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 PRECISION IA SWINGS RAIL-TO-RAIL ON SINGLE 5V SUPPLY By R. Mark Stitt (602) 746-7445 You can combine a precision instrumentation amplifier (IA) and an inexpensive CMOS op amp to get the best of two worlds—the precision of a bipolar IA and 5V single-supply operation. Using the INA131 gives 50µV max voltage offset, 0.25µV/°C max offset drift, and 110dB min commonmode rejection. How close the output swings to the power supply rail depends on which “rail-to-rail” CMOS op amp you use. With a TI TL2272 you can expect the output to swing within 100mV of the rails. The circuit is shown in Figure 1. INA131 is fixed Gain = 100 IA specified to operate on power supplies as low as ±2.25V. Its output can swing ±1.25V on ±2.5V supplies. Adding a gain-of-2 “rail-to-rail” CMOS op amp, A4, inside the feedback loop boosts output swing to ±2.5V (0 to 5V on a single 5V supply). Because the CMOS op amp is inside the feedback loop, its errors are divided-down by the loop gain of A3 in the INA131 so they contribute negligible error to the composite amplifier. The gain of the CMOS amplifier is set by R1, R2, and R3. Gain = 1 + R1 • R 2 + R 2 • R 3 R1 • R3 With the values shown, Gain ≈ 2V/V. Remember, since A4 is in the INA131 feedback loop, its exact gain is unimportant. The INA131 will still have a precise gain of 100V/V ±0.024%. Using the R1, R3 divider to set the gain forces the INA131 output swing to be centered midway between the +5V supply and ground for rail-to-rail output swing. Compensation capacitor, C1, provides high-frequency feedback around A4 to assure loop stability. It is important to choose A4 with at least 2MHz small-signal bandwidth for good loop stability. +5V +VS R1 100kΩ INA131 –In Over-Voltage Protection Bridge Sensor 5kΩ R2 47kΩ 25kΩ A1 C1 100pF 25kΩ A4 R9 A3 2.63kΩ 25kΩ External Gain-Set Resistor Optional +In 5kΩ Over-Voltage Protection A2 VOUT VO R3 100kΩ 25kΩ REF –VS FIGURE 1. Adding a “Rail-to-Rail” CMOS Op Amp in its Feedback Loop Allows the INA131 Output to Swing Rail-to-Rail on a Single 5V Supply. © 1993 Burr-Brown Corporation AB-056A Printed in U.S.A. July, 1993 Figure 2 shows the output swing of the composite IA. In this triple exposure, the output sine-wave signal is superimposed with the ground rail and +5V rail. Gain can also be increased by connecting an optional gainset resistor, RG, as shown in Figure 1. Gain-set resistors in the INA131 are trimmed for precise ratios, not to absolute values. Absolute accuracy of the internal gain-set resistors is ±40%. To compensate for this tolerance, the value of the external gain-set resistor may need to be adjusted from device to device. Nominal gain with an external RG can be calculated as follows: Although the output of the composite IA will swing rail-torail, its inputs will not function at ground. This is not a problem for the bridge application shown. In this application, the bridge is biased from the +5V power supply to ground. With a balanced bridge, the IA inputs are at 2.5V (midway between the +5V power supply and ground). Because the INA131 uses a gain-of-five difference amplifier, the inputs to A3 are at 2V when the INA131 inputs are at 2.5V. This allows the INA131 to operate properly with common-mode input voltages from 2V to 3V. Gain = 100 + 250kΩ 250 kΩ RG Where: RG is the external gain-set resistor (Ω) Accuracy of the 250kΩ term is ±40%. As with most rail-to-rail op amps, the TLC2272 is a dual op amp. If the second op amp is not used elsewhere, it can be used for filtering or added gain. +5V Rail Ground FIGURE 2. Triple Exposure Showing Rail-to-Rail Output Swing of Composite Precision IA. 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