Saturated signal in a daqmx task without wrong wiring

[sciu]

Hello

I've installed daqmx-driver on a PXIe-8108 controller in order to acquire analoge signals from several load cells. The task I created used to work until I've started to use an FPGA-Card to perform other tasks in the program.

Now, when I start my program, the output of the load cells is always 10.45...Volts which means more than the possible 10V output. There is no drift but just a "saturated" signal. Since I've not yet found a wrong wiring, grounding or something similar, I don't have any other idea what could cause this signal problem.

Thanx for any suggestions.

[Henrik_Volkers]

I would guess an open contact.

Test the channel with a short cut (and a bias to AGND) , does it measure ~0V ?

[sciu]

Hi Hendrik

if I make a shortcut from the signal source to AI Ground the signal dropes close to zero. I get a signal from the load cell even when I remove the shortcut - at least for a few seconds...

[Henrik_Volkers]

That indicates a missing bias path.

So at least one input seems to be floating...   or both... How is the bridge connected and supplied?

[sciu · Accepted solution]

Hi Henrik

I'm using a SCB-68 Connector block (R Series). The amplifier output signal (see attachment) is connected to the AI+ and AI- pins (#68 and #34).

I've added a 100kOhm resistor between AI+ and AI-Ground and AI- and AI-Ground. This seems to work. Interestingly even a 100kOhm resistor between AI+ and AI-Ground seems to be sufficient...

Greets and many thanks

[johnsold]

1. You do not need a differential input with that amplifier. It produces a single-ended output.

2. How are the grounds connected? Signal conditioner Supply GND (pin 2), signal conditioner Signal GND (pin5), DAQ system ground?

3. Is any part of the load cell or the device to which it is mounted connected to any ground, including earth ground?

4. What is the actual analog input device (not the controller or the connector block) you are using?

Lynn

[Henrik_Volkers]

One resistor 100k is fine. but should connect AI- to AGND    (or powergnd (=signal gnd?) to AGND.

Your amplifier seems to have a floating powersupply ... if it worked in the past...  any changes in the configuration there?

[sciu]

Hi Lynn

I am using a NI-6363 and a NI-7853R card in a 1078 Chassis of NI. Pin 2 and 5 are connected with a PULS power supply. This power supply is connected to an earth ground. There is no connection to the chassis or to any of the NI-cards. The cards themeselves are mounted in the chassis and share the same system ground (internally).

I've tried to acquire a RSE signal, but somehow the differential way worked better...?!?

thanx a lot

[sciu]

Hi Henrik

It used to work only when using NI-6363 DAQ-card alone (w/o the FPGA card). As soon as I implemented the FPGA card the signals of the 6363 card raised up to 10.4..V

Greets