NI USB 6225 M-Series Number of Mux and assigned ports

[Haui]

Hi Guys,

I'm currently working on setting up a pressure scanner array with 32 pressure sensors for my institute's transsonic compressor test rig (specifics on the hardware setup are noted below).

Basically everything is up and running but I'm experiencing odd sensor returns which seem to be dependant on sample rate(5-30K) and the position of the sensor in the circuit itself. I've attached 4 screenshots showing the dependancy on the sample rate. Furthermore, the first sensor of every circuit is very sensitive to the touch or slight vibrations while the following sensors are not.

Given these issues, I want to check whether or not the internal multiplex might be responsible for this. So my specific question is:

How many multiplexers are build into the USB 6225 and which ports do they address?

Am I right in assuming that there are 8 Mux and each addresses 5 ports?

Any input is highly appreciated!

Happy Holidays!

Info on the hardware setup:

Power:

4 parallel circuits each utilizing a tempco constant current source (using LM134H triodes) and powering one row of sensors. Each row holds 8 sensors which are conected in series. So the first sensor sees the highest voltage (Vin) and every following sensor experiences a lower voltage in relation to the voltage drop over the previous sensor(s).

Sensors:

The sensors are piezoresistive membranes driving a tuned wheatstone bridge.They are floating, meaning that they have no ground connection and are directly connected to the ai+&- ports (differential measurement) of the NI USB 6225.

Grounding:

Every AI GND port (ai+&-) and the chassis gound lug are connected to the casing of the entire array which in turn is connected to the ground of the supplying wall socket.

[CorinneDoppmann]

Hi

Have a look at the M-Series Manual - chapter "Multichannel Scanning Considerations"

http://www.ni.com/pdf/manuals/371022l.pdf

You also might be interested in this Knowledge Base:

How Do I Eliminate Ghosting from My Measurements? - National Instruments
http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF

Also have a look at your wiring to be sure to avoid any bias currents.

Field Wiring and Noise Considerations for Analog Signals - National Instruments
http://www.ni.com/white-paper/3344/en/

Happy Holidays!