9234 Sensor Return to Chassis Resistance

[bcombs]

My question is related to the NI-9234 module and its return terminal (AI-) resistance to chassis. On the side of the module, the schematic shows a 50 ohm resistance to chassis. However, when measured using a multimeter, the resistance is much higher (on the order of 500 kohms.) I am hoping someone might be able to explain this discrepency. My only thought is that, based on the datasheet, there are current-limiting diodes present between the return and the 50 ohm resistor that could be affecting the measurement. I don't understand how this would be possible, however. Any help is appreciated.

[AnalogKid2DigitalMan · Accepted solution]

Page 11 of the manual:

http://www.ni.com/pdf/manuals/374238c.pdf

shows back to back diodes in series with the 50 ohm. They are probably affecting your measurement if your DMM does not exceed the conduction voltage of them (not sure what the specs are on those diodes).

-AK2DM

[Bigredd0087]

AK2DM, that makes sense. Measuring with different DMM models yields different results, indicating it is due to the differing amounts of current being pushed by each model. Neither will fully bias the diodes. Thanks.