[sdiy] Silly simple question, input switching

Rutger Vlek rutgervlek at gmail.com
Thu Jan 8 11:56:20 CET 2015

Hi guys,

I have a very silly, simple question that I don't know the answer to. Perhaps you know?

Let's say I have an inverting op-amp (on bipolar supply) with 100K in the feedback path and I want it to have an input that can be switched (with a SPDT) between 50K DC or 100K AC-coupled, where would I place the switch?

Two scenario's:
a) Signal flows into two resistors 50K and (a series cap+) 100K connected in parallel, the switch comes after and selects which one will get a conductive path to the inverting input of the op amp.

b) The signal goes into the switch first and depending on the position gets fed into a 50K or (cap +) 100K resistor, both of which are always connected to the inverting input of the opamp.

Which scenario would you choose? I'm mostly concerned about the PCB trace length through the switch and possible chances for picking up noise in the case of a). Intuitively that feels like a bigger problem than op amp input resistors that are left floating with a moderately high impedance in the case of b). The question is, is my intuition right? And does everything hold when the switch becomes an electronic switch (for instance a DG-40x series), rather than a manual toggle?



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