Thomas Mosteller Introduction microcontrollers become more compact pow

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Thomas Mosteller Introduction microcontrollers become more compact pow

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Application Note August 1992 Interfacing Microprocessor Based Systems
Thomas Mosteller Introduction microcontrollers become more compact powerful, opportunity arises designer these digital powerhouses pack more more functionality onto small PCBs. number devices include even 10-bit A/Ds, allow painless conversion from analog realm into neat simple digits. situations where onchip appropriate nonexistent, many external systems-on-a-chip available. Linear Technology LTC1099, LTC1096/LTC1098 LTC1196/ LTC1198 8-bit A/Ds; LTC1090 thru LTC1096 10-bit A/Ds; LTC1290 through LTC1296 12-bit A/Ds. attractive this possibility sounds, seems input signal never quite form needs easy digitization. It's either small, noisy, impedance high, it's referenced ground, some combination these factors. problem varies, seems there's always something that requires little signal conditioning prior digitization. Operational amplifiers obvious method solving these problems, there snags. most common design constraints microcontroller based PCBs only supply. Modern single supply amps come close allowing operation under these constraints, with important exceptions successful designer should aware This note will outline some these pitfalls, along with techniques solving them. Search Rail-to-Rail first approach simply find which will handle rail-to-rail inputs outputs. common method CMOS amp, it's panacea thought some designers. general, CMOS output stages suffer from fixed voltage drops inherent bipolar devices. Instead, losses ohmic nature. result, when operating into high impedance loads, output CMOS amplifier typically comes very close probably within millivolts power supply rails. output problem. news, which resides inputs. While most CMOS amplifiers handle inputs down negative supply rails always with perfect accuracy), they can't close positive rail. class, input common mode range limit generally falls about 2.5V below positive supply. there more problems. general, CMOS amplifiers have very good input leakage current characteristics, especially temperatures. This makes them well suited high source impedance applications. kicker, though, input offset voltage, which generally millivolt range. This limits CMOS device's usefulness gains. Unfortunately, many sensors have output voltages which require rather high gains. course, there around these large input offset voltages. Zero drift amps retain desirable characteristics CMOS output stage while continuously autozeroing input offset voltage. While many designers prejudiced against zero drift amps (especially those have used older devices), modern "choppers" have eliminated most pitfalls. They provide level stability over temperature time other technology match. George Erdi's Design Note revealing discussion. between these extremes accuracy there lies middle ground, which entails making composite amplifier job. shown Figure using good single supply bipolar front end, with inexpensive 4000 series CMOS gate back end, excellent results achieved. Williams outlined
Note Design Notes, Chopper Bipolar Amps Unbiased Comparison George Erdi, December, 1990.
AN50-1
Application Note
this method AN182, works quite well. front amplifier's output simply sits CMOS gate's input threshold voltage, relieving requirement swing rails. economize package count here versus AN18 circuit. Since we're usually driving CMOS A/D, with inherent high input impedance, excellent results obtained using just CMOS gate instead six. Tests circuit using gate, shown Figure show that within 1LSB LSBs both rails, even 10-bit circuits, load impedances 100k Thus, combine good single supply like LT1013/LT1014 LT1077/LT1078/ LT1079, along with 4069 inverter, very good accuracy combined with rail-to-rail output swing. clever designer combine leftover gates with amps, giving "free" performance enhancement.
Note Application Note Power Gain Stages Monolithic Amplifiers Williams, March, 1986.
Three things watch here are: careful noise have route analog signal over noisy digital data paths. non-inverting gate, reverse sense inputs shown Figure aware that power supply current CMOS gate will since it's operating linear region. tested random parts, found average quiescent current hundreds microamps range. Micropower system designers need keep this mind. This composite amplifier still leaves short mark common mode range. Single supply bipolar devices like LT1013/LT1014 have common mode input range limit about 1.5V below positive supply rail.
