Application Note 43 June 1990 Bridge Circuits Marrying Gain and Balance Jim Williams Bridge circuits are among the most elemental and powerful and stability of the basic configuration. In particular, trans- electrical tools. They are found in measurement, switch- ducer manufacturers are quite adept at adapting the bridge ing, oscillator and transducer circuits. Additionally, bridge to their needs (see Appendix A, “Strain Gauge Bridges”). techniques are broadband, serving from DC to bandwidths Careful matching of the transducer’s mechanical charac- well into the GHz range. The electrical analog of the me- teristics to the bridge’s electrical response can provide a chanical beam balance, they are also the progenitor of all trimmed, calibrated output. Similarly, circuit designers electrical differential techniques. have altered performance by adding active elements (e.g., amplifiers) to the bridge, excitation source or both. Resistance Bridges Figure 1 shows a basic resistor bridge. The circuit is Bridge Output Amplifiers usually credited to Charles Wheatstone, although S. H. A primary concern is the accurate determination of the Christie, who demonstrated it in 1833, almost certainly differential output voltage. In bridges operating at null the preceded him.1 If all resistor values are equal (or the two absolute scale factor of the readout device is normally sides ratios are equal) the differential voltage is zero. The less important than its sensitivity and zero point stability. excitation voltage does not alter this, as it affects both An off-null bridge measurement usually requires a well sides equally. When the bridge is operating off null, the calibrated scale factor readout in addition to zero point excitation’s magnitude sets output sensitivity. The bridge stability. Because of their importance, bridge readout output is nonlinear for a single variable resistor. Similarly, mechanisms have a long and glorious history (see Ap- two variable arms (e.g., RC and RB both variable) produce pendix B, “Bridge Readout—Then and Now”). Today’s nonlinear output, although sensitivity doubles. Linear investigator has a variety of powerful electronic techniques outputs are possible by complementary resistance swings available to obtain highly accurate bridge readouts. Bridge in one or both sides of the bridge. amplifiers are designed to accurately extract the bridges A great deal of attention has been directed towards this differential output from its common mode level. The circuit. An almost uncountable number of tricks and tech- ability to reject common mode signal is quite critical. A niques have been applied to enhance linearity, sensitivity typical 10V powered strain gauge transducer produces only 30mV of signal “riding” on 5V of common mode level. 12-bit readout resolution calls for an LSB of only 7.3μV…..almost 120dB below the common mode signal! Other significant error terms include offset voltage, and R A RC its shift with temperature and time, bias current and gain DIFFERENTIAL stability. Figure 2 shows an “Instrumentation Amplifier,” EXCITATION OUTPUT VOLTAGE which makes a very good bridge amplifier. These devices + VOLTAGE are usually the first choice for bridge measurement, RB RD and bring adequate performance to most applications. Note 1: Wheatstone had a better public relations agency, namely himself. AN43 F01 For fascinating details, see reference 19. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Figure 1. The Basic Wheatstone Bridge, Technology Corporation. All other trademarks are the property of their respective owners. Invented by S. H. Christie an43f AN43-1 Application Note 43 In general, instrumentation amps feature fully differential 100, with additional trimmed gain supplied by A1B. The inputs and internally determined stable gain. The absence configuration shown may be adjusted for a precise 10V of a feedback network means the inputs are essentially pas- output at full-scale pressure. The trim at the bridge sets sive, and no significant bridge loading occurs. Instrumenta- the zero pressure scale point. The RC combination at A1B’s tion amplifiers meet most bridge requirements. Figure 3 input filters noise. The time constant should be selected lists performance data for some specific instrumentation for the system’s desired lowpass cutoff. “Noise” may amplifiers. Figure 4’s table summarizes some options originate as residual RF/line pick-up or true transducer for DC bridge signal conditioning. Various approaches responses to pressure variations. In cases where noise are presented, with pertinent characteristics noted. The is relatively high it may be desirable to filter ahead of A3. constraints, freedoms and performance requirements of T his pr even t s any pos sible signal in fideli t y due to nonline ar any particular application define the best approach. A3 operation. Such undesirable outputs can be produced by saturation, slew rate components, or rectification effects. When filtering ahead of the circuits gain blocks + remember to allow for the effects