Certain designs need a programmable-gain amplifier with a wide gain range and high accuracy and common-mode rejection. Usually, it's wise to exploit a programmable-gain instrumentation amplifier, such as an AD625. Unfortunately, the gain range of such standard parts is fixed at certain values, limiting their flexibility. Figure 1 shows a multichannel, eight-level-program-mable-difference-amplifier circuit. IC1, an AD623, operates from a single supply. This amplifier is a low-power, low-cost instrumentation amplifier that offers good accuracy.
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| Figure 1. | You choose the weighting resistors to obtain the optimum gain ranges for your application. | |
A single external resistor sets the gain from 1 to 1000. IC2, a CD4051, is a programmable, low-voltage 1-of-8 analog multiplexer, which connects to eight weighting resistors, R0 to R7, to increase the gain range of the circuit. The overall gain of the circuit depends on the value of the selected weighting resistor. You can compute the weighting resistors, R0 to R7, for a given input-output signal range as follows:
where RON is the on resistance of the CD4051, typically 125 Ω. RK is the 50-kΩ internal feedback resistor of the AD623, and RX is one of the selected weighting resistors. IC3, a CD4052, is a 2-of-8 programmable-difference-input IC. You can control the port-select pins, Z0 and Z1, of IC3 and Z2 to Z4 of IC2 with a µC. With the aid of some software, the circuit can provide self-adjusting gain.