0.020 LT1014 LT1014/4069 BUFFER 0.015
VOUT
220pF
0.010
0.005
VOUT
LT1014
4069 100k 0.0022µF
LOAD
100k
LTAN50 TA02
Figure Saturation Performance Minimum VOUT
100k
LT1014 0.013 0.013 0.013 0.012 0.012 0.011 0.010 4.390 4.380 4.360 4.340 4.320 4.290 4.240 LT1014/4069 4.991 4.971 4.951 4.932 4.893 4.797 4.606
LOAD 200k
VOUT
LTAN50 TA01
LT1014 LT1014/4069 BUFFER LOAD 100k
LTAN50 TA03
Figure LT1014 4069 Composite Amplifier
Figure Saturation Performance Maximum VOUT
AN50-2
Application Note
Giving Search: Alternate Approaches easiest method ensuring proper operation over full range cheat higher power supply voltages. Consider case where your circuit local voltage regulator provide +5V. utilize input this regulator analog circuitry. 6.5V power supply enough ensure that devices like LT1013 LT1078 will handle their inputs outputs. While supply have unknown noise characteristics, this usually handled amp's power supply rejection ratio, which over 100dB frequencies. you're running switching power supply whose output full high frequency noise, small filter supply line will help this problem. Figure illustration this technique.
VSUPPLY VANALOG 100µF
Realizing this, make RS-232 device serve dual purpose without requiring additional parts many designs, pulling power from interface chip's plus minus rails. instance, charge pump LT1180 RS-232 RX/TX chip easily provide 10mA external load while still meeting RS-232 interface standard. Noise these lines reduced filter. This will provide approximately 100:1 noise reduction power supply ripple frequency. Even don't like idea generating additional power supplies your board, have choice. instance, wish have reference your converter, you're going need more positive supply that reference. those plan simply line their reference, beware! This supply typically noisy rapid current changes caused logic chips. addition, poor initial tolerance, which amounts nearly counts error 8-bit system. Adding trimmer adjust system proper readings only partial solution, since doesn't allow regulator's unspecified time temperature drift, which will also poor. These factors problems systems which transducers with ratiometric output that power supply both transducer drive reference, thereby cancelling errors voltage. reduce this problem Linear Technology's precision fixed output voltage regulators, which have tighter output voltage specs than industry standard parts. instance, LT1086-5 initial tolerance, over temperature, cutting initial error counts. Compared industry standard regulators like 7805, these devices suffice reference some applications. this route, consider using filter reference path help kill noise caused digital load. those decide real reference, your problems aren't over just yet. shown Figure typical power supply tolerance results minimum voltage 4.75V. Using LT1029 LT1019 reference, maximum output voltage 5.01V. have system where, worst case, reference voltage 0.26V higher than power supply. This problem A/D.
REGULATOR
VLOGIC
LTAN50 TA04
Figure Clean Analog Supply from Input Supply Voltage
these voltages aren't available from input power supply, LT1026 along with couple caps provides enough current drive allow dozens power amps. currents, LT1026 will provide ±9V, which more than enough ensure proper operation inexpensive precision amps like LT1097 over full range both input output. usual objection this approach that adds cost requires another This true, important factor cost designs. Also, some designers feel that purity beauty their design spoiled addition other power supply voltages. However, these aesthetic properties manage survive affront incorporating only RS-232 interface chips, even though these devices utilize same type charge pump generate bipolar RS-232 signals.
AN50-3
Application Note
VSUPPLY 7805 VMAX 5.25V VMIN 4.75V
4.7k VOUT
1N914 1N914
OPTIONAL BUFFER
LT1019-5
VMAX 5.01V VMIN 4.99V
CONVERTER
LT1013
LTAN50 TA05
LT1013
Figure Power Supply Voltage Higher Than Reference Voltage
this isn't only place where this issue crops LT1026 voltage converter, your amps running ±9V. With these supply voltages, they capable merrily swinging their outputs, inputs present same unhappy possibilities. Even allowing inputs beyond power supply range does damage latch A/D, will almost always cause improper readings other channels. best solution here system dependent. Some will require Shottky diode clamps input capable exceeding power supply range prevent latch-up possible damage. Others only require series input resistors analog input, must test such combination make sure that: conversion speed accuracy degraded formed input resistor A/D's intrinsic capacitance. Errors introduced input leakage current, especially elevated temperatures. multiple channel A/D, over range input will affect other channels. must ensure that signal does beyond A/D's power supply range, employ circuit shown Figure These clamps will hold input within legal range long input signal's slew rate does exceed clamp amplifier's ability come saturation limit. buffer amplifier necessary applications that require output impedance. Ensuring that reference does exceed power supply slightly different story. While series resistor prevent damage A/D, accuracy high voltage inputs will degraded. Here different approach required.
LTAN50 TA06
Figure Precision Limiter Analog-to-Digital Converter Inputs
both initial long-term accuracy requirements low, ratiometric system used, previously discussed filtered approach simplest. This automatically solves VREF overvoltage problem. have stable reference, there other approaches. need local regulation your board, utilize LT117 LT1086 circuit shown Figure superior reference used these parts allows them voltage which will always higher than reference, lower than 5.25V maximum logic specified for.
