Re: ad7793 weirdness

On Mon, 17 Dec 2007 21:52:00 +0100, Arlet Ottens <usenet+5@xxxxxxxxxx>
wrote:

Arlet Ottens wrote:
jakeb221@xxxxxxxxx wrote:
I'm prototyping an RTD based temperature sensor using the analog
devices AD7793 24-bit ADC. I seem to be seeing some unexpected
results when feeding a small fixed voltage (340 millivolts) and
varying the gain of the AD7793. Reference voltage is fixed at approx
2.26 volts.

If I set the ADC for unity gain, I see a reading of 2,531,837...
If I set the ADC for 2x gain, I see a reading of 5,071,765... Which
is reasonable...
Now if I set ADC for 4x gain, I get an unusual reading of
6,116,603... Not double the 2x reading that one would expect.
If I then set the ADC for 8x gain, I see a reading of 12,050,792...
Which is about double the 4x reading, but not 8x the unity reading
that one would expect.

I've tried doing internal fullscale and zero calibrations without any
real change.

I did find a line in the data*** which worries me: "When the in-amp
is active (gain >= 4), the common-mode voltage (AIN(+) + AIN(-))/2
must be greater than or equal to 0.5 V." Surely that is a mistake.
That requirement would render the gain function nearly worthless since
small input values is when you are going to want to use the higher
gains.

Can anyone help me figure out what I'm overlooking?

It means that for higher gain settings, you must bias the inputs such
that the average is above 0.5V. For instance, you could have 1.000V on
AIN- and 1.340V on AIN+.

For RTD measurement, check fig. 21 of the data***. The Rref value must
be high enough such that (Iout1 + Iout2) * Rref > 0.5 V

By the way, if you're not using this 3-wire RTD configuration, you may
find the built-in bias voltage generator useful. See bits 14/15 of the
configuration register.

I don't see how that would help, unless he has a completely isolated
current source, which seems unlikely.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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