I recently laid out a printed circuit board to connect to the USB 6009. There is a single ground plane layered in the board for both the digital and analog circuits on the board. The digital IO is activating a relay (via port 1) while port 0 is broken down to three digital outputs(P0.0-P0.2) and one digital line (P0.3) Furthermore, there is a 1-5 volt signal that I'm monitoring (I've tried RSE & Differential) on AI0.
The problem is that when I have my digital lines & ground hooked up to the board, I get a 0.8 volt reading on my analog input. To further clarify, I have no power running to the circuit board when I get this reading but I get the same reading regardless. If I disconnect the digital lines, the analog input reads 0 volts as it should. I should also point out that the voltage that I'm trying to read from my circuit board is from the output of an op amp/buffer.
I've used the USB 6009 (or similar NI products) for many years now but this has me stumped.