Strange analog output from USB-6211

[skuo1008]

I just received USB-6211 to replace USB-6001 for setting external sample clock on analog output for LED light control. The external sample clock part is working fine, but the analog output voltage is strange. In order to do self-testing, I have connected LED control pin to AO0 & AI0 for monitoring in NI MAX test panel, and LED control ground to AO-GND & AI-GND. Since I have both USB-6001 and USB-6211, I carried out tests on both of them with the same wire setting. When I generate sinusoidal wave -5V to 5V at AO0 (from NI MAX test panel), USB-6001 can monitor the same signal from AI0, but USB-6211 shows -3.4V to 3.4V truncated voltage. I have done the test separately (wiring one device at a time), so there is no interference between the two devices. USB-6211 passed self-calibration and self-testing. I've also done reset devices. I don't know why they would behave differently with the same setup, and hopefully someone could help with this issue. Thanks.

[Roohcifer · Accepted solution]

Hi skuo1008,

The USB-6001 can support +/- 5 mA current output from the analog output terminals, whereas the USB-6211 can only provide +/- 2 mA current output.  It's likely that the impedance of your load is too low, causing the 6211 to hit its current compliance and thus clip the voltage.  If you try swapping your load with a resistor of at least 5V/.002A = 2500 Ohms, you should be able to see the full +/- 5V sine waveform.  I suspect your DUT has an impedance of about 3.4V/.002A = 1700 Ohms.  You could either use a device with higher output current drive or use a buffer circuit to source more current.  If you don't need a bipolar output, you could also consider using the digital lines to control the LEDs.

Regards,

[skuo1008]

I see. Thanks for reply. We are looking at another card: NI USB-6341. It has output current drive +/- 5 mA and supports sample clock on Analog output. Would this one be a good replacement? Asking in case that I missed something. Thanks.

[Roohcifer]

Hi skuo1008,

I don't know your full project specifications, but the 6341 certainly can accept an external sample clock for AO tasks and has +/- 5 mA current drive on the AO channels.  You may also want to consider sample rates, channel counts, expandability, connectivity, signal conditioning, resuability for future projects and other things when you purchase any product.  If you have any questions about these things not answerable in spec sheets, I would reach out to your local sales rep.

Also, it couldn't hurt to verify the impedence of the LED to confirm that that is indeed why the signal is clipping.

Regards,

[skuo1008]

Hi David,

We are now simply working with single analog output with required sample rate around 200 Hz, so this should be more than sufficient for us.

Thanks again.

[Nate@UT]

I was informed previously that none of the bus-powered USB Analog Output cards supported external sample clocking.  But your post is implying that this is incorrect. Will the 6211 actually work with AO in an externally clocked mode?  If so, could you tell me how to configure this setting via DAQmx call?  Thanks!

[bahannah]

Hey Nate@UT,

I would recommend creating a new forum post for this question so that it gets more views and is more easily searchable for our online community.

However, if you have a PFI line, which the USB 6211 has, you should be able to use an external sample clock. We have an example VI on our website about using an external clock source with analog input. You should be able to follow the example to figure out how to apply it to an analog output situation.

Regards,

Hannah

Applications Engineering 

National Instruments 

www.ni.com/support