Re: HCT4051 leakage

On 30 Nov 2005 17:16:24 -0800, "Mark" <makolber@xxxxxxxxx> wrote:

>consider the capacitance at the input of the A/D.
>
>when the mux changes channels, it has to charge/discharge to the new
>value through the series resistance
>
>it might take a long time for the cap to reach final value accurate to
>24 bits.
>
>this leads to what appears to be crosstalk between the channels
>
>you can improve the situation by setting the mux to channel with the
>fixed ref voltage in bettween reading active channels
>
>it seems hard to belive a small c could do this but if you are going
>for 24 bits, it may be part of the problem
>
>Mark


I am letting things settle for about 4 milliseconds before I kick off
the ADC, which should let RC tau's settle pretty well. Impedances are
low here.

Our system spec is 250 PPM, and we're getting 10's, even 1's of PPM
for resistances near what we calibrate with, with a systematic trend
of error versus Rx, peaking at around 100 PPM at our operating
extremes. So we're OK, but I'd like to understand the residual errors
and fix them. We just did a channel-channel crosstalk check, and
changing channel N over its full resistance range affects N+1 about 1
PPM, hard to resolve, so thermals in the cmos switch are looking
unlikely.

We'll build and test a few more boards. If the errors seem systematic,
we can always toss a little software curvature at it, and never really
have to understand the physics.

Y = Y + K * Y^2

has cured more sins than the College of Cardinals.

John

.

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