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Old 2nd June 2003, 01:23 PM   #31
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Hi Mlloyd,

you are right, #1 has little variation of voltage ACROSS the switch, the residual voltage depending only on the ratio of r_on to 1 Meg. However, there is a huge voltage variation against the switch substrate. This will to some degree influence r_on (which is only relevant in terms of delta r_on / 1 Meg, i.e. pretty irrelevant), but it is bound to influence the capacitance to substrate or ground. This is why this circuit should only used with very low source impedances.

Circuit #2 has virtually no voltage and hence capacitance variation against ground, but the voltage varation will be higher than in #1 due to the higher current.

The best way, IMHO, is what I called #3, i.e. to place the switch between the inverting input and the point where input and feedback resistor meet. This way, there will be no current and no voltage variation. If you are concerned about the residual (static) capacitance on the input, use a 100 R isolation resistor in series.

Regards,

Eric
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Old 2nd June 2003, 07:40 PM   #32
mlloyd1 is offline mlloyd1  United States
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capslock:

thanx! I understand. I was looking to have the contributions of the various effects separated to make the discussion a little easier.

mlloyd1
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Old 4th June 2003, 04:23 PM   #33
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looking at the DG211 datasheet it is not readily apparent to me
what effect these might have on a signal
has any one measured / heard a net effect? THD/something else?
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Old 22nd June 2013, 06:47 PM   #34
RonV is offline RonV  Netherlands
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Analog Devices has some switches with real low R-on. ADG411-series has 25 ohms and ADG451 even 4 ohms. Does anybody have any experience with those switches in audio? I'm using ultralow-distortion OPA134 opamps, and I assume those low R-on switches make a good match.
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Old 23rd June 2013, 12:39 PM   #35
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This is an ancient thread, but some of the original comments apply. Lower resistance goes with either larger stray capacitance or lower working voltage (more likely these days). There are parts on the market that cannot handle several of the hotter CD player output levels
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Old 23rd June 2013, 03:26 PM   #36
RonV is offline RonV  Netherlands
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Yes, I know the original thread is ancient, but I suppose the subject itself is not outdated. I hope for recent insights on the subject. Thanks anyway for the tip about capacitance (I already selected only devices with large enough voltage swing).
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Old 23rd June 2013, 04:04 PM   #37
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I just got a few of those and many other assorted types of analog multiplexers and opamps.
I haven't had the chance to try them out yet.
I will be shortly though.

jer
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Old 23rd June 2013, 07:24 PM   #38
RonV is offline RonV  Netherlands
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Great! Hope to hear about your experiences soon. I'm still in doubt whether to use relays or solidstate switches.
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Old 24th June 2013, 03:48 PM   #39
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Relays vs solid state depends on your application. Relays are more ideal low capacitance and on and off resistance, but can suffer from contact oxidation in dry switching
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Old 26th June 2013, 01:08 PM   #40
RonV is offline RonV  Netherlands
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I'm aware of the oxidation issue. I already have a number of relays suitable for dry circuit switching (sealed, AuAgNi contacts). I agree they are the best considering capacitance, R-on and R-off, but on the other hand they are relatively bulky. So I'll continue doubting for a while...
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