With the circuit shown in figure 1 it is possible to digitally control the gain of an operational amplifier. In this configuration, the 12-bit Analog PM-7541 DAC was used, which gives a control range from 0 to 1024 with 10 bits and up to 4098 with 12 bits. The OP-41 operational amplifier with JFET input has a very high input impedance, which guarantees the accuracy of the gain control. Note the need for a symmetrical power supply and the DAC adjustment input voltage range is from 12.5 mV to 10 mV depending on the desired gain range. With input 0 we have the operation in open loop and with input 4095 the gain is -100024. In the table below we have the scale of gains obtained.
The same configuration can be adapted to operate with other operational amplifiers and also with DACs of other characteristics.
An important application for this circuit is in instrumentation, where very low voltages can be accurately measured. These voltages can reach the microvolt range.
