AVR-Chat

At 02:35 PM 8/16/05 +0000, Dave VanHorn wrote:
>I'm not sure if this is an on-chip problem, or off chip..
>
>I have a T26, measuring current by a differential measurement, using
>the G=20 amplifier.
>
>Explaining the whole thing in detail would be tedious, but I'm doing
>high side current measurement with resistors scaling down the inputs
>so that I don't rail out the ADC.  I am using an external 2.5V
>reference, and I've verified that it's in tolerance. (2.458V)
>
>In the existing setup, I am seeing about 180 counts on the ADC, but
>running the math on the external parts says that I should be seeing
>closer to 90 counts.  IOW, the system thinks the current is about
>twice what it really is.
>
>This is a mature system, not a prototype, and has been in production
>for some time.. Date codes on the chips don't seem to matter (I've
>tried various ones all the way back to "ES"), and the existing
>firmware works fine in older "known good" boards. This does seem to
>be a problem on some number of boards.  Others seem ok, but I am
>waiting on confirmation of that. The ADC is not overclocked.
>
>
>What could cause the ADC readings to be almost (but not exactly)
>twice what they should be, assuming that it's not a firmware change,
>and that the components pass visual inspection, plus the voltage
>measurements confirm the component values and PCB connectivity?


Noise?  I don't know why that would be different between boards but I've 
seen narrow spikes (<1ns) cause A/D readings to change quite a bit from the 
baseline, more than 2x in same cases.  Have you checked the inputs with a 
'scope?  DMMs and scopes set on slow sweeps can miss the spikes quite 
easily. Small timing differences in when the A/D samples and steps could be 
quite important in the case of noise spikes on the input.

Robert

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