AVR-Chat

(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?

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

" 'Freedom' has no meaning of itself.  There are always restrictions,   be 
they legal, genetic, or physical.  If you don't believe me, try to chew a 
radio signal. "  -- Kelvin Throop, III
http://www.aeolusdevelopment.com/

Dave -

When you say that this is a mature system, does that mean
that it used to work properly and isn't now?

One of the potential problems with differential measurement
is keeping the common-mode voltage within allowable limits.
Normally, however, you would expect lower sensitivity
rather than greater if this were violated. But... do you
have a ground reference connection in addition to the two
signal connections?

Jim
---------------------------------------------------------------
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http://www.thinkdifferentstore.com/
For All Your Mac Gear
---------------------------------------------------------------

One real simple test to isolate the problem: Swap CPUs between a working
board and a failing board.

My money is on the board, not the CPU, as the source of the problem.

-----Original Message-----
From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Jim Wagner

When you say that this is a mature system, does that mean
that it used to work properly and isn't now?

One of the potential problems with differential measurement
is keeping the common-mode voltage within allowable limits.
Normally, however, you would expect lower sensitivity
rather than greater if this were violated. But... do you
have a ground reference connection in addition to the two
signal connections?

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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