I am designing a circuit where I need to measure the mid-point of a voltage divider with a 1kHz square wave, where the output is buffered with an op amp. The upper resistor is a fixed resistance, the lower is a sensor. The simplified circuit looks like this:
So this all works fine, and is very straightforward.
However, I need to measure a wide range of resistances, so for accuracy, I want to switch in a different fixed resistor as a "drive" resistor when needed. I decided on using MOSFETS for this. The circuit now looks like this:
Which isn't too difficult to understand. If the 1k resistor is needed, M2 is switched on, the others are off, and so on.
The problems happened when using the 100k resistor. My op-amp output had a curve to it, rather than a nice square wave, like it did with all the other ranges. It was almost like some stray capacitance was about, so I did some investigating and eventually realised it was down to the capacitance of the MOSFETS and being in parallel, the problem was obviously worse.
The MOSFETS being used are the IRLML6402 which has an input capacitance of 633pF per FET. With 4 of them, that's just over 2nF, which made sense when I analysed the curve a bit closer, so I am assuming it is the input capacitance here. Someone please correct me if I am wrong.
So, I have been looking at FETS with much lower input capacitance to try out, but it will be a few days before they arrive (yay to component shortages!), so in the meantime, is there anything I can try? I am not even 100% sure this will fix the issue, as I haven't been able to try any other components, so can anyone see any problems with this approach? Any assistance or help is appreciated.