# Resistor Between Voltage Reference and + Input of Op-Amp? Why?

something I commonly see in schematics which include op-amps set up as inverting amplifiers is the inclusion of a resistor between the non-inverting "(+)" input of an op-amp and a strong voltage reference such as ground, power, or a voltage reference set up using an op-amp buffer.

What is the purpose for this? The (+) input of op-amps are high impedance, and the references mentioned above are strong sources of current, which aren't vulnerable to crosstalk in the way a weak reference would be.

• All opamps have a small but not entirely negligible bias current at their inputs. There is also something called an offset current (which is the difference between the two input bias currents, which are never exactly the same.) These bias currents cause small, additional voltage drops across resistances. If you use a similar resistance for both inputs, then the bias currents will generate "about the same" added voltage drops across the external resistances. This improves behavior. To remove effects from the remaining offset current requires calibration.
– jonk
Commented Jan 31, 2018 at 23:27

Figure 1. Both op-amp inputs have a bias current. If R3 is zero ohms then there will be a difference in the offsets between the two inputs. Source: ECircuitCenter.

From the same article:

One of the golden rules of op amp analysis says this: no current flows into either input terminal. This concept is key for analyzing an amplifier's signal gain. However, in reality, a small current flows into both inputs to bias the input transistors. Unfortunately, this bias current gets converted into a voltage by the circuit's local resistors and amplified right along with the signal. The result is an output error in your circuit. What can you do about it? A clever choice of resistor values can help you cancel most of the output error. The remaining error can be adjusted to zero if necessary.

• You can add a capacitor in parallel with R3 to avoid extra noise. Commented Jan 31, 2018 at 23:54

There's three reasons.

Firstly, an input current to an operational amplifier may be non-negligible (up to a microamp). That often does not matter, except that the input current has a stronger thermal drift in many designs than the offset voltage. So, a low-offset op amp such as would be used with an accurate reference will have temperature sensitivity unless both the inputs have similar input resistance (Thevenin source resistance).

Secondly, some references feed comparators and operational amplifiers that have input clamps, often to power supply rails, sometimes to other input pins OPA277 inputs clamped. Clamp currents can disturb the reference value.

Third, some references may be capable of destructive currents during startup or shutdown sequencing: you want to protect the op amp input pins from that.