Differential measurements on 6036E via SCXI-1302 and SCXI-1180

[Joseph_D._Rudmin]

I am trying to do differential measurements using direct connection to the 6036E card.  I have both the NI 6035E/NI 6036E pinouts and the corresponding pins on the SCXI-1302 terminal block.  In particular, I am trying to use ai0 and ai8 as one differential pair, and ai1 and ai9 as the other differential pair.  So, I am using SCXI-1302 lines 3, 4, 5, and 6, respectively, and lines 1 and 2 for the aigrnd, to which I connect 1.2kohm bias resistors.  To test the system, I put 40Hz 1V peak-to-peak on ai0, and a few volts DC on ai8.  In MAX I configure a task to use ai0 and ai1 as differential, and just observe the test screen at the task in MAX.  I see a 1mV oscillation on ai0 and ai1, at 40Hz, or whatever frequency I use from the function generator, but no effect of the DC offset, although ai0 and ai1 have different DC offsets of 10s of mV.  When I include in the task channels 8 and 9, I see the 1mV oscillation on all channels, but the DC offsets differ greatly whether 8 or 9 are configured as RSE or NRSE. (See table below.)  I also sometimes observe +5V on the physical ai0 line, when the power supply providing the DC voltage is set to provide less than 5V.  This behavior mystifies me.  What am I doing wrong?

 DC Voltage on channels for different MAX configurations of channels 8 and 9.

Chan  8&9RSE 8RSE&9NRSE 8&9NRSE 8NRSE&9RSE

ai0      -100mV   +1.20V              +1.33V        -120mV

ai1      -205mV   +1.25V              +1.43V        -240mV

ai8      -145mV   -2.00V               +3.24V        +4.55V

ai9      -158mV   +2.8V                +3.25V        -230mV

Joseph D. Rudmin

JMU ISAT MSC4106

701 Carrier Dr.

Harrisonburg VA 22807 USA

540-568-2784

[mallorim]

Hello Joseph,

I am not surprised that you a receiving unexpected values for your differential readings. However, your system set up will actually determine the procedure to remove this behavior.

What other modules do you have in SCXI chassis with the 1180, and how is your 1180 cabled to the 6036E card? Are you using a cable adapter like the SCXI-1349 at the back of the SCXI chassis?

The reason I ask for a better description of the system is that channel AI0 of the DAQ card is reserved for multiplexing all the acquisition channels in the SCXI chassis, so taking differential measurements against this channel will not work.

This is described in the SCXI Getting Started manual, pg 1-17, 1-18. Some other Knowledge Base Documents that describe this behavior are:

Using the Digital I/O Lines on an E or M Series DAQ Board While Controlling an SCXI Chassis

Why Do I Lose Access to Certain Analog Input Channels When Attempting to Use My SCXI-1180 Feedthroug...

Can I Use the SCXI-1180 Feedthrough Panel with an SCXI-1520 Module to Access My Unused DAQ Device Ch...

I hope this information helps,

Mallori M.

[Joseph_D._Rudmin]

Dear Mallorim,

Thank you for kind assistance.  I will read the links that you suggest.  I think that the use of ai0 for multiplexing probably IS the problem.  Also, I have already advised the instructor who asked for this configuration that I think that direct connections this way can damage the DAQ card, and that he can achieve the same result by subtracting signal from channels using the SCXI1100 which has better protection. 

In order from left to right in the SCXI1000 chassis I have:

SCXI1100 (Multiplexes the channels.)

SCXI1180

SCXI1162

SCXI1121

Sincerely, Joseph D. Rudmin