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AD7799 - values "jump" at a specific, non-zero point when instrumentation amplifier is active

Question asked by EGIF on May 29, 2017
Latest reply on Jun 7, 2017 by JellenieR

Hello,


I am new to the AD7799. I am creating a prototype measuring pressure from a piezo transducer.


Expected behavior : Measured values would rise smoothly and linearly from 0 to 250 bar with linear pressure increase.

Observed behavior: Starting from 0 bar, measured values start rising linearly, then jump at a specific point (near 5 bar), then continue to rise linearly.

 

My setup is:

 

  • AD7799 connected to a Renesas RL78 through SPI
  • Targeting 0.1 bar resolution from 0-250 bar.
  • VDD/ADD = 5V, REF voltage is 0-VDD (for now).
  • Transducer is 2mv/V sensitivity, and connected to AIN1+/AIN1- in bipolar mode.
  • Readings attempted in Single mode and at 16hz continuous.
  • Internal calibration performed after setting amplification (4x,64x)
  • 128x (ideal) amplification also tried (presumably with factory calibration as stated in the datasheet).


The pressure transducer is connected to a fine pressure regulator, I can increasting and decreasing the pressure from 0-6 bar in fine increments.

 

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Measuring the pressure transducer directly with 4x amplification (inAmp not active) - I get smooth, linear and repetable (if a bit noisy and imprecise) .. raw values from the AD7799. This is workable for ~0.5 bar precision in my case.

 

When I activate the InAmp (Amplification of 128x or 64x for example) - then starting at 0 bar, I slowly crank up the pressure - at a specific point, the data jumps all of a sudden. Converting this to pressure, it's as if the pressure jumps from 5 to 22 bar in the space of 0.1 bar.

 

De-activating the InAmp (returning to 4x) - the linearity returns, is nice and stable and as expected from 0 to X bar.

 

Is there a limitation of the AD7799 I am missing? What can be done to troubleshoot this?

Thank you very much.

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