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Gain-programmable circuit offers performance and flexibility

-November 25, 2004

You can use a standard precision instrumentation amplifier, such as the INA118 or AD623, as a gain-programmable amplifier with high accuracy and wide gain range. However, the gain range of such parts is fixed at certain values, limiting their flexibility. To solve the problem, a usual way is to use a gain-adjustable circuit controlled by a microcomputer (Figure 1).

IC2 is a programmable 1-of-8 analogy multiplexer that connects to eight weighting resistors, R1 to R8, to improve the gain range of the circuit based on IC2, a general-purpose precision amplifier. The overall gain of the circuit depends on the value of the selected weighting resistor, as follows:

where RON is the on-resistance of IC2, and RX is one of the selected weighting resistors, R1 to R8. You control the port-select pins Z0 to Z2 of IC2 with a microcontroller to provide self-adjustable gain according to the selected weighting resistor. Unfortunately, the performance and quality of the circuit cannot provide good performance and high quality due to the on-resistance of IC2, which you also cannot control, especially as the temperature changes.

The modified gain-adjustable amplifier circuit in Figure 2 uses the same IC1 but changes IC2 to a programmable 2-of-8 difference-input analog multiplexer, which connects to four balancing resistors, R01 to R04, and eight weighting resistors, RG1 to RG8, to improve the gain range of the circuit. By controlling the port-select pins Z0 to Z1 of IC2 with a microcontroller, the circuit provides self-adjustable gain with high quality. The overall gain of the circuit is:

where RGA is one of the selected weighting resistors, RG1 to RG4, and RGB is one of the selected weighting resistors, RG6 to RG8.

Analog multiplexer IC2 is on the input side of amplifier IC1. Resistors R01 to R04 balance the signal-input channel to decrease the level-shifting because of the on-resistance of multiplexer IC2 and minimize the effect of that resistance. Additionally, two operational amplifiers, IC01 and IC02, act as followers to improve the overall driver performance and common-mode-rejection capacity of the circuit.

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