gEDA-user: analog switch idea

[gene glick]

Noise and distortion are my main concerns.  The more stuff that gets in 
the way, the more it gets worse.  Having said that, maybe the ADC 
distortion and noise is the worst - maybe not, have to look into that. 
I think I need to come up with a good noise budget and work from that. 
But it is hard to argue that the simpler the analog chain, the better - 
in terms of noise and distortion.

Hey, I am curious, noise adds as the root of the sum of the squares, 
right?  How does distortion accumulate?  Seems to me that it just adds 
linearly.

gene


> Many years ago I did some of that ..  As I recall, the "analog 
> switch IC's" are really mosfets, series or shunt.  The 
> distortion spec was made under certain operating conditions.  
> Since you didn't say what you are using, or the circuit, I 
> can't judge how to make it better.
> 
> This type of switch works best at a virtual ground, so the 
> voltage is zero or constant.  Since it is used as a series 
> element, the distortion can be minimized by using a larger 
> series resistor.  This makes the nonlinear part of the 
> resistance a smaller part of the total.  This may increase 
> noise, which may or may not be an issue.  You need a pair of 
> switches, or a single-pole-double-throw, for each signal. You 
> need to consider what happens during switching.  Make sure that 
> you never short the op-amp input to ground (which effectively 
> removes negative feedback) and you never let the voltage get 
> big.  Either of these problems will result in a pop or click 
> during switching.
> 
> You also need to consider that distortion numbers usually 
> increase as the signal gets larger.  Running at a lower signal 
> level will usually result in lower distortion.  For the devices 
> I used, the distortion was almost all second order, which is 
> the least audible type.
> 
> Considering all that, by the time all optimization was done, the 
> performance impact of the switch IC was essentially zero.  Its 
> distortion and noise were masked by other distortion and noise 
> in the system, and that was so low that the distortion was 
> difficult or impossible to measure, and noise was essentially 
> what has predicted by theory and dominated by other stages.
> 
> I don't remember what brand or device we used.  I do remember 
> that it was mainstream and cheap.
> 
> 
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