplifier differentially sense output provide proper gain input Converter. Ratiometric mode operation Converter eliminates error introduced variations excitation current. circuit shown Figure excitation current supplied operating from +5V. would appear that this current accurate enough high precision temperature measurement. And, that true except trick that played utilizing ratiometric mode operation with Converter.
TABLE TEMP. (°C)
IEXT VOUT
84.3 100.0 138.5 175.8
IEXT (mA) 1.22 1.22 1.21 1.20
VRTD (mV) 84.3 100.0 138.5 175.8
CODE OUT10
+100 +200
FIGURE
EL8173 INVS- VOUT (VOUT)
OSC1 AVDD VRHI VRLO VINHI VINLO AGND DGND AVSS
10MHz OSC2 DVDD SCLK SDIO SYNC DRDY MODE CLOCK DATA DATA SYNC DATA READY RESET
PT100 3-WIRE
FBGAIN
Rw1, Rw2, LEAD RESISTANCE WIRE 0.0168/Ft
GAIN
HI7190 SIGMA DELTA CONVERTER
FIGURE
AN1298.2 2009
Application Note 1298
CONVERTER REFHI REFLO Iext
Code 2N*(VOUT 0)/(IEXT*R1) Code 2N*Gain*IEXT*RTD/(IEXT*R1) Code 2N*Gain*RTD/R1 Now, output code only dependant gain EL8173 value variations IEXT cancelled ratiometric operation Converter. Also, there error created wire resistance from leads from voltage sensing point. Therefore, RTDs often connected with 3-wire 4-wire lead configurations reduce effect wire resistance. far, most common configuration 3-wire connection, many general purpose 3-wire RTDs available. Three different configurations wiring summarized following. However, even with 3-wire configuration, there still error associated with voltage drop caused wire resistance. circuit incorporates technique which provides 4-wire accuracy with 3-wire RTD, effect wire resistance eliminated completely. voltage drop created wire resistance, multiplied same gain EL8173, then differential input Converter (U5) subtracts effect wire resistance,
VOUT
INEL8173 GAIN
FIGURE
simplified circuit shown Figure IEXT VCC/(R1 RTD) VRTD IEXT*RTD, VOUT Gain*IEXT*RTD Converter, digital output code,
CODE -REF
(EQ.
Where Resolution REFHi REFLo IEXT*R1
Iext PT100 2-WIRE VOUT PT100 3-WIRE
Iext VOUT
Iext
PT100 4-WIRE
VOUT
VOUT Iext*(RTD 2*Rw) ERROR Iext*2*Rw VOUT Iext*(RTD ERROR Iext*Rw
VOUT Iext*RTD ERROR
FIGURE 47A. 2-WIRE CONNECTION
FIGURE 47B. 3-WIRE CONNECTION FIGURE
FIGURE 47C. 4-WIRE CONNECTION
AN1298.2 2009
Application Note 1298
0.005 OUTPUT VOLTAGE
1.2V OUTPUT REMOTE SENSE AFTER POINT LOAD
SENSE POWER SUPPLY CIRCUIT OUTPUT VOLTAGE 0.1µF 0.1µF VOUT FBVS4 48.7k VOUT +2.5V 0.25V/A 3.3V INEL8173
GAIN
FIGURE
Voltage High Side Current Sense
rail-to-rail input stage EL8173, high side current sensing very easy implement, shown Figure This circuit appropriate power supply circuit with without remote sense capability. output current measured current sense resistor, that scaled desired output voltage resistor power rating. simple pass filter attenuate power supply output ripple noise. Resistors gain EL8173 desired full scale output voltage.
(EQ.
there often current sense resistor used monitor inductor current. this case, output current measured with that current sense resistor since average value inductor current equal load current buck regulator. circuit shown Figure example using current sense resistor, that already part current mode control loop sense load current. There ripple voltage across that inductor ripple current inductor ripple current usually load current switching frequency inductor value. This ripple voltage essential current mode control loop, must filtered obtain output load current; this filter performed Notice that even though input voltage DC/DC converter +12V, common mode voltage that applied EL8173 output voltage; this case, 1.2V. long output voltage <5V, input voltage much higher. inductor current sensed using switching FET's resistance current sense element, this method possible with EL8173 circuit. output current also sensed using inductor's Resistance) current sense element shown following circuit. This example Figure shows using EL7566, this method applies buck mode switching regulator.
this circuit, Equation shows full scale voltage 2.5V.
