APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 ±200V DIFFERENCE AMPLIFIER WITH COMMON-MODE VOLTAGE MONITOR By Art Gass and R. Mark Stitt (602) 746-7445 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. Using on-chip high-voltage resistor dividers, the INA117 rejects common-mode signals up to ±200V and translates a 0V to ±10V differential input signal to a 0V to ±10V ground-referenced output signal. In some applications it is also necessary to monitor the common-mode level of the input signal. A common-mode level monitor can be implemented with the addition of an external op amp or two. Even though standard signal level op amps are used, the circuit remains protected for momentary common-mode or differential overloads up to ±500V. If precision is not required, the circuit shown in Figure 1 can be used to monitor the common-mode voltage with a maximum error of about ±5V. This implementation actually monitors the common-mode level of the INA117 noninverting input (pin 3). The circuit works by measuring the current in reference pins 1 and 5, which are normally connected to ground. Amplifier A 1 forces the reference pins to a virtual ground through feedback resistors R 6 + R7. The V2 R1 R2 2 380kΩ 380kΩ 3 380kΩ 6 R5 R4 20kΩ INA117 8 1 5 100pF R6 R7 5kΩ 10kΩ A1 OPA27 If R6 + R7 = R4 || R5 then A1 OUT = –V3/20. Where I1 + 5 = total current flowing out of INA117 pins 1 and 5 [A] R4 || R5 = parallel combination of R 4 and R5 [Ω] R4 || R5 = (R4•R5)/(R4+R5), nominally 10.27kΩ A1 OUT = A1 output voltage [V] Although the resistor ratios in the INA117 are accurately laser trimmed, the absolute resistor values can vary by as much as ±25%. For better accuracy, the circuit must be calibrated. To calibrate the gain, short pins 2 and 3 of the INA117 to ground, offset adjust A 1 for 0V at its output, connect pins 2 and 3 to a known V REF (such as +10V or +100V), and adjust R 7 for an A1 output of –VREF/20. VOUT = V3 – V 2 21.1kΩ I1 + 5 = (V3/20)/(R4 || R5) The signal is scaled by 1/20 so the output of A 1 does not exceed its maximum of ±10V with common-mode inputs of ±200V. If smaller maximum common-mode voltages are to be monitored, the value of R 6 + R7 can be increased for more gain. R3 V3 normal operation of the INA117 is unaffected since its reference pins are connected to virtual ground. Resistors R 3 and R4 in the INA117 form a voltage divider so that the top of R4 is at V3/20. Feedback of the op amp in the INA117 forces the voltage of its inverting input to be equal to its noninverting input so that the top of resistor R 5 is also at V3/ 20. The common mode level of V 3 is therefore related to the current flowing out of pins 1 and 5. By definition, the true common-mode input voltage of the INA117 is (V2 + V3)/2. The actual common-mode voltage can be monitored with the addition of a second op amp as shown in Figure 2. The second op amp is connected to sum the –V3/20 output of A1 at a gain of –1V/V with the V 3 – V2 output of the INA117 at a gain of –1/40V/V to produce an output of VCM/20. –V3 /20 FIGURE 1. INA117 with V 3 Common-Mode Voltage Monitor. 1990 Burr-Brown Corporation AB-010A Printed in U.S.A. September, 1990 Calibrate the Figure 2 circuit as before, adjusting R 7 for an A2 output of V REF/20. Then, ground pin 3 of the INA117, connect pin 2 to +10V and trim R 9 for 0.025V at the output of A2. If resistors R8, R9, and R10 accurately ratio match, adjustment of R9 is unnecessary. 3 and 4 can be used to monitor the common-mode input voltage. With these circuits, calibration is not required if accuracy commensurate with the tolerance of R 6, R7, and R8 is acceptable. As before, either R 7 or R8 can be omitted to monitor the common-mode voltage of just one input. If R 7 or R8 is omitted, double the value of R 6. Of course, if connection of additional components to the INA117 inputs is acceptable, the circuits shown in Figures V2 R1 R2 2 380kΩ 380kΩ 3 380kΩ R3 V3 6 VOUT = V3 – V 2 R5 R4 21.1kΩ 20kΩ R9 400kΩ 100pF INA117 8 1 5 R10 100pF R6 R7 R8 5kΩ 10kΩ 10kΩ 10kΩ A2 OPA27 VCM /20 A1 OPA27 FIGURE 2. INA117 with True Common-Mode Voltage Monitor. V2 2 R1 R2 R1 R2 380kΩ 380kΩ 380kΩ 380kΩ V2 R3 V3 3 2 R3 6 380kΩ VOUT = V3 – V 2 V3 3 6 380kΩ VOUT = V3 – V 2 R5 R4 R5 21.1kΩ 20kΩ R4 21.1kΩ 20kΩ INA117 8 1 INA117 5 8 1 5 R6 R8 R7 1MΩ 1MΩ 24.9kΩ R8 R7 1MΩ 1MΩ OPA177 VOUT = +VCM /20 OPA177 R6 VOUT = –VCM /20 26.1kΩ FIGURE 3. External CMV Monitor, Inverting. FIGURE 4. External CMV Monitor, Noninverting. 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