Thomas, I had indicated the same thing as below several messages ago. Anti-aliasing is only a concern when trying to reconstruct a signal. Since you have no intent on reconstructing the signal, you have no need for an anti-aliasing filter. However, what you should care about is noise entering your circuit. If appreciable noise enters your circuit, then your samples may alternate between x and x+1 even though no knob is being touched. If you remember, this thread was originally posted by me because I could not get a stable 7 bit value even though I was using an 8 bit A/D (a noise issue, not an aliasing issue - the same as in your design). To eliminate the 60 Hz hum you would need to either filter *each* pot *before* it enters the multiplexed (4051, correct?) or at the voltage source. You cannot filter it out after the multiplexed because the 'signal' frequency at that point is way beyond 60 Hz: if you sample at 1 kHz (you wanted a sample rate of 1 ms, correct?) and each successive pot alternates between Vdd and GND, then the output of your multiplexed will be a 0.5 kHz square wave (or there about). Your 60 Hz filter would introduce both a time delay and significant smoothing of you signal. Filtering each pot is expensive, however. Therefore, your only practical alternative for this is filter at the source (a good practice regardless of the project). Cost, a couple of capacitors. I will admit that a 1 kHz sampling rate seems high to me, but it is your project and you know it's requirements better than anyone else. I will also say that sampling at less than 60 Hz seems much too slow (musicians can be very exacting and fickle), so that although you may be able to inexpensively filter out the 60 Hz hum in this case, you are doing it at the expense of your original intent for the circuit. To eliminate digital ground signal noise, your filtering needs are very much less - a filter of ~10 kHz will probably work just fine. This one can be located after the multiplexed since the signal at that point will be lower in frequency content. In this case you will require only 2-3 low pass filters (at each multiplexed) of minimal order (first should be more than fine). Cost 2-3 resistor and 2-3 capacitors. Of course, you may just take Harold's advice, and make sure your ground plane is laid out properly - this, for my case, may be the simplest solution. And if you do get a bouncy signal, just implement a simple digital filter or do some averaging. Cost, a well laid out PCB or some filtering code. The last source of noise that you may or may not care about is thermal. The biggest passive source of it is from resistors. The higher the resistance, the higher the thermal noise. Be careful though, lowering the resistance in you case (upto 32 pots) will use more power. One last thing. I have to agree with Lance. The etiquette in the conversations seems to be degenerating for no apparent reason. If we all had all the answers, there wouldn't be a need for this list. And if any single person out there has all the answers, please be kind enough to provide them completely the first time around to minimize any chance for conversing or for having your absolute answers questioned. Mario Thomas Brandon wrote: > > Thank you. This is what I wanted to hear. I could not see how aliasing could > effect my application. That certainly makes it a little easier, if only > conceptually. > > Tom. > > ----- Original Message ----- > From: Bill METZENTHEN > To: > Sent: Monday, September 27, 1999 11:50 AM > Subject: Re: A/D Challenge Summary > > > Thomas, > > > > I think that aliasing is a red herring for your application. > > > > I think that other people have pointed this out in postings to this list. > > > > If you are only interested in the settings of the pots at your sampling > > instants then you don't > > need any anti-aliasing filtering. What you want is what you will get. > > Aliasing would only be an > > issue if you wanted to be able to re-construct the signal (e.g. by digital > > processing and/or > > writing your samples to a D/A and filter) between the sampling instants. > > > > > > Cheers, > > Bill -- If it wasn't for the last minute, nothing would get done.