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Output Issues with interfacing NI PCIe 6361 + BNC 2110 with an external device

Hi everyone,

I am experiencing a strange issue with Analog Outputs on my NI-PCIe 6361 board (with BNC 2110). I am working on an AFM device and I am trying to do system identification using the NI board. I am able to receive signals properly. However, there is a problem with Analog Output (both A0 and A1).

I used oscilloscope to monitor the AO0/AO1 signal (with AFM unit not in-loop) and it works fine. However, the moment I connect the signal to the AFM unit, the signal becomes zero (as seen on oscilloscope) and the drive current is constant 21m. However, when I use an external function generator to generate the same sine wave with AFM unit in loop, the drive current varies sinusoidally with 6m pk-pk current as expected and I was able to actuate the device properly. Let me know if I am missing something, probably APFI0 connection?

I tried to flip the FS/GS switches on the BNC 2110, but that doesn't make any difference. The AFM control signal is a single-ended input.

Thanks,
Mayank

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Hey mayankb,

 

I noticed this post exists on some other threads. Have you tried their recommendations yet? I believe Lynn from one post linked below suggested that you check the grounding since you may have a ground loop involved. Have you tried this yet?

 

http://forums.ni.com/t5/Multifunction-DAQ/Strange-problem-with-analog-output-on-PCI-6251-and-BNC-211...

 

Regards,

 

Ryan

Ryan P.
CLA
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Hi Ryan,

I guess overloading is one of the reasons. I am going to try a voltage follower (the post recommends voltage subtractor --- i guess it possibly means voltage follower).

Thanks,
Mayank

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Hi Ryan,

All the devices - the PCIe card, oscilloscope and AFM unit have the same grounds. Let me explain you the scenario more precisely.

I have an AFM unit and the input to AFM unit is a voltage signal from -10V to 10V. I think the input of the AFM unit is internally pulled low (as it reads -10V with the multimeter when turned on). Before feeding in the voltage to AFM from the PCIe board, I first scoped the signal on an oscilloscope and it read the correct signal. However, the moment I feed the signal to the AFM unit, the signal drops down to zero and the AFM piezos do not get actuated.

Then, I used a function generator (again sharing the same ground) with 50ohms output resistance and generated the same sinusoidal signal and the AFM unit works fine.

I have tried using a voltage follower for the input signal to the AFM unit in case the AFM unit was trying to draw an excess current from the PCIe card but that didn't help either.

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The output drive current limit for the PCIe-6361 is 5 mA. At 10 V that corresponds to a load resistance of 2000 ohms. The fact that the function generator will drive the AFM and the DAQ device will not sounds like a loading issue.

 

Unless your voltage follower can drive a 50 load (200 mA at 10 V), it may have the same problem as the DAQ device.  Most standard op amps cannot produce that much current,

 

Check the AFM specifications to see what its drive requirements are.

 

Lynn

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