+VIN 6.73V LT1086 VOUT 5.23V 5.13V 5.02V 10µF
LTAN50 TA07
Figure Regulator with VOUT 5.25V
However, your doesn't local regulation, brings nominal supply from external source, you're stuck. typical power supply tolerance means that, unless take steps raise supply voltage your board, can't work with signal "straight in." Rescaling Output 2.5V This accomplished realizing that normally really don't have input swing A/D. Most
AN50-4
Application Note
A/Ds provide both high reference inputs, rather than tying them power supply terminals. fact, given noisy state average digital power supply, manufacturer generally choice! take advantage this using lower reference voltage that within swing capabilities unaided single supply amp. Most single supply amps swing within positive power supply when lightly loaded. Looking over range available references leads pick 2.5V convenient value. References like LT1004-2.5 LT1009 inexpensive give good accuracy this voltage level. This reference voltage allows plenty margin output swing high end, well keeping trouble with input common mode range. addition, it's impossibly A/D. Most 10-bit designs will work adequately this reference voltage. end. Linear Technology bipolar single supply amps unique output topology which allows them pull their output down less than 10mV with good linearity. 8-bit, 2.5V reference system, this amounts error approximately count. This grows counts 10-bit systems. However, keep mind, can't really A/D's full range most time have allow some margin circuit tolerances both high ends A/D's
2.49k LT1006, LT1013, LT1014, LTC1047, LTC1049, LTC1050, LTC1051/LTC1053, LTC1052, LT1077, LT1078, LT1079, LT1178, LT1179 VREF_HIGH
range. Most 10-bit systems will noticeably compromised having counts unavailable end. suppose can't deal with that additional error. There simple those lowest counts back, using technique known synthetic ground. Rather than using power supply's ground terminal common point amplifier chain, resistor divider create arbitrary ground point offset above power supply ground. This shown Figure Since only need millivolts offset here, value resistor bottom divider chain. This gives stable voltage synthetic ground without drawing current. amp, it's exactly though have power supply voltages say, 4.95V 0.05V. take advantage lower reference voltage input feed synthetic ground into there. Thus, counts will always equal value synthetic ground, full scale will equal synthetic ground plus reference voltage. Some designers feel uncomfortable without hardwired ground terminal. most systems, ground arbitrary point, such bottom bridge rectifier power supply. long synthetic ground power supply ground both tied common point, such power supply ground, there's problem. This
LT1009 2.5V)
ANALOGTO-DIGITAL CONVERTER
VREF_LOW POWER SUPPLY RN1, BECKMAN #698-3-R100K-D
LTAN50 TA08
SYNTHETIC GROUND
OPTIONAL BYPASS
Figure Synthetic Ground
AN50-5
Application Note
technique especially good when voltage being measured generated floating transducer whose output committed ground. we're completely free gotchas. You'll have remember make voltage measurements referenced synthetic ground, rather than instinctively clipping your lead power supply ground. This difficult point across troubleshooting personnel. Also, watch ground loops when hook powered voltmeters oscilloscopes your synthetic ground point. most valid complaint that cable running from remote transducer your circuit will have "hard" ground. This true, ohms "real" ground could troublesome some cases. However, shielded cable, shield isolated from circuit ground connected power supply earth ground. Additionally, synthetic ground point coupled ground through bypass cap, giving ground. sure pick value which will funny things your circuit parameters frequencies interest. now, using this technique, finally handle input with only supplies that started this discussion first place using input Figure Here, signal through precision divider which formed matched resistor package, such Beckman 698-3-R100K-D. These resistors matched well enough allow 8-bit accuracy with trimming. detailed above, this shifts output more manageable 2.5V. Further, output divider committed synthetic ground, rather than power supply system ground. Using LT1009 2.5V reference gives 8-bit accurate system with adjustments. only caveat remaining that signal source containing must share common ground, otherwise must abandon synthetic ground approach lose bottom two. Resolution Issue: When Eight (Bits) Enough Resolution, along with memory megahertz, some things digital systems never have enough Linear Technology manufactures line A/Ds which address this need nicely. what bits isn't quite enough, need just more bit? It's hard justify cost another A/D, especially when microcontroller you're using board already. circuit Figure will another resolution cheaply. uses quad same precision resistor package referenced above "fold" 2.47V input range 1.235V 2.47V while providing comparator sense whether input upper lower half range. amps U1B, along with RN6, form full wave rectifier with "center" point equal reference voltage, VREF LOW. Input voltages