Common ground for NI USB-6341 (BNC)

[chikabc]

Hi All,
I have electromyogram analogue output through BNC from biometric system. In addition to the BNC, there is also unterminated common ground.

I want to perform the recording of the EMG using NI USB 6341 (BNC) unit.

I can connect the BNC output of Biometrics through the analogue input of the NI USB 6341 (BNC) unit. Recording without the common ground causes the signal to fluctuate up and down.
But how do I connect the common ground from Biometrics to the NI USB 6341 (BNC) unit?

Thanks

[Cavarval]

Hi,

I am tempted to say you could connect the common ground to the AI Gnd terminal on the USB-6341, but I want to ask you a couple questions before jumping to conclusions.

1. What position is the FS/GS switch in?

If it is in the GS position, try FS without connecting the common ground. This will make the card internally reference the signal to its own ground and may solve the drifting issue depending on the input configuration you are using, which brings my next question.

2. What input configuration, RSE, Differential or NRSE, are you using?

3. Does the problem only happen with the specific device you are trying to measure? Try shorting the USB-6341 channel by putting the inner and outer conductors from the BNC connector in direct contact and perform a reading. The value should be essentially zero. Does the signal still drift?

Regards,

[chikabc]

Thanks Cavarval for your reply.
I have tried both FS and GS; RSE, NRSE and Differential and the issue persists.

This issue seem to occure only when using biometric amplifier to record electromyogram signal which require a separate the ground;  other signals like goniometer which does not have separate ground seem to work fine.

[Cavarval]

Hello,

I am still tempted to connect the common ground from your device to the AI Gnd from the USB-6341, measuring with a differential configuration. However, I would prefer to take a look at the biometric amplifier specifications to make sure there wont be any issues. Do you have a specifications document you can share? Having at least the make and model may be helpful.

Regards,

[chikabc]

Here is the link with specification:

http://www.biometricsltd.com/datalink.htm

I decided to do what you are tempted to ask me to do and connect the common ground to the AI GND on the USB 6341.  As soon the ground is connected, the drifting stops for recording with NI and signal seem like fine electromyogram. When I do this the USB connection for the biometric system disconnects  and only connects back when I disconnect the common ground but does not affect the good signal recording with NI whether the USB is connected or not. It will be nice to confirm that such a connection does not damage anything or creates a latent issue otherwise it could solve the drifting as you suggested?

[Cavarval]

Hi,

I was wandering through the website and I think I am still missing information.

Are there any sensors involved? I found Surface EMG and MyoMeter among the products but I don't know if you are using some of that.

The device from the link seems to be a data acquisition system so I am unsure about the connections right now. Are you connecting an output from that device to the USB-6341? Or is there a sensor connected to both the USB-6341 and the biometrics system?

Having a detailed description and / or diagram of any connections between the involved parts may be helpful.

About the ground connection disabling the USB link from the device to the computer, I really don't know if this may mean a problem or not, but it does seem odd. Since I can now assume the device is not USB powered, is it connected to the same power plug as the USB-6341? If it is, at least there should not be any risk of a ground loop.

I found some sensors from that website show a "Please contact us for full technical information" when they will be connected to 3rd party hardware. Have you tried to reach Biometrics Ltd? They may be able to provide some feedback about whether the USB disconnection behavior is something bad.

I wish I could provide a more direct answer, but the exhibited behavior concerns me a little bit. Would not want any equipment to break because we overlooked something Biometrics Ltd may be able to answer.

Regards,

[chikabc]

In the link i sent to you, below the base unit is a subject unit which they listed as accessory. The subject unit links to the base unit. Here is a link to the surface EMG which links to subject unit:

http://www.biometricsltd.com/semg.htm

Please find the attached sketch of my setup.

I will try to contact Biometrics Ltd again but they always say they will get back and never do. I have contacted them around about the same time I posted the question here and am still waiting for them; you have already helped me more.

The USB connection of the biometric base unit to PC is not required during analogue recording; my guess is that this may be while it is being diconnected when the ground is connected; but I am not sure and hopefully they can confirm this for me.

Thanks

[Cavarval · Accepted solution]

Hello,

Thanks for the details. I believe you are right. Just found this in the Subject Unit description.

The Base Unit connects to the PC using a USB port where the data may readily be stored on disk or passed real time into other applications such as Microsoft Excel or Microsoft Visual Basic using the DLL (dynamic link library). Analog outputs may be obtained using an optional output cable (R2000i) attached to the Base Unit allowing the DataLINK to be interfaced to virtually any analog data recording system.

 Just as you say, the device may be disabling the connection when it detects a "proper" connection to an "analog data recording system". If Biometrics Ltd confirms this is a normal behavior, I would say you are good to go.Hope you hear from then soon.

If there is still anything we can help you with, just let us know and we'll try to do it.

Regards,

[chikabc]

Thanks