USB-2408 for mixedThermocouple and Analog input noise

[Joe S]

With 8 differential or 16 single ended channels available, my device is measuring seven Type J thermocouples and one analog 0 to 5V signal which is an output from a strain gauge process meter (2 wire output).  The seven TCs are differential.  The remaining two channels are currently configured as single ended channels.  The positive lead of the analog output from the meter is connected to one SE channel and the negative lead is connected to analog ground.  The result is noise on all TC channels.  Please confirm the preferred connection and configuration.  The user guide describes the differential connection with a 100Kohm resistor between the channel low and analog ground. 

[JRys]

Hello,

You can configure the last channel as two single-ended channels. Connect the process meter (+) to channel 7 (pin 25) and the process meter (-) to analog ground (pin 26).  If doing this makes the temperature inputs noisy or unreadable, then you have a ground loop between the thermocouples and the process meter. For this situation, switch channel 7 back to differential mode and connect the process meter (+) to pin 25, the process meter (-) to pin 24 and connect a 100k ohm resistor from pin 24 to pin 26. If the ground loop is severe, the only solution may be to use a second device for the process meter measurement or to use some sort of signal conditioning to electrically isolate the signal.

best regards,
John