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.
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.