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AD7147: Using an External Sensor With the Evaluation Board

Question asked by Dave_Tripp on Jun 27, 2016
Latest reply on Jul 1, 2016 by Maurice

Hello,

 

I have the AD7147 evaluation board, and I'm trying to use it to read capacitance values from an external AD7147 chip on a PCB. I'll include below the schematic I am using for my external sensor. The external sensor is designed to measure force, a  copper pad on a PCB connects to CIN0 of the AD7147. This copper pad is covered in a layer of dielectric (silicon) and then a piece of aluminum rests on top. By applying force to the aluminum (and squishing the dielectric) I expect a change in capacitance. If there is a problem in my schematic design please let me know.

 

I have been able to connect the external sensor PCB to the evaluation board, and they are communicating, however there is a lot of noise and fluctuations on the output. Without any load the values jump between 32500 and 35500 randomly. Applying force to the sensor also does not really seem to be changing things. 

 

The external PCB has two copper pads, one connected to CIN0 and one to CIN2, (the blank space of CIN1 was just to make soldering easier). The rest of the pins are connected to BIAS as suggested in the data sheet, with the exception of CIN1. CIN1 is connected to nothing, this again was just to make soldering easier. Is it possible this is causing a problem?

 

Right now I have set up the evaluation software as follows:

 

Register Configuration> CIN Configuration>Stage 0, set CIN0 to +input path, all others to Bias. Then clicked Get Offset.

Register Configuration> CIN Configuration>Stage 1, set CIN2 to +input path, all others to Bias Then clicked Get Offset.

All other stages have everything set to BIAS, but I am only using stage 0 and stage 1, correct?

 

Then I am going to Register Configuration>CDC converter to read the CDC. I was looking for a way to directly read the CDC_Result_Sx register, but couldn't find it. Is this the same thing though?

 

Thanks,

Dave

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