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AD7730 in bipolar application not self-calibrating

Question asked by sborden on May 14, 2013
Latest reply on Jun 19, 2013 by ChrisD.Rama

I corrected a problem with ACX !ACX lines driving analog switches in a bipolar ac excitation load cell application.  All seemed OK.

 

But always results come as 0.  I tried running combinations of internal and system self-calibrations and the 7730 hangs.  This is the second IC I have tried.  All voltages are nominal as AFAIK, even 5.3V max between AGND (set to -2.3V) and VDD (3.0V).  AVDD is set for 2.3V.

 

I read that  the AD7730 REF inputs are not buffered.  This is a 6-wire load cell, so the REF inputs are going through a string of resistors to drop from +/-5 to +/- 2.2V (approx).  The resistors are  9.1K - 16K - 9.1K.  IC hangs using both AC mode and non-AC mode.  I cannot manually set the gain and offsets either.  If I just run the IC without calibrating, everything cycles, I can change frequency, etc.  But it will NOT calibrate. CHOP="1" (sometimes "0" with non-AC modes), HIREF is "1", !B/U is "0".

 

I have not had any problems with this IC in 0-5V applications.  Are there any particular settings that need to be observed in bipolar mode?

 

Is there an alternative, more modern IC I should be using instead?

 

Best regards,

 

Stuart Borden

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