Circuit provides flexible gain ranges
Luo BenCheng, Chinese Academy of Sciences, Beijing, China -- EDN, June 21, 2001
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. 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: VOUT=VIN(1+2RK/-(RX+RON)), where RON is the on resistance of the CD4051, typically 125W. RK is the 50-kW 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, such as an AT89C51 or an 80C196. With the aid of some software, the circuit can provide self-adjusting gain.