of bias current induced errors caused by the filter’s series resistance. This can be – a significant consideration because large value capacitors, mNO FEEDBACK RESISTORS USED particularly electrolytics, are not practical. If bias current mGAIN FIXED INTERNALLY (TYP 10 OR 100) OR SOMETIMES RESISTOR PROGRAMMABLE induced errors rise to appreciable levels FET or MOS input mBALANCED, PASSIVE INPUTS AN43 F02 amplifiers may be required (see Figure 3). Figure 2. Conceptual Instrumentation Amplifier To trim this circuit apply zero pressure to the transducer and adjust the 10k potentiometer until the output just DC Bridge Circuit Applications comes off 0V. Next, apply full-scale pressure and trim the 1k adjustment. Repeat this procedure until both points Figure 5, a typical bridge application, details signal con- are fixed. ditioning for a 350Ω transducer bridge. The specified strain gauge pressure transducer produces 3mV output Common Mode Suppression Techniques per volt of bridge excitation (various types of strain-based transducers are reviewed in Appendix A, “Strain Gauge Figure 6 shows a way to reduce errors due to the bridges Bridges”). The LT®1021 reference, buffered by A1A and common mode output voltage. A1 biases Q1 to servo the A2, drives the bridge. This potential also supplies the bridges left mid-point to zero under all operating condi- circuits ratio output, permitting ratiometric operation of tions. The 350Ω resistor ensures that A1 will find a stable a monitoring A/D converter. Instrumentation amplifier operating point with 10V of drive delivered to the bridge. A3 extracts the bridge’s differential output at a gain of This allows A2 to take a single-ended measurement, LTC1043 PARAMETER LTC1100 LT1101 LT1102 (USING LTC1050 AMPLIFIER) Offset 10μV 160μV 500μV 0.5μV Offset Drift 100nV/°C 2μV/°C 2.5μV/°C 50nV/°C Bias Current 50pA 8nA 50pA 10pA Noise (0.1Hz to 10Hz) 2μVP-P 0.9μV 2.8μV 1.6μV Gain 100 10,100 10,100 Resistor Programmable Gain Error 0.03% 0.03% 0.05% Resistor Limited 0.001% Possible Gain Drift 4ppm/°C 4ppm/°C 5ppm/°C Resistor Limited <1ppm/°C Possible Gain Nonlinearity 8ppm 8ppm 10ppm Resistor Limited 1ppm Possible CMRR 104dB 100dB 100dB 160dB Power Supply Single or Dual, 16V Max Single or Dual, 44V Max Dual, 44V Max Single, Dual 18V Max Supply Current 2.2mA 105μA 5mA 2mA Slew Rate 1.5V/μs 0.07V/μs 25V/μs 1mV/ms Bandwidth 8kHz 33kHz 220kHz 10Hz Figure 3. Comparison of Some IC Instrumentation Amplifiers an43f AN43-2 Application Note 43 CONFIGURATION ADVANTAGES DISADVANTAGES +V Best general choice. Simple, CMRR, drift and gain stability RATIO straightforward. CMRR typically may not be adequate in highest OUT >110dB, drift 0.05μV/°C to 2μV/°C, precision applications. May require gain accuracy 0.03%, gain drift second stage to trim gain. + 4ppm/°C, noise 10nV√Hz – 1.5μV OUT for chopper-stabilized types. Direct ratiometric output. – INSTRUMENTATION AMPLIFIER AN43 F04a +V CMRR > 120dB, drift 0.05μV/°C. Multi-package—moderately RATIO Gain accuracy 0.001% possible. complex. Limited bandwidth. OUT Gain drift 1ppm with appropriate Requires feedback resistors to set resistors. Noise 10nV√Hz – 1.5μV gain. for chopper-stabilized types. Direct + ratiometric output. Simple gain trim. Flying capacitor commutation OUT provides lowpass filtering. Good – choice for very high performance— monolithic versions (LTC1043) available. OP AMP AN43 F04b CMRR > 160dB, drift 0.05μV/°C to Requires floating supply. No direct 0.25μV/°C, gain accuracy 0.001% ratiometric output. Floating supply possible, gain drift 1ppm/°C with drift is a gain term. Requires appropriate resistors plus floating feedback resistors to set gain. supply error, simple gain trim, Noise 1nV√Hz possible. + + OUT – AN43 F04c OP AMP +V CMRR ≈ 140dB, drift 0.05μV/°C to No direct ratiometric output. 0.25μV/°C, gain accuracy 0.001% Zener supply is a gain and offset possible, gain drift 1ppm/°C with term error generator. Requires appropriate resistors plus floating feedback resistors to set gain. supply error, simple gain trim, Low impedance bridges require noise 1nV√Hz possible. substantial current from shunt + regulator or circuitry which OUT simulates it. Usually poor choice if precision is required. – AN43 F04d OP AMP –V Figure 4. Some Signal Conditioning Methods for Bridges an43f AN43-3 Application Note 43 CONFIGURATION ADVANTAGES DISADVANTAGES +V CMRR > 160dB, drift 0.05μV/°C to Requires precision analog level RATIO 0.25μV/°C, gain accuracy 0.001% shift, usually with isolation OUT possible, gain drift 1ppm/°C with amplifier. Requires feedback appropriate resistors, simple gain resistors to set gain. trim, ratiometric output, noise 1nV√Hz possible. + + OUT – + OP AMP AN43 F04e +V CMRR ≈ 120dB to 140dB, drift Requires tracking supplies. RATIO 0.05μV/°C to 0.25μV/°C, gain Assumes high degree of bridge OUT accuracy 0.001% possible, gain symmetry to achieve best CMRR. drift 1ppm/°C with appropriate Requires feedback resistors to set resistors, simple gain trim, direct gain. ratiometric output, noise 1nV√Hz + possible.