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Recording counter output with analog input of same device

Hi,
 
I am using a USB6218 under Measurement Studio for an electrophysiology rig.  I have 2 analog inputs and one counter output task. I am using the counter output (CO) to provide precise off/on/off timing of an LED light.  One analog input  (AI1) records the voltage from the tissue I am stimulating with the LED. The other analog input (AI0) was to be used to record the counter output so that I had stimulus and response voltages side by side and could display when the pulse(s) were sent to the LED.  However, when I connect the counter output triggering the LED to AI0, the voltage recorded at AI1 drops noticeably when the counter output goes high.  It does not do this when the counter output is disconnected from AI0, but still connected to the LED driver (a Buckpuck).  I have tried connecting CO to AI0 using a resistor (in case AI0 was drawing too much current) and the drop was less but still there (and the pulse smaller in voltage). I also tried both single ended and differential voltage modes for AI.  So, is there a way to record a counter output pulse with an Analog Input line on the same device without this voltage drop artifact appearing on other AI lines?
 
Thanks,
 
Nathan
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Hello, Nathan!

Wouldn't it be easier to simply record the counter value as you ouput it and have it side by side with the voltage you measure from the tissue? This way you won't need to wire the counter output signal back to an AI channel.

Otherwise please take a look at the following document for proper wiring considerations.
http://zone.ni.com/devzone/cda/tut/p/id/3344

Cheers, 



Message Edited by Kalin T on 01-24-2008 03:00 PM
Kalin T.
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Hi Kalin,

Thanks for replying. Could you tell me how to record the counter output value without actually connecting it to an AI channel?  I know I can calculate what the counter state _should_ be from the variables I give the Counter Output Task, but I was hoping to be a bit more accurate with the timing, just in case the counter output pulse appears e.g. 101ms after triggering rather than the 100ms I have specified.

I have read though the article on analog signal wiring, but I am still no wiser as to why recording the CO with one AI affects the voltage on other AI channels, even with both inputs in differential configuration, which I'm guessing should avoid problems with DGND and AGND being different?

Cheers,

Nathan

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Hi Nathan,

Just to clarify when you changed the mode to "differential" did you also change the physical wiring as shown in the wiring diagram in MAX? I assume you did, but that's something I need to verify.

The following Knowledge Base: How Do I Eliminate Ghosting From My Measurements? will help you in eliminating the ghosting that you are seeing.

Based on your description, it seems like there is a delay between the time you output a signal with your Counter Output task and reading the response from the tissue. This is the delay of your unit under test. The counter output signal most probably will be read faster on AI 0 than the tissue's reponse on AI 1. When you use a multifunctional DAQ, there is some interchannel delay when you scan multiple channels as there is only 1 ADC for all AI channels. If you want to have a perfect simultaneous sampling, you will need a S series board, which features 1 ADC per AI channel. That all depends on the goals of your experiements.

Hope this helps! 

Kalin T.
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Hi Kalin,

Thanks for that - the article on ghosting is a great help.

Nathan

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