> From:         Mark A. Samuels[SMTP:msamuels0@COX.NET]
> Sent:         Wednesday, February 11, 2004 2:43 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [PIC:] A/D module gain

> I'm having some issues with 10-bit A/D conversions that I've never seen
before, and wanted to know if anyone on the list had ever > seen, or would
have any insight into...

> I'm seeing a variable gain in the A/D result, apparently dependent on the
frequency of the sampled signal.  I'm primarily > interested in reading a 1
to 6 hz ac signal.  The ADC works perfectly at DC, and works perfectly
above 20hz.

> My test setup right now is the frequency generator and the scope are both
connected directly to the AN0 input, on the PIC pin > itself.  The only
other thing on the board that is on this pin is a voltage divider, made up
of 2 1k resistors to set my bias at 2.5v, > and an open connector where the
normal analog signal would be applied.
> I'm sampling at 192hz, which is plenty slow, and time samples as follows:
> turn on the A/D module, delay 24uS (calculated min. Taq of 11.5uS), do
the conversion, delay 100uS, turn the A/D module off, > store ADRESH and
ADRESL, delay remainder of 5.2mS necessary for 192hz sample rate, and
repeat.  I've also tried this with a > full millisecond aquisition time,
and a full millisecond to do the conversion, with the same results.  Also
tried witl all possible > ADC clock sources, with identical results.  Once
I've read all 192 samples, I spit out all the sampled values via the serial
port, > and can capture and plot the data.

> Waveforms look perfect, just the amplitude is off.  The gain that I see
is anywhere from 2 to 5 times, depending on the frequency > ranging between
.5hz and 10hz.

The common elements in all of this are the signal generator and the scope.
Perhaps the output
of the generator changes with frequency. You may say that you are using the
scope to keep
the output constant as the frequency changes. Here is a subtle point: the
frequency range
from 0.5 to 10 Hz may be at the low end of the AC range for your scope. If
you are using AC coupling,
you may be seeing the fall off of the scope response, which you are
compensating for by
cranking up the output of the generator. By the time you get to 20Hz, the
input is above the
knee of the input response. Perhaps you are using AC coupling to prevent
the 2.5 volt DC bias
from complicating the display. Try using DC and shifting the display
accordingly.

John Power

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