On Thursday 03 August 2006 18:38, gene glick wrote: > I'm looking for some ideas on how to multiplex some > single-ended audio signals. The plan is to connect the mux > to a differential amp, which in turn drives an ADC. > > The obvious (at least I think it's obvious) choice is to use > some variety of analog switch IC. ON-semi has some decent > ones actually, but they have a THD numbers around 0.01%. Not > bad, but not good enough either. I'd like to get at least an > order of magnitude better. > > I recently saw a "design idea" in EDN where the author used > an analog switch to change the gain of an op-amp to zero. > Cute idea actually. But, he used a really expensive > low-noise op-amp. So I guess my next criteria is low cost. > How low? not sure.
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. _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