48.7K 0.005 0.25 (EQ.
accurate output voltage obtained since remote sense used connecting after sense resistor, remote sense possible, care should exercised minimize voltage drop across previous circuit uses external sense resistor monitor output current. DC/DC buck converter which uses internal controller with current mode control,
AN1298.2 2009
Application Note 1298
0.005 Cout
1.2V OUTPUT
CURRENT MODE CONTROLLER
3.3V 0.1µF 0.1µF VOUT FBVS4 48.7k VOUT +2.5V 0.25V/A EL8173
GAIN
FIGURE
EL7566
COILCRAFT, DO3316P-272HC 2.7µH 150µF
2.5V OUTPUT
EL7566 DEMO BOARD 0.1µF 0.1µF VOUT FBVS4 40.2k VOUT +3.0V 0.5V/A EL8173
GAIN 41.7
FIGURE
AN1298.2 2009
Application Note 1298
circuit shown Figure filter extremely important because remove voltage square wave (swinging between input voltage ground) that applied inductor. values should selected attenuate signal level that appropriate VOUT noise that acceptable. Since inductor being used current sense resistor, there several factors which degrade accuracy this approach. First, most inductors specified only maximum DCR; example, Coilcraft DO3316P-27HC shown above specified 12m, maximum. Actual measurements should Vishay offers product line inductors with specified tolerance inductor DCR; example, IHLP2525CZ-07 product family guarantees with tolerance. Second, inductor's internal winding's have temperature coefficient +0.393%/°C (copper wire) which large error source inductor allowed from ambient temperature self-heating core losses power loss. error from this source critical application, thermistor could mounted close proximity inductor used compensate temperature coefficient copper windings. High current (>30A) DC/DC converter outputs microprocessor cores present very unique challenges
0.33µH VOUT 1.2V Cout 0.33µH FB+A IHLP-2525CZ-07 TOLERANCE 3.2m 0.1µF 0.1µF 0.1µF 0.1µF ISL28273 PHASE Iout Vout1 200mV/A 4.99k TOTAL Iout Vout +3.0V 100mV/A 4.99k PHASE Iout Vout2 200mV/A
sensing output current. high currents, current sense resistors become very impractical their values minimize their power dissipation. example, with output current, current sense resistor must <400µ keep power dissipation <1W. addition, very difficult from layout viewpoint break high current power plane insert current sense resistor, that forces plane neck-down very narrow current flow path. High current DC/DC converter outputs take advantage advances multi-phase DC/DC converters where multiple lower current DC/DC conversion stages connected parallel obtain necessary high output current. Rule thumb operates each phase that output current either 3-phases operated parallel. With multi-phase DC/DC converters output current each phase measured with current sense resistor current sensing. output from each current sense circuit summed together total load current. additional feature this scheme that current balance each phase monitored. circuit using ISL28273 (dual EL8173) current sense circuit shown Figure 30A, phase circuit.
IN+A OUTA IN-A FB-A 61.9k
IN+B
OUTB 61.9k
ISL6568 600kHz
IN-B FB-B
GAIN 62.5
FB+B
FIGURE
AN1298.2 2009
Application Note 1298
output current each phase measured explained previous example. VOUT1 VOUT2 proportional output current each phase with scale factor 200mV/A. outputs, VOUT1 VOUT2, summed together with give total output current, Total IOUT, with scale factor 100mV/A. This basic scheme extended number phases extremely high output currents exceeding 100A. this application, assumed VOUT would measured with microprocessor Converter with full scale voltage 2.5V. Each channel scaled output voltage, VOUT, equal 2.0V maximum load current provide overload measurement capability 25%. each amplifier, Sensed voltage, IOUT*Rs VOUT Gain*Vs VOUT Gain*Rs*IOUT
TABLE SENSED VOLTAGE VOUT SENSITIVITY (V/A) 0.10 0.20 0.50 0.27 VOUT LOAD CURRENT
Multiplexed Voltage Current Sense
multiple voltage computer power supply, often necessary monitor output current from each DC/DC converter with Converter. When ISL28271 ISL28272 enabled (i.e., device shutdown), output stage goes into high impedance state. This allows multiple VOUT pins connected together multiplexed output applications. Since output stage high impedance state, only feedback resistors (Rf, required amplifiers operating with same gain. Likewise, different gains required each amplifier, separate feedback resistors used unique gain each amplifier. Figure demonstrates multiplexing scheme power system with four output voltages; 1.2V 20A, 1.8V 10A, 3.3V 5.0V 7.5A. dual instrumentation amplifier, ISL28271, used minimize parts count circuit size. Each power supply load current monitored with value current sense resistor (RS1, RS2, RS3, RS4) minimize voltage drop across resistor. Input protection resistors limit input fault current <5mA case short circuit connection.
