[sdiy] Silly simple question, input switching

David G Dixon dixon at mail.ubc.ca
Thu Jan 8 20:50:27 CET 2015


That sounds like a great idea, but I'm wondering whether there shouldn't
also be a high-R shunting resistor to ground on the bypassable path to
discharge the cap when it is bypassed...?
 

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Rutger Vlek
> Sent: Thursday, January 08, 2015 11:39 AM
> To: Synth diy
> Subject: Re: [sdiy] Silly simple question, input switching
> 
> Thanks! Great idea! I'll give it a try. Though for DC 
> blocking the caps the first element in the series of course;).
> 
> Rutger
> 
> 
> On 8 jan 2015, at 12:35, Tom Wiltshire wrote:
> 
> > 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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> 
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