9 years, 8 months ago.

Problem with AD7746

Dear All

For one of my project, I am trying first time AD7745 in single ended mode and sensor capacitance is approximately 4pF. I programmed with dsPIC30F4013 and communication looks OK.

I used CapDAC value of 0x7F so that I can use it for capacitance up to approx. 21 pF.

However checking results with debugger I found that CapHIGH, CapMED & CapLOW all showing 0xFF value. I am using signal from RDY pin to read CapDATA while transition from high to low. In the program I have initialized CapHIGH, CapMED & CapLOW to 0x00. I checked with oscilloscope that excitation signal is 31.8KHz.

Finally I disconnected sensor and checked it's value which measures 3.56pF. After disconnecting supply to PCB, I measured capacitance between pads EXCA and CIN+ with agilent capacitance meter it shows capacitance between these pads to be 37.3 pF with out senor. I cleaned AD7745 pins on the PCB with acetone but no difference. I understand that It could be due to Parasitic Capacitance to Ground and pin EXCA & CIN+ but should not result in to

So I would like to know if there is a procedure to check the fault.

Thanks in advance