AVR-Chat

Dave VanHorn wrote:

> (apparently this didn't make it, the first two times I sent it, so 
> I'm plugging it in again from the web interface.)
> 
> 
> 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, Switching spikes, loss of sync between the switching element and 
the adc ?

Regards,

Kat.


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