Odd resistor in op amp circuit - Page 1
Forum section: electronics / projects

author: BrotherTheo
author_rank: Newbie
timestamp: May 23, 2023, 07:16:38 pm
content: Hi, I got a schematic from the internet (yes, I know) and there is an odd resistor in it. The circuit is an electronic switch feeding an inverting amplifier with a resistor from the neg op amp input to ground. The neg pin on an inverting amp is a virtual ground so the resistor seems to have no purpose. The only thing I can think of is the resistor is there to keep the amp from going crazy while the switch is switching.In the attached circuit RX is the odd resistor. I need to know if I am correct in thinking that RX resistor does not affect the gain of the op amp.Thanks!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 23, 2023, 07:31:16 pm
content: It forms a divider with Ra and Rb but I can't see any other purpose to it.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2023, 07:37:49 pm
content: The resistor Rx effectively reduces the signal to the op-amps and this way needs more gain of the OP-amp.The resistor indeed looks odd, but it depends on the purpose of the circuit. It could make sense of the OP-amp is not unity gain stable.

author: TimFox
author_rank: Super Contributor
timestamp: May 23, 2023, 09:59:55 pm
content: For an ideal op-amp, the resistor RX from inverting input (virtual ground) to common has no effect on the gain of the inverting amplifier, which is the ratio of the feedback resistor and input resistor (see last sentence for bandwidth).The amplifier should not "go crazy" with no resistor to ground at the inverting input, so long as there is a feedback resistor, unless (as Kleinstein points out), the amplifier is not unity-gain stable.Except for stability questions, one should avoid conductance to ground at the inverting input, since that increases the "noise gain" of the circuit, and therefore the output noise, with no benefit.For bandwidth calculations, consider RX to be in parallel with whichever input resistance is switched in, and do the normal calculation involving unity-gain frequency and feedback fraction.

author: moffy
author_rank: Super Contributor
timestamp: May 23, 2023, 10:38:19 pm
content: Switches like the DG449 have charge injection when they switch, the resistor might limit the switching spike due to injection when it is switched. Not really convinced that is it, you could also put a small cap across the feedback resistor for the same reason.

author: dietert1
author_rank: Super Contributor
timestamp: May 24, 2023, 04:50:52 am
content: In order to somewhat absorb switching spikes, the resistor can be in series with a capacitor. Opamps are sensitive to those spikes and don't have enough bandwidth to handle them properly. People resort to passive parts, even with the disadvantages involved.One use case is the integrator in a DVM, where we already have a large cap from output to negative input. When you connect another cap (like 470 pF in the HP 3456A) across the inputs, this will be a capacitive load to the opamp. So one better uses a small resistor in series with a capacitor.Regards, Dieter

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 24, 2023, 09:09:54 am
content: The added resistor between op-amp inputs will modify the noise gain of the circuit, without having a large impact on the overall circuit gain. This means more noise, less bandwidth, more phase margin and better ability to drive capacitive loads. Not sure this is what they intended here, but it's one of the practical implications of having a resistor in this position.

author: jwet
author_rank: Frequent Contributor
timestamp: May 24, 2023, 02:51:15 pm
content: A likely function is to keep the amplifier from going unstable if one of the sources on the left side of analog switch is selected but absent. In that case, the off switch looks like a small C, working against the feedback R of the op-amp creating a classic op-amp oscillator. That R swamps the capacitance in that condition.There are no values but I imagine Rx is fairly large to keep from affecting gain much. In normal operation (closed loop), its essentially bootstrapped away.