4.1 Resistor Selection

Resistors are almost always used as part of the feedback network for operational amplifier circuits. In most cases, the resistors in the feedback network are critical elements that determine the behavior of the circuit. Because of this, resistor tolerance can cause errors in the expected behavior of the circuit. Each resistor has a specified accuracy tolerance - the better the tolerance, the more expensive the resistor is. However, in most cases, the absolute accuracy of the resistor is not crucial: only the ratiometric accuracy with respect to the other resistor(s) in the circuit determines the circuit's accuracy.

On devices with an internal resistor ladder, the absolute accuracy of the resistor network is not tightly controlled, however the ratiometric accuracy on the ladder is more stable, as both resistors were fabricated on the same die in the same wafer lot. The internal resistor ladder is recommended for use in area-constrained applications and in applications where gain accuracy is not very strict (or can be calibrated out). The gain of the resistor ladders can be measured using the method discussed in Microchip Application Note AN3633, "Gain and Offset Calibration of the Analog Signal Conditioning (OPAMP) Peripheral" (DS00003633A) to calibrate out some of the errors in the ladder.