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capacitance measurements from AD7147 are noisy

Question asked by Morgan on Apr 17, 2013
Latest reply on Aug 14, 2015 by retti

I'm using an AD7147 to measure the capacitance of four different sensors. I'm communicating with the chip over SPI using an MSP microcontroller. At the moment, I'm just trying to record the raw capacitance values that the sensors measure and plot them. When I do this, I see that the capacitance values are pretty noisy.

 

The capacitance values tend to look ok, but every once in a while there's a spurious spike in the reading. The spikes happen on all the channels, and they seem to be related to the ambient capacitance of the system. If I wave my hand around the system, I can find a location where all of the spikes stop. In general the spikes are about 200 counts large, and I've seen them go both positive and negative from the nominal capacitance value. I'm attaching an image that shows what's happening for only one channel.

 

Is there a configuration step that I'm missing that would take care of this? I'd like to be able to get consistent readings no matter where my hand is near the sensors.

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