I am reading voltage inputs from a number of sensors using a cDAQ-9174 fitted with a 9205 module. I am using 2 differential inputs connected to shunts and 28 single ended inputs to measure an array of sensors.
Nearly half of these sensors are thermocouples that are conditioned using AD594 ICs. A handful of these sensors are giving incorrect voltage readings; the readings do not agree with a multimeter or with the ambient conditions (AD594 output is 10 mV/ degC, so I generally expect 200-250 mV when measuring ambient; the inputs that do not agree give readings of ~130 mV or less.)
These particular channels are configured as single ended (RSE) connections, 0-1 volts with a continuous sampling rate of 240 Hz. I have swapped chips and thermocouples on the same input channel with no effect; a chip/thermocouple combination may give rasonable readings on one channel and then incorrect readings on another. Further, a chip/thermocouple combo that was giving incorrect readings on once channel may give correct readings if I simply change the input channel. It's hit or miss as to which channels tend to read correctly. The thermocouples are grounded through the wiring of the AD594. I am using a 5vdc 600mA power supply with a floating ground.
I do not have any of these issues with my other sensors. Any ideas as to what is going on? Thanks in advance.
