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Old 3rd March 2003, 10:31 AM   #11
dhaen is offline dhaen  Europe
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Hi,

If you must use analogue mux's, I'd recommend DG211 / DG212. They are a world away from the first generation analogue switches, without the cost penalty of the SSM's.
I have used them for source switching by putting them at the "virtual earth" (inverting) input of an opamp. There is an input resisitor for each source.
In my experience, this scheme gives minimum crosstalk and minimum distortion.
If this isn't clear, I'll post a schematic.

With regard to silent switching, there are 2 issues here:
If you switch a waveform fast, the discontinuety will sound. Zero swiching will help. Specialised (expensive) switches are the answer.
Most analogue switches crosstalk from the control pin to the signal. Speed limiting the control signal, and good earthing practice will help to reduce this.

Cheers,
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Old 3rd March 2003, 02:26 PM   #12
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Default thd switches

The switches distort because their on-resistance varies with signal level. Assuming a sine wave across the on-switch, the on-resistance increases when the sine goes to max and vice versa.
This can be limited by 1 - operating the switch at max supply voltage (should be clear from ds), and 2 - limit the signal level across the switch.

Example: use a 50 ohms switch at virtual ground with a 10k in series. At 1vrms, the switch sees nominal 50/10000 * 1V = 5mV.

The amount of thd can be estimated by looking in the data sheet for Ron vs signal level.

You will for instance see that at signal level variation of 1V, the on res varies from 50 to 55 ohms (just making these numbers up, check the ds).

On the total res this would be a variation of 10050 to 10055 ohms or some delta of 500ppm.

In the above example, 5mV across the switch, it would probably be sub-ppm.

The crosstalk etc influences the linear distortion (freq response), and can be limited by switch construction and keeping all impedances as low as possible (which is in conflict with min thd which asks for larger impedances).


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Old 27th March 2003, 06:14 AM   #13
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Default Analog Switches and Muxes

If you are looking to use quality parts for will find that many of the Siliconix switches and muxes introduce no distortion and have no leakage voltage or linearity problems. While working at TI, I used many different switches and muxes made by Siliconix and Harris and found their performance to be extremely good.

You can find many of these parts isolation and cross talk greater than 100db. In fact many of their switches are great for sample and hold applications and multi channels high performance and video systems. Of course a 4066 and 4053 are cheap parts and should not be used if your expecting quality audio performance.
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Old 27th March 2003, 07:06 AM   #14
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Quote:
Originally posted by peranders
Is it possible to make slow on and offs with a 4066? I don't think so. Isn't the inputs buffered (= high gain)?

It's peculiar that AD not have mentioned any distortion figures at all....
Often the distortion figures are not specified in the traditional way. Rather, what you will see is a graph of Ron versus input level. That shows that the larger the input level, the larger the Ron. That again means that the Ron varies through the signal cycle. Note that the variation is lowest with highest supply voltage.

So, if you use such a switch as input switch, where the signal level can be up to several volts peak, there will be appreciable Ron modulation which can give distortion, depending on the input resistance (it is an attenuator with attenuation varying through the signal cycle which gives 3rd harmonic distortion). You can do some rough calculation to check the variation to get a feel for the distortion.

On the other hand, if you use them say as level switches at the inverting (virtual earth) input of a feedback (op)amp, the signal level is extremely small (fractional mV) so the distortion is zero for practical purposes.

Jan Didden

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Old 1st April 2003, 10:17 AM   #15
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In addition to the resistance vs. voltage issue, there is probably an (typically unspecified) capacitance vs. voltage issue because there must be some parasitic capacitance to the substrate.

1. Connecting the switch directly to the non-inverting input of an op-amp means it sees practically no load other than the pull to ground resistor which can be > 1 Meg with a FET-input op amp, so there is no resistance issue. On the other hand, it sees a large voltage swing, so I would use this only if the switch is driven from a low (< 500 R?) source impedance.

2. Putting the switch into the inverting feedback network means minimized voltage swing but some current flows, so the resistance issue is back.

3. Using an inverting configuration and putting the switch between the virtual ground point and the inverting input is probably ideal (close to no current and no voltage), but it means you need a resistor pair for each input.
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Old 22nd May 2003, 12:46 AM   #16
mlloyd1 is offline mlloyd1  United States
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capslock:
Just doing a really quick eyeball ...
Looking at the circuits below and assuming (for the sake of discussion) :
1. the driving element has "adequately low" (i.e. less than 100ohms) Zout,
2. Op amps gain blocks have same distortion even though different configuration (yeah, I know Circuit 2 should actually have lower THD thanks to lower common mode signal, but I have to add another op amp and pair of resistors to get the correct signal polarity back )
3. For Circuit 2, I'm not going to add another switch in the feedback loop to compensate for the input leg switch's Ron

I'm thinking the switch in Circuit 1 has less current through it and less voltage across it (so better performance?) than the switch in Circuit 2. I'm also worried about hanging a parasitic capacitance at the inverting input of the op amp.

What did I miss?
mlloyd1

Quote:
Originally posted by capslock
In addition to the resistance vs. voltage issue, there is probably an (typically unspecified) capacitance vs. voltage issue because there must be some parasitic capacitance to the substrate.

1. Connecting the switch directly to the non-inverting input of an op-amp means it sees practically no load other than the pull to ground resistor which can be &gt; 1 Meg with a FET-input op amp, so there is no resistance issue. On the other hand, it sees a large voltage swing, so I would use this only if the switch is driven from a low (&lt; 500 R?) source impedance.

2. Putting the switch into the inverting feedback network means minimized voltage swing but some current flows, so the resistance issue is back.

3. Using an inverting configuration and putting the switch between the virtual ground point and the inverting input is probably ideal (close to no current and no voltage), but it means you need a resistor pair for each input.
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Old 22nd May 2003, 12:57 AM   #17
dhaen is offline dhaen  Europe
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Hi,

I favour circuit 2 because the common point of all the switches is virtual earth. This configuration, with the series resistor, gives minimum source crosstalk.

Cheers,
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Old 22nd May 2003, 05:43 PM   #18
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Default mux stuff

The resistance issue is a non-issue, as long as it is a constant resistance (what's 100 Ohms between friends, eh?). The resistance MODULATION is an issue, meaning the resistance varies through the signal cycle. That means distortion. The modulation comes from the voltage swing of the mux. So, fig 2, with minimal voltage swing, is the better one.

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Old 22nd May 2003, 05:55 PM   #19
dhaen is offline dhaen  Europe
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Jan,

If you mean the switch's resistance I agree.
The series resistor however, is best to be large in comparison with the switch's "on" resistance. That way, the change of resistance throughout the cycle (of audio), will have less significance. Thus the distortion will be lower.

Cheers,
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Old 22nd May 2003, 06:00 PM   #20
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Default mux stuff

Hi John,

I'm not sure I get it. You mean the switch off resistance compared to it's on resistance? The off resistance doesn't vary through the cycle, does it? (Although it of course varies with freq due to cap crosstalk).

Jan Didden
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