# Why does a resistor have this value in a circuit with an operational amplifier?

I'm analyzing this circuit to condition a PT100.

I clearly understand what the circuit does, but the current flowing through R7 is very small and negligible.

So in theory it could be any value (generally from 1k to 100k), but I don't understand why the value should be 49.9k.

There is no information on why the value of R7 where said circuit is described.

Any suggestion or comment is welcome.

The circuit is described here: Precision Temperature Sensing with RTD Circuits

• It is there because of something called the amplifier bias current. Note that it is about equal to the parallel of those two $100\:\text{k}\Omega$ resistors? There's a reason for that. If curious, take a look at a typical bipolar opamp input stage (diff-amp, current mirror, currrent source/sink, and a few bits and pieces around that area.) – jonk May 9 at 23:39
• So 100kΩ || 100kΩ and there is no 50kΩ, so not strange at all. And I though I saw a cosmic black hole pass through here. – StainlessSteelRat May 9 at 23:56
• Yes, you're right, I remember I learned that when I studied op-amps, but had forgotten. Thanks @jonk – Fabián Romo May 10 at 1:04
• @FabiánRomo I'm so glad to hear that all this settled back in, after some years! It's wonderful when just a reminder or two is all that's needed to get things back into place! And it's a good reason that justifies this site and the time that people put into it, too! – jonk May 10 at 2:12