AD7730 in bipolar application not self-calibrating

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

Parents
  • I am still waiting for a new board with second op-amp for the reference.  In the meanwhile, I lowered the 9.1K and 16K resistors to 680 ohm and 1.3Kohm, this should allow *plenty* of current (>3.5mA) flowing through the loop for a reasonable ADC to munch on

    AVDD=2.7V AGND=-2.3V DVDD=3.0V, REF+=1.3V REF- = -1.3V.

    If I so much as *READ* the offset or gain registers, the AD7730 locks up.  Same if I perform calibrations or perform writes to the offset or gain registers.  If I leave these alone, then all works, but, of course, only "0" comes up as data.  Occasionally I am able to read either  the gain or the offset register after a RESET and the values match what the data sheet says the POR values should be, but I can never get BOTH registers read.

    As this is the second IC I have tried, I am reluctant to try a third.

    My opamp for the AGND and AVDD is an AD8639.  I am also going to use an AD8639 for reference voltage unless a better suggestion is offered.  In the new board, to avoid IC heating, I separated the negative and positive voltages (AVDD/REF+ on one AD8639 and AGND/REF- on the other AD8639).

    I am desperate for suggestions.  I cannot believe I am the only one who has encountered this issue.  What am I missing?

Reply
  • I am still waiting for a new board with second op-amp for the reference.  In the meanwhile, I lowered the 9.1K and 16K resistors to 680 ohm and 1.3Kohm, this should allow *plenty* of current (>3.5mA) flowing through the loop for a reasonable ADC to munch on

    AVDD=2.7V AGND=-2.3V DVDD=3.0V, REF+=1.3V REF- = -1.3V.

    If I so much as *READ* the offset or gain registers, the AD7730 locks up.  Same if I perform calibrations or perform writes to the offset or gain registers.  If I leave these alone, then all works, but, of course, only "0" comes up as data.  Occasionally I am able to read either  the gain or the offset register after a RESET and the values match what the data sheet says the POR values should be, but I can never get BOTH registers read.

    As this is the second IC I have tried, I am reluctant to try a third.

    My opamp for the AGND and AVDD is an AD8639.  I am also going to use an AD8639 for reference voltage unless a better suggestion is offered.  In the new board, to avoid IC heating, I separated the negative and positive voltages (AVDD/REF+ on one AD8639 and AGND/REF- on the other AD8639).

    I am desperate for suggestions.  I cannot believe I am the only one who has encountered this issue.  What am I missing?

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