USB-6351 channel ghosting

[kbriggs]

I am using a USB-6351 to read from two channels (current and voltage) at 625kHz each, so it's maxxed out. I am stepping the voltage every 10ms (analog output) and recording both the current and voltage response of my system at 625kHz. When I apply the votlage step there is capacitance excited in the system, so the current I read out has a short transient which I am seeking to record.

I am getting a ghosting effect in the voltage channel wherein there is a very short transient overshoot (usually maybe 1 or two data points) right at the leading edge of voltage steps in the voltage I read back. I have verified that it is not present in the applied voltage, but rather it is an artifact of the reading end of things, due to the fast switching between channels. For the voltage channel I don't care as long as I know it's not real I can ignore it, but what is not clear to me is whether or not there is a similar effect present in the current reading, since there is a real transient there that would be convoluted with any artifacts from the hardware.

The solution that one of my colleagues found is to sample from a third grounded channel in between each read of the current and the voltage. But this has the effect of reducing my max sampling rate to 416kHz per channel, and I really do need 625kHz. 

Is there a solution to remove the ghosting effect that won't compromise my sampling rate? Or better yet, is the ghosting effect based on gradients in time in the channel being recorded, so that I can confidently say that there is no effect on the current channel, since the voltage is constant in time over each step?

I hope I've explained this clearly, if any additional information or clarification is needed please let me know.

[Aaron_W.]

kbriggs,

There are some documents on NI's site that will help you out.

How do I Eliminate Ghosting from My Measurements?

One of the suggestions is to use a Unity Gain Buffer to address your high impedance source.

Depending on your device and your measurement configuration (RSE vs Differential). You could try and move your second signal onto the the back half of the Analog Input channels (for 16 channel AI channels there is an ADC for channels 0-7 and then another ADC for channels 8-15). If you are using a RSE configuration you could put one channel on AI0 and the other on AI8 and see if that helps.

[kbriggs]

Thanks for the link, the article seems like just what I was looking for. I am using differential mode. Will using AI0 and AI8 not help in that case?

[Aaron_W.]

With the 6351 you only have 16 AI channels in Single Ended mode, meaning you have 8 differential channels. Channels 8-15 get used as the other differential line, so you don't "have" those channels to use. In differential mode one ADC is used for your AI+ channel the other ADC used for your AI- channel.