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Precision current sensing amps with drift.
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Only National guarantees TCVOS tri-temp testing each device
LMP8270 Typical applications diagram
4.75V 5.5V Battery
LMP8270
RSENSE Proprietary level-shift circuitry
Gain
Gain 100k
Load
Optional filter network
National Semiconductor Corporation, 2005. National Semiconductor,
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LMP8270, LMP8271, LMP8272 Features:
Input offset voltage TCVOS µV/°C CMRR (min) Gain temp drift ppm/°C Extended CMVR Operating temperature range -40°C 125°C; (bare die) -40°C 150°C LMP8270/72 Gain 20/14 LMP8271 Bidirectional measurement Supply voltage 4.75V 5.5V Supply current Rail-to-rail output voltage swing Available SOIC-8 packages
Ideal automotive, industrial, medical, test instrumentation, consumer PCs, notebooks
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CURRENT MEASUREMENT
Tuite, Analog/Power Editor
Take Right Steps Achieve Accurate Measurements
Starts With Ohm's (Then Gets Interesting)
urrent measurements used variety equipment control safety functions. Measuring voltage drop across low-value resistor
most common method measuring current flowing circuit (Fig. current flow through load also flows through classic approach current measurement shunts milliammeter with resistor RSENSE resistor, also scale current through meter. known shunt resiscontemporary approach amplifies voltI tor, which creates voltage drop across resistor applies Load amplified voltage meter analogage drop, across to-digital converter. resistor. Figure depicts current-measurement RSENSE approaches. Ammeters VM(1 R2/R1) were developed RSENSE early history electronMeter ics, they're still used (Shunt) today. Typically, meter itself read full-scale when current through read larger currents, connected parallel with resistor that shunts most current around meter. That way, meter scaled measure magnitude current.
Online 9940 Supplement Electronic Design/March 2005 Sponsored National Semiconductor
Resistor Sizing Op-Amp Gain
more contemporary approach measuring current uses operational amplifier, amp, amplify voltage dropped sense resistor. Often, output connected
analog-to-digital converter (ADC). first steps choose value sense resistor amplifier's gain. example, suppose current through load varies from designed maximum input signal Because current relatively high, very low-value shunt resistor required. simplify calculations, choose 0.025 maximum current voltage, across shunt 0.025 physical size shunt resistor determined power dissipation, I2R. Using values from above, 0.025 power rating more should used minimize heat shunt resistor. satisfy ADC's input requirements, gain required amplifier 5.0v 0.250v
signal from shunt resistor applied non-inverting input amplifier, gain plus ratio feedback input resistors. Choosing value value sets gain
Consider Shunt Resistor Thermal Characteristics
above example points several areas that need additional consideration. heating shunt resistor power dissipation ambient temperature changes affects measurement accuracy changing resistance shunt. various metals used resistors offer different temperature coefficients (see table).
Temperature Coefficients Metals Used Resistors
Material Copper Nickel Platinum Manganin Nichrome Resistivity 10-6 10-6 10-6 10-6 10-6 Temperature coefficient 0.004041 0.005866 0.003729 0.000015 0.00017
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LMPamplifiers deliver high precision over time temperature
extended temperature range, noise, single/dual/quad precision amplifier family
LMP2011, LMP2012, LMP2014 features: guaranteed offset voltage over temperature Very input offset voltage drift (0.015 µV/°C) Extended operating temperature range from -40°C 125°C Slew rate V/µs) Wide gain-bandwidth product MHz) noise High open-loop voltage gain (130 High CMRR (130 PSRR (120 supply current (0.930 LMP2011 available space-saving SOT23-5 SOIC-8 packages LMP2012 available MSOP-8 package LMP2014 available TSSOP-14 LLP-14 packages
Voltage noise frequency
1000
Voltage Noise (nV/Hz)
10.0 100.0 1000.0 10000.0
Frequency (Hz)
CMRR frequency
Ideal medical, industrial, test measurement, scales weigh stations, automotive, telecommunications applications.
CMRR (dB) Frequency (Hz)
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100k
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BASICS
Design
Kelvin Sense Resistors
second area concern connection between shunt resistor amplifier's input. exam-
we've been using, ordinary resistor would have 20-gauge copper leads. Assume leads trimmed either side resistor soldered place. Twenty-gauge copper wire resistance about 0.001 /in., which represents error, relative resistor's nominal value. avoid introducing such error, additional features needed: Kelvin connection shunt resistor differential-input amplifier. Kelvin connection four-wire connection with force leads sense leads (Fig. wires force leads, they connect shunt resistor high-current side circuit. other wires sense leads, they connect voltage across shunt resistor amplifier. This arrangement separates load-current-carrying wires from measurement circuit. voltage drops removed from measurement voltage across shunt resistor. take full advantage Kelvin connection, differential amplifier, Figure necessary.
RSENSE
avoid measurement errors caused voltage drops across conventional resistors' leads, Kelvin-connection sense resistors supplied with four leads: forcing current, others, connected right resistance element, high-impedance measurement circuit. Kelvin resistors come number configurations.
Load RSENSE (Shunt) (VW3 VW4)(R2/R1)
optimal measurement accuracy, differential amplifier avoids ground-referencing current-sense voltage.
Sponsored National Semiconductor
Low-Side High-Side Sensing
lacing sense resistor between load ground called lowside sensing. Low-side sens-
acceptable when input voltages positive close ground. However, problems arise when amps used those conditions met. LowDifferential amplifier Load Load side current measurements cause prob- amps exceeding negative common-mode voltage limit amp. they designed, amps Differential amplifier only work correctly when input common-mode voltage within certain range between positive negative powerControl unit supply voltages. rail-to-rail-input that operates from single supply might have datasheet common-mode input voltage spec example low-side measurement Locating sense resistor between load negative common-mode that would create common-mode problems, circuit's common node, practice called lowvoltage limit most side sensing introduce measurement inaccuraconsider distributed-load system cies when parasitic resistances between sense amplifiers. Unless Figure control unit (dashed line) uses resistor common node significant. amplifier being used MOSFETs control power loads. differential designed Parasitic resistances, RP2, between negative common-mode voltages, will oper+ power source, loads, control unit repDC correctly this circuit. (Another disadvantage resent ground wiring resistance con- nector resistance. real circuit, they easily have low-side sensing this case that wiring required between shunt resistors inputs differvalues several tenths ohm. this example, ential amps.) assume Differential amplifiers High-side current sensing solves these current1 just like differential amplifiers measurement problems (Fig. shunt resistors Figure supply side power source instead Figure current each load shunt resistors (RS1 RS2) 0.025 con- common side. This arrangement permits shunt resistors mounted control unit with differentrol unit's current When load tial amplifiers MOSFET switches. wiring from voltage point volts more positive than control module also simplified. point (0.2 volts this case, differential amplifiers have more positive then VG2. result, inputs differable handle input common-mode voltage equal ential amplifier -2.2 relative common voltage supplied power source. connection control unit. This greater then
handle this operate differential amplifier from power source amplifier that input common-mode voltage specification that includes positive supply. Another amplifier that designed high input commonmode voltages. These have advantage operating from same supply voltage other control circuits when inputs connected higher positive voltage. general, measuring current flow electrical circuits requires amplifiers with input common-mode voltage specifications that extend beyond positive negative supply voltages amplifier while maintaining input offset voltage gain accuracy.
Load Load Differential amplifier Control unit Differential amplifier
High-side sensing eliminates disadvantages low-side sensing, requires differential amplifiers with inputs that accommodate supply voltage within their common-mode input range.

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