IOUT(AMPS)
GAIN
gains amplifiers both single feedback resistor divider network, R10. gains different order same VOUT sensitivity using standard current sense resistor values. Gain R12, Gain R14. lines (EN1, EN2, EN3, EN4) select desired measurement channel.
AN1298.2 2009
Application Note 1298
0.001 1.2V 1.8V IN+A IN-A IN+B 0.002 1.2V OUT, 1.8V OUT, ISL28271 VOUTA VOUT
IN-B
VSEN2
FB+A
FB-A
FB+B
FB-B
VOUTB
97.6k
GAIN GAIN
1.0k
0.005 3.3V 0.002 5.0V IN+A IN-A IN+B IN-B VOUTA ISL28271 5.0V OUT, 7.5A 3.3V OUT,
VSEN4
FB+A
FB-A
FB+B
FB-B
VOUTB
97.6k 133k
GAIN GAIN 133.5
1.0k
1.0k
FIGURE MEASUREMENT POSITIVE NEGATIVE CURRENT FLOW
AN1298.2 2009
Application Note 1298
LOAD CURRENT MUST 0.01 EL8170 BAT54S FBR4 1.0k 1.0k VOUT 250k (20mv) 100k VOUT IOUT
4.7k
4.7k
FIGURE CURRENT MONITORED WITH VALUE RESISTOR
Bi-Directional Current Sense
pins EL8170 make ideal choice bi-directional current sense circuit battery gauging current monitor H-bridge configuration shown following circuits. Figure current monitored with value resistor protect EL8170 from overvoltage which would applied with excessive load current short circuit output, amplifier gain with offset 20mV center mid-range output voltage VOUT. Sensed voltage shown Equation
LOAD Gain Gain LOAD
TABLE LOAD CURRENT (ILOAD) VOUT 0.0V +2.0V +4.0V
range measured current easily changed proper selection Gain voltage. circuit Figure shows EL8170 set-up battery gauge monitor both charging current discharging current. this circuit, when battery charging, current will negative (i.e., flowing from X1). EL8170 output voltage will between +2V. When battery being discharged, current flow will from EL8170 output voltage will between +4V.
LOAD 0.01 0.02 LOAD (EQ.
AN1298.2 2009
Application Note 1298
BATTERY CHARGER, POWER SUPPLY
0.01
SYSTEM LOAD
LITHIUM-ION (4.2V) 4.7k 4.7k EL8170 FBVOUT 250k
BAT54S
VOUT IOUT 100k
1.0k
1.0k
FIGURE EL8170 SETUP BATTERY CHARGER
more direct measurement current polarity increased output voltage sensitivity required, circuit shown Figure used. Figure used measure positive current flow used measure negative current flow X1). polarity current detected which being used zero crossing detect comparator. pins EL8170 (U1, used turn proper amplifier depending current flow positive negative. above circuit, current monitored with value resistor protect EL8170's from over-voltage which would applied with excessive load current short circuit output, When this bi-directional current sense circuit used application such H-bridge, added filter signal average current value. amplifiers gain with minimum sensed current EL8170 offset voltage.
0.25mV -0.01 25mA (EQ.
Figure shows bi-directional current source circuit configured monitor motor current H-bridge circuit. direction motor (CW, CCW) monitored polarity depending direction current flow motor. rail-to- rail input capability EL8170 allows current sensing ground level ON). pulse width modulation used control speed motor, filter capacitors should used obtain average value motor current. value capacitors should selected based frequency desired overall accuracy.
AN1298.2 2009
Application Note 1298
0.01 4.7k TEXT
BAT54S
4.7k TEXT ISL28271 IN+Iout ISL28271 IN-Iout VOUTB FB+B FB-B 1.0k ISL28271 INPOLARITY DETECT VOUTA FB+A FBA- VOUTA FB+A FB-A
VOUT 1V/A 100k
+Iout -Iout 2N7002
FIGURE MEASUREMENT POSITIVE NEGATIVE CURRENT FLOW
AN1298.2 2009
Application Note 1298
0.01
MOTOR
VOUT POLARITY
VOUT Imotor CLOCKWISE COUNTER CLOCKWISE
EL8170 CIRCUIT (GAIN 100)
FIGURE BI-DIRECTIONAL CURRENT SOURCE CIRCUIT
Intersil Corporation reserves right make changes circuit design, software and/or specifications time without notice. Accordingly, reader cautioned verify that Application Note Technical Brief current before proceeding.
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AN1298.2 2009

Application Note 2009 AN1298.2 Introduction Instrumentation Ampli

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