[sdiy] Silly simple question, input switching

Tom Wiltshire tom at electricdruid.net
Thu Jan 8 12:35:08 CET 2015


I like Richie's single pole switch idea. Put two 50K resistors in series with the cap, and then short out the one resistor and the cap for the DC coupled input.

That'll work well for a electronic switch too, since the on resistance of the switch will be small compared to the 50K, so Ron variations won't affect anything much.

On 8 Jan 2015, at 11:25, Richie Burnett <rburnett at richieburnett.co.uk> wrote:

> Scenario b for exactly the reasons you say.
> 
> You could use a multiple pole switch and arrange for the floating input path to be grounded if you were worried about picking up interference. However, doing this will increase the noise gain of the amplifier, so probably best to leave the unused input floating and put the whole thing in a screened metal box.
> 
> There is also another way of doing it. You could feed the input through a series combination of 50k+50k+cap and then use a single pole single throw switch to optionally short out (bypass) the last 50k+cap section when you want the 50k dc coupled option. Conversely with the switch open you get 100kHz plus your account coupling capacitor. Just another way of doing it that doesn't result in a floating high impedance input, if you're worried about that! 
> 
> -Richie, 
> 
> 
> Sent from my Xperia SP on O2
> 
> ---- Rutger Vlek wrote ----
> 
>> 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?
>> 
>> Best,
>> 
>> Rutger
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