Null terminals

Some opamps include a pair of terminals which allow the user to compensate for the input offset voltage at a given temperature and common mode voltage. The offset voltage may be just the opamp voltage offset, or it may include other system offsets, as the adjustment span is usually larger than necessary to compensate for just the device offset. To use the null terminals, a trim pot should be connected between the two terminals, with the wiper terminal connected to the positive or negative supply. The diagram shows the circuit for an opamp with null terminals on pins 1 and 5, which are connected to the negative supply.

The chooser application uses an 'n' prefix for null terminals which should be connected to the negative supply and a 'p' prefix for null terminals which should be connected to the positive supply. The subsequent numbers are the pin numbers. As an example, 'n15' means that the trim pot connects to pins 1 and 5, and the wiper goes to the negative supply. Similarly, 'p213' means that the trim pot connects to pins 2 and 13, and the wiper goes to the positive supply.

Check the device datasheet for exact details on recommended trim pot values. Datasheets don't always give the available adjusment range, and rarely indicate the polarity of the adjustment. In general, only single devices have null terminals.

Null terminals can sometimes be used for other purposes. They have comparable sensitivity to the input pins, which means that they must be treated with care. They do however have a relatively low impedance and a negligible common mode voltage range.