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'Analog MUX ON Resistance Variations [Tech]'
2000\03\23@183512 by Brandon, Tom

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Looking at a ADC application measuring voltages generated by pots. Looking
at something like 64 channels so 2 levels of 8ch Analog MUXing. I'm
wondering, what sorts of variations am I going to see in ON Resistance? The
Analog Mux I'm looking at (Philips 74HC4051) specs. something like 9 Ohms
variation in ON resistance between any 2 channels.

What I want to know is:
1) What sort of variations am I going to get between channels on different
ICs? They spec ON resistance as varying by up to 180 Ohms from the specs.
2) What factors will effect the ON Resistance of the MUX during operation?
i.e. is temperature going to have a large effect on ON Resistance? What
about the signal, wil the signal type effect ON resistance?

The application only needs 7bits accuracy (having 14bits would be nice too)
as it's MIDI but it needs to be a very consistent 7bit. Cause there's only
128 positions, 1 position is important. There can't be any drift and ideally
all channels would be highly equivalent in terms of position -> value.
Obviously some sort of calibration will be neccesary I'm just trying to get
an idea how often I'll need to calibrate and if it's wirth compensating for
external conditions.

Anyone have any data on this sort of thing (just general datas fine doesn't
have to be specific to this chip)? The philips data sheets don't provide any
info. on this sort of thing. About the only graph in the whole datasheet is
supply operating ranges.


2000\03\23@195337 by Harold M Hallikainen

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       The variation in ON resistance has no effect until you load the ouput of
the mux. The input of a PIC A/D is a pretty high resistance, much higher
than the mux. The other concern is OFF resistance or leakage current. How
will the voltage on nonselected channels affect the selected one?
       I have used one level of 4051's with 10K pots and gotten at least 8 bit
resolution with a 16c74 (I can get a stable reading with variation only
at the "bit boundary" areas of the pot).
       To get 64 analog inputs, how about a single level of 4051's driving 8
analog inputs on the PIC?  I did this for 24 inputs.


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