AnsweredAssumed Answered

AD7794 Counts change

Question asked by ajaber on Nov 11, 2013
Latest reply on Nov 25, 2013 by MaryMc

Dear Sir/Madam,

 

I have an application of a pressure gauge using AD7794. A Pressure sensor and an RTD is connected in Ratiometrec configuration. ADC settings are as follows:

For Pressure sensor:

Channel 1, External Reference 1, Bipolar range, Buffer Enable, Update rate 33.2 Hz, gain 4, Continuous conversion, Chop enable.  

For RTD:

Channel 2, External Reference 2, Unipolar range, Buffer Disable, Update rate 33.2 Hz, gain 2, Continuous conversion, Chop enable, Excitation current is disabled.

Internal calibration is made before each sample. Both samples are taken by switching between the channels of the ADC every 1 second.

A reference voltage is supplied from a microcontroller  to a low pass filter ( 100nF, 1Kohm) to give a 2.5V reference and the same voltage is supplied to the pressure sensor. 

The RTD reference is taken across a 3.32K Ohm resistor in series with 1K Ohm RTD.

After the samples are taken the supplies for the pressure sensor and RTD are turned off.

 

The problem is that the counts of the pressure sensor from the ADC keeps rising continuously and slowly after temperature of the pressure sensor, RTD and PCB has stabilized using a calibration oven and at atmospheric pressure. The counts from the ADC could go up by 500 to 2000 counts after temperature has stabilized. What could be causing this rise in counts ? please find attached some test results.

 

Any help is greatly appreciated

Best Regards,

Ali Jaber

Attachments

Outcomes