above VREF passed through rectifier unchanged, while those below VREF inverted shifted upwards according equation: VOUT VREF accuracy this section held within bits accurate matching package's resistors errors amps shown figure. lower power consumption required, LT1014 amps replaced LT1079 LT1179. formed comparator U1C. output this comparator will trigger standard CMOS gate, value read into free channel. Note that when low, readings biased upwards inverted. proper sense restored reading software. A/D's positive reference voltage (VREF_HIGH) exactly twice LT1004-1.2 reference's voltage U1D. gain this circuit precisely resistors resistor package. Thus, sees input signal changing between 1.235V 2.47V. This circuit does require negative voltage operate, although does have regulated. Even More Resolution There times when even more than bits resolution required. example when monitor variable which take number different ranges, each which requires good precision. instance,
AN50-6
Application Note
2.47V
4.7k
2.7k
BECKMAN #698-3-R100K-D DIODES 1N914 AMPS LT1014, LT1079 LT1179
LT1004-1.2
Figure 8-Bit 9-Bit Converter
industrial temperature monitors many different kinds thermocouples, each them used over wide range. instrument required have display range 2000°F resolution 0.1°F without requiring gain offset changes, you'd need with least 20,000 counts. This implies 17-bit A/D, expensive proposition. There less costly approach you're monitoring slowly changing parameter like temperature. voltage frequency converter used convert input variable into precision pulse train. counting these pulses over fixed time length, very accurate voltage reading made. This particularly easy software, most microcontrollers have input capture function which allows them count incoming pulses conveniently. simple this shown Figure This variation circuit Williams showed AN33, Figure major modification made here that
Note Application Note Applications Switched-Capacitor Instrumentation Building Block Williams, July, 1985.
13.7k 0.22µF
2.2k
4.7µF
LT1004 1.2V
LT1018
1N5712
Figure Single Supply Voltage Frequency Converter
Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
VOUT A/D) 1.235V 2.47V VREF_LOW +2.47V VREF_HIGH
LTAN50 TA09
LTC1043
fOUT 0kHz 5kHz
0.01µF* 1N4148 560pF START-UP *POLYSTYRENE
68pF
LTAN50 TA10
AN50-7
Application Note
single supply used rather than dual supplies employed earlier circuit. This necessitates switch single supply comparator, LT1018. circuit operates balancing current flowing into input with discrete packets charge delivered flying capacitor (C2). This capacitor charged 1.2V reference level when LTC1043 analog switch's connected pins respectively. input voltage charges capacitor inverting input LT1018 comparator rises slightly above ground, forcing comparator's output low. This causes analog switch change states bringing C2's positively charged ground forcing negative. This negatively biased charge then balances positively biased charge stored input capacitor. delay, provided ensures that C2's charge transferred Clamp diode protects comparator's noninverting input from excessive negative excursion. input voltage rises, input capacitor charges more quickly, thus requiring more frequent "hits" C2's charge balance circuit. This action forms voltage frequency converter with following equation:
Although 5kHz full-scale frequency shown, output frequencies tens possible rescaling both. Applications requiring very high resolution require long pulse accumulation times. slowly changing variables like temperature this generally problem. Start-up overdrive cause this circuit's coupled loop latch. output comparator forced extended period, analog switch forces inverting input comparator before charged This stable condition, oscillation will commence. problem cured addition When comparator's output low, LTC1043's internal oscillator "sees" ground begins oscillating this state persists. free running oscillation pumps charge until normal operation commences. Under normal conditions, comparator's output state controls analog switch through Another potential problem that reversed sense forces LTC1043 inverting input LT1018 below ground. This usually poor practice. However, size input capacitor been chosen limit excursion below ground safe level. input capacitor should made smaller without careful testing ensure linearity high temperatures. This circuit meets ideals outlined beginning this note that only requires supply handle input signal. fact, input swing levels greater than power supply, since amp/comparator used inverting mode summing junction held close ground times.
VREF
(See note
Typical performance specifications are: FOUT 0kHz 5kHz Linearity 0.0025% PSRR 0.1%/V 200ppm/°C
Note Note accidental inversion term equation.
literature call (800) 637-5545. applications help, call (408) 432-1900, Ext. 777.
AN50-8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7487
(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977
BA/GP 0393
LINEAR TECHNOLOGY CORPORATION 1993

Thomas Mosteller Introduction microcontrollers become more compact pow

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