Reading AD7745 with Arduino Mega 2560

I am trying to read a 1pF ceramic cap using AD7745 (datasheet. I am talking to the chip with Arduino Mega 2560 via i2c. My code has been borrowed from this post with modifications as needed: Reading AD7747. When I read out the values on my computer, I notice that the high bit (@ address 0x01) is always read as 2(tried with two different AD7745) regardless of what cap I connect between EXCA and CIN+. The reading I am getting for a 1pF cap is too low and is almost the same as the reading I get when I don’t connect anything (parasitics?). This tells me I am probably setting up something wrong. Any pointers on what I might be doing wrong? .ino file attached

Capsensing.ino (1.6 KB)


Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks Tom.... :slight_smile:

Hi, I have attached the schematic

A picture would also help, to see your component layout, at 1pF, layout, short leads are all very important.

Also a .1uF bypass capacitor across pins 13 and 14.

Tom... :slight_smile: