Calculating CMS & DRL Values

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allendukes
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Joined: Wed Jun 20, 2018 3:22 pm

Calculating CMS & DRL Values

Post by allendukes » Mon Dec 17, 2018 5:15 pm

I'm in the process of writing my own acquisition software and had a question about CMS and DRL. I'm using speedmode 4, a water bucket test, and a signal generator, I get nice smooth curves in my data, so I know I have the buffers and calls correct thanks to the forum post: viewtopic.php?t=452. What I don't understand is how to reference the signals based on CMS/DRL. Those values don't seem to be a channel in the stream of data.
According to: https://www.biosemi.com/faq/cms&drl.htm, "we suppose it is 100uV" but if you always assume it's the same value, why do you connect it? Unless it is 100uv, and has to be connected to have that value maybe?
I'm more of a software developer than an electrical engineer, so I may be missing something. Just looking to see if a forum member could provide more concrete information.

Thanks!

Coen
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Re: Calculating CMS & DRL Values

Post by Coen » Tue Dec 18, 2018 2:46 pm

The data stream consists of measured voltages. A voltage is a difference in potential. In case of the ActiveTwo, the electrode potentials are measured with respect to the potential of a point between the CMS and DRL electrodes, see viewtopic.php?f=1&t=177&p=391 .

You are basically asking: what are the voltages of CMS and DRL ? Such a question can only be answered with a counterquestion: with respect to what ?

Consider a map showing the heights of mountains measured with respect to sea level. Then a similar question would be: what is the height of the sea level. The issue is that there is not a point of absolute zero potential in this universe. There is not a point of zero motion either, but because there is an absolute maximum speed equal to the speed of light, things are different when looking at relative speeds, as Einstein explained.

Best regards, Coen (BioSemi)

allendukes
Posts: 2
Joined: Wed Jun 20, 2018 3:22 pm

Re: Calculating CMS & DRL Values

Post by allendukes » Tue Dec 18, 2018 10:06 pm

I (think) I totally get it now. What's reported is the difference between the electrode in the datastream and the point between CMS and DRL. The point between CMS and DRL is the zero point from which all other points are referenced. Got it!

Thanks Coen!

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