Designing with a New Family of Instrumental Amplifiers Design Note 40 Jim Williams A new family of IC instrumentation amplifiers achieves performance and cost advantages over other alternatives. Conceptually, an instrumentation amplifier is simple. Figure 1 shows that the device has passive, fully differential inputs, a single ended output and internally set gain. Additionally, the output is delivered with respect to the reference pin, which is usually grounded. Maintaining high performance with these features is difficult, accounting for the cost-performance disadvantages previously associated with instrumentation amplifiers. Figure 2 summarizes specifications for the amplifier family. The LTC®1100 has the extremely low offset, drift, and bias current associated with chopper stabilization techniques. The LT®1101 requires only 105μA of supply current while retaining excellent DC characteristics. The + – q NO FEEDBACK RESISTORS USED q GAIN FIXED INTERNALLY (TYP 10 OR 100) OR SOMETIMES RESITOR PROGRAMMABLE q BALANCED, PASSIVE INPUTS q OUTPUT DELIVERED WITH RESPECT TO OUTPUT DN040 F01 OUTPUT REFERENCE PIN REFERENCE Figure 1. Conceptual Instrumentation Amplifier PARAMETER Offset Offset Drift Bias Current Noise (0.1Hz-10Hz) Gain Gain Error Gain Drift Gain Nonlinearity CMRR Power Supply Supply Current Slew Rate Bandwidth CHOPPER STABILIZED MICROPOWER HIGH SPEED LTC1100 LT1101 LT1102 10μV 100nV/°C 50pA 2μVp-p 100 0.03% 4ppm/°C 8ppm 104dB Single or Dual, 16V Max 2.2mA 1.5V/μs 8kHz 160μV 2μV/°C 8nA 0.9μV 10,100 0.03% 4ppm/°C 8ppm 100dB Single or Dual, 44V Max 500μV 2.5μV/°C 50pA 2.8μV 10,100 0.05% 5ppm/°C 10ppm 100dB Dual, 44V Max 105μA 0.07V/μs 33kHz 5mA 25V/μs 220kHz Figure 2. Comparison of The New IC Instrumentation Amplifiers 10/90/40_conv FET input LT1102 features high speed while maintaining precision. Gain error and drift are extremely low for all units, and the single supply capability of the LTC1100 and LT1101 is noteworthy. The classic application for these devices is bridge measurement. Accuracy requires low drift, high common mode rejection and gain stability. Figure 3 shows a typical arrangement with the table listing performance features for different bridge transducers and amplifiers. Bridge measurement is not the only use for these devices. They are also useful as general purpose circuit components, in similar fashion to the ubiquitous op amp. Figure 4 shows a voltage controlled current source with load and control voltage referred to ground. This simple, powerful circuit produces output current in strict accordance with the sign and magnitude of the control voltage. The circuit’s accuracy and stability are almost entirely dependent upon resistor R. A1, biased by VIN, drives current through R (in this case 10Ω) and the load. A2, sensing differentially across R, closes a loop back to A1. The load current is constant because L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. +VBIAS + OUT – BRIDGE TRANSDUCER AMPLIFER VBIAS COMMENTS 350 Strain Gage (BLH #DHF –350) LTC1100 10V Highest Accuracy, 30mA Supply Current 1800Ω Semiconductor (Motorola MPX2200AP) LT1101 1.2V Lower Accuracy & Cost. <800μA Supply Current Figure 3. Characteristics of Some Bridge TransducerAmplifier Combinations A1 + VIN 0q±10V A = 5V/DIV LT1006 – 0.05μF B = 5mA/DIV IK = 10k A2 + IK R* 10Ω LT1102 A = 100 – VIN R w 100 LOAD HORIZ = 20μs/DIV DN040 F05 DN040 F04 Figure 5. Dynamic Response of the Current Source * = PRECISION FILM TYPE Figure 4. Voltage Programmable Current Source is Simple and Precise 15V 27k 15V 10k* + A1A 1/2 LT1078 LT1009 2.5V 274k* – 250k* 0.1μF 15V 2k 50k ZERO + A2 LT1101 A = 10 88.7Ω* – *1% FILM RESISTOR Rp = ROSEMOUNT 118MFRTD TRIM SEQUENCE: SET SENSOR TO 0°C VALUE ADJUST ZERO FOR 0V OUT. SET SENSOR TO 100°C VALUE. ADJUST GAIN FOR 2.500V OUT. SET SENSOR TO 400°C VALUE. ADJUST LINEARITY FOR 10.000V OUT. REPEAT AS REQUIRED. Rp 100Ω AT 0°C RTD – A3 LT1101 +A = 10 + 5k LINEARITY A1B 1/2 LT1078 – 8.25k* 0V-10VOUT = 0°C-400°C ±0.05°C 2k GAIN 13k* 10k* DN040 F06 Figure 6. Linearized Platinum RTD Bridge. Feedback to Bridge from A3 Linearizes the Circuit A1’s loop forces a fixed voltage across R. The 10k-0.5μF combination sets rolloff, and the configuration is stable. Figure 5 shows dynamic response. Trace A is the voltage control input while trace B is the output current. Response is clean, with no slew residue or aberrations. A final circuit, Figure 6, combines the current source and a platinum RTD bridge to form a complete high accuracy thermometer. A1A and A2 will be recognized as a form of Figure 4’s current source. The ground referred RTD sits in a bridge composed of the current drive and the LT1009 biased resistor string. The current drive allows the voltage across the RTD to vary directly with its temperature induced resistance shift. The difference between this potential and that of the opposing bridge leg forms the bridge output. The RTD’s constant current drive forces the voltage across it to vary with its Data Sheet Download www.linear.com Linear Technology Corporation resistance, which has a nearly linear positive temperature coefficient. The non-linearity could cause several degrees of error over the circuits 0°C-400°C operating range. The bridges output is fed to instrumentation amplifier A3, which provides differential gain while simultaneously supplying non-linearity correction. The correction is implemented by feeding a portion of A3’s output back to A1’s input via the 10k-250k divider. This causes the current supplied to Rp to slightly shift with its operating point, compensating sensor non-linearity to within ±0.05°C. A1B, providing additional scaled gain furnishes the circuit output. To calibrate this circuit, follow the procedure given in Figure 6. Details of these and other instrumentation amplifier circuits may be found in LRC Application Note 43, “Bridge Circuits – Marrying Gain and Balances.” For applications help, call (408) 432-1900 dn40f_conv IM/GP 1090 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1990