Design Ideas: March 3, 1994
You can combine an instrumentation amplifier and an inexpensive CMOS op amp to get the best of two worlds: the precision of a bipolar instrumentation amp and the 5V single-supply operation of the CMOS op amp. The characteristics of such a circuit depend first on the instrumentation amp you use. The INA131 in Fig 1 delivers 50-µV max voltage offset, 0.25-µV/°C max offset drift, and 100-dB min common-mode rejection. Next, how close the output swings to the power supply's rail depends on which rail-to-rail op amp you use. A Texas Instruments TL2272, for example, swings outputs to within 100 mV of the rails.
In the circuit in the figure, instrumentation amplifier IC1 has a fixed gain of 100 and operates from power supplies as low as ±2.25V. IC1's output can swing ±1.25V from ±2.5V supplies. Adding a gain-of-two rail-to-rail CMOS op amp, IC2, inside the feedback loop of IC1's output amplifier boosts the circuit's output swing to ±2.5V (0 to 5V on a 5V supply). IC2 contributes negligible error because the circuit divides IC2's errors by the loop gain of IC1's output amplifier.
R1, R2, and R3 set the gain of the CMOS amplifier:
Using the values in Fig 1, GAIN (approximately)='2V/V. Remember, because IC2 is in the feedback loop, its exact gain is unimportant. The composite amplifier still has a precise gain of 100V/V ±0.024%.
The R1-R3 divider sets the gain and forces IC1's output swing to center midway between the +5V supply and ground for rail-to-rail output swing.
Compensation-capacitor C1 provides high-frequency feedback around IC2 to ensure loop stability. For good loop stability, you must choose a device for IC2 that has at least a 2-MHz small-signal bandwidth. EDN BBS /DI_SIG #1371