Incorrect reading in RSE mode USB-6009

[jasonjps]

I'm using a USB-6009 in conjunction with a K-type thermocouple and AD597 conditioner/amp.  I initially had 4 signals plugged in which correctly reported the 10mV/degC signal using the differential setting, however, I needed to add one more reading to the experiment.  Since the USB-6009 only has 4 differential channel and 8 RSE channels, decided to make the switch to accommodate the 5 readings.  I changed all of the inputs to the correct terminals (The negative output lead of the amps are all common and connected to the USB-6009 ground and the positive leads are connected to the AI# terminals) and switched the Labview settings to RSE, but all the readings in Labview went to ~0.33V.  This result was indifferent to whether the amps were powered by battery, outlet or even unplugged as well as if the thermocouples themselves were disconnected.  A multimeter across the terminal port shows the same ~0.33V.  If the leads of the powered amps are disconnected from the USB-6009 terminals, the multimeter shows that the signals from the thermocouple amp are correct again (room temp -> ~0.22V) and adjust to heat changes.  Returning everything back to differential mode also corrects everything again.  We have several USB-6009 units so I disconnected everything and retried on a different USB-6009 and obtained the same erroneous results.   

[BROhan]

Hi jasonjps,

When using the USB-6009, it is not uncommon to get noisy results, as mentioned in this knowledgebase article. I would recommend moving to another form factor like the USB-9211. With an RSE configuration, you will be even more subjected to noise than a differential configuration.

[jasonjps]

It seems that this is not a noise problem.  The RSE voltage is virtually static at ~0.33V.  It's not like it's near the correct value and reacts to temperature changes with a large margin of error.  

[BROhan]

Hi jasonjps,

Actually, you should generally not use an RSE configuration for such a low voltage input value, as noted in the following knowledge base:

Field Wiring and Noise Considerations for Analog Signals.

There is a warning note in the knowledge base that states:

Single-ended input connections can be used when all input signals meet the following criteria.

• Input signals are high level (greater than 1 V)
• Signal cabling is short and travels through a noise-free environment or is properly shielded
• All input signals can share a common reference signal at the source

Differential connections should be used when any of the above criteria are violated.

[greentea2]

Hello All,

I am still confused on RSE and Differential Mode. I have a pressure sensor with a measurement range of +/-5V which I want to connect to USB-6009.  Since there is only one pressure transducer I attached the positive end to AI1 and the negative end to ground.  Is this RSE or Differential mode? (If I select Differential Mode in MAX, the signal is less noisy then in RSE).

Thank you

greentea2

[Dan_N]

Hello,

This is wired in an RSE configuration, as you have the positive input wired to the AI0 and the negative input wired to ground. The field wiring guide (https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...) does a good job of explaining how the different types of measurements are wired. A differential measurement would require wiring the positive terminal to AI0+ and the negative terminal to AI0-. 

Best,

Dan Nelson

Applications Engineer