APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (520) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (520) 889-1510 • Immediate Product Info: (800) 548-6132 INCREASING INA117 DIFFERENTIAL INPUT RANGE By R. Mark Stitt (520) 746-7445 1 The INA117 is a monolithic difference amplifier with the unique ability to accept up to ±200V common-mode input signals while operating on standard ±15V power supplies. Because the gain of the INA117 is set at 1V/V, and because the output would saturate into the rails at about ±12V, the maximum specified differential input range is ±10V. Since the common-mode input range is ±200V, it makes sense that some designers would also like to use the part for differential inputs greater than ±10V. Figure 1 shows the recommended circuit. Adding resistors to the input may seem simpler, but there are some problems with that approach. R1 380kΩ V2 4 R1 380kΩ V2 V3 3 V3 – V2 1+ 8 R4 20.0kΩ 1 19 R7 V1 R6 R6 R7 6 5 V5 *VOUT = V3 – V2 + 19 V5 – 18 V 1 INA117 5 *VOUT INA117 VOUT = R4 20.0kΩ 6 + 6 + 1 – R5 21.1kΩ 3 8 5 2 2 R5 21.1kΩ R2 380kΩ R3 380kΩ – – OPA27 + FIGURE 1. INA117 with Increased Differential Input Range. The performance of the INA117 depends on extremely precise resistor matching (0.005% for 86dB CMR). Resistors added to the input must be adjusted to at least this accuracy to maintain high performance. Both gain error and CMR must be adjusted. Maintaining 86dB CMR over temperature requires 1ppm/°C resistor TCR tracking. Significant resistance added external to the INA117 would require the same performance. By using the circuit shown in Figure 1, internal resistor matching is preserved, and the INA117 CMR and CMR drift with temperature are maintained. Gain can be set independ- 1990 Burr-Brown Corporation 7 FIGURE 2. INA117 Shown as a Four-Input Summing Amplifier. 8 CMR is preserved and the gain is reduced if a small portion of the output signal is inverted and fed back to pin 5 with V1 set to zero (V1 grounded). 9 Where: VOUT = V3 – V2 + 19 • V5 – 18 • V1 10 If, V5 = –VOUT • R7/R6, then VOUT = 11 V3 – V2 1+ 19 • R7 12 R6 SELECTED-GAIN EXAMPLES 13 GAIN(1) (V/V) R7 (kΩ) R6 (kΩ) 1/2 1/4 1/5 1.05 3.16 4.22 20.0 20.0 20.0 NOTE: (1) INA117 is not stable in Gain < 1/5. © 2 To understand how the circuit works, consider the INA117 to be a four-input summing amplifier as shown in Figure 2. R2 380kΩ R3 380kΩ V3 ently of CMR by adjusting the inverter resistors, R6, R7. Gain drift is preserved so long as R6 and R7 track with temperature. Furthermore, noise at the output is improved by the gain reduction factor whereas it is unchanged with the other approach. AB-001B Printed in U.S.A. June, 1995 14 15 If CMR adjustment is desired, add a 10Ω fixed resistor and a 20Ω pot as shown in Figure 3. Adjust CMR by shorting together pins 2 and 3 of the INA117 and driving them with a 500Hz square wave while observing the output on a scope. Using a square wave rather than a sine wave allows the AC signal to settle out so that the DC CMR can be seen. The R1 380kΩ V2 The INA117 is now available in three standard 8-pin packages: hermetic TO-99, plastic DIP, and the small surface-mount SOIC package. R2 380kΩ 2 – R3 380kΩ V3 CMR trim will change the gain slightly, so trim CMR first, then trim gain with R6, R7 if desired. 6 VOUT + 3 R4 20.0kΩ R5 21.1kΩ INA117 8 1 R6 R7 5 – 20Ω OPA27 + 10Ω FIGURE 3. INA117 with Increased Differential Input Range with CMR Trim. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. 2