1794-IT8 - Removing Bias Current

I am using an specific IR temperature sensor device that requires basically NO bias current (they spec it in the nano amp range). Unfortunately, the IR device manufacturer didn't tell us that until after the devices (all 24) were installed and wired. Thus our accuracy is way off.

Has anyone figure out what resistor to trim to remove the bias (open circuit) current on the 1794-IT8? One of my co-workers reverse engineered the circuit until the multiplexer but thats were we "lost the trail".

Any another ideas would be appreciated as well. We are considering amplifers, or making a unity gain buffer with op amps...unfortunatley we don't have much room left to mount addition devices. Plus we are trying to control our rework costs.

Thanks,
Chad

More info on the sensor specs.
Have you tried grounding one side of the signal?


The 1794-IT8 is a temperature/mV measuring module that accepts inputs from a variety of thermocouples and from the mV source in the range of ±76.5 mV. Use cold-junction compensators (cat. no. 1794-CJC2) in thermocouple mode. Two cold-junction compensators are shipped with the 1794-IT8.

The sensor we are using is made by exergen. The complete spec sheet is here: http://www.exergen.com/industrl/PDFs/uirtcsv.pdf

We were trying to use it just like a standard thermocouple because thats how the manufacturer advertises it. We were using it with cold junction compensations.

We checked to see if the mV setting had a bias current and it does.

Thanks,
Chad

The spec sheet says it is type K wiring. Is the hook up wire you used
type K thermocouple wire. If not that will creat a junction at each
connection. Each connection will then act like a thermocouple and
generate a voltage. Also any crimp connectors, terminal strips,
plugs or jacks must be for type K thermocouples.

Yup...check all that.

We talked directly too exergen and they stated the bias current of the analog card is too high. It needs to be in the nano amp range.

CG