sgrobler

OpenLogger ADC resolution + exporting

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I'm looking at the .csv file captured using OpenLogger and Waveformslive and the output looks something like this:

image.png.70c6d8ce0ca2dc88ed1ac19dc6298d94.png 

The analogue input range is +/-10V or 20,000mV so with a 16bit ADC we should have a resolution of about 0.3mV or 0.0003V.

Why is the export limited to 1mV resolution? Can the export be updated to provide more decimal places so that the 0.3mV resolution is available?

Thanks

Edited by sgrobler
typos

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Hi @sgrobler,

I have asked another engineer more familiar with WaveFormsLive to see if there is some limitation on that end regarding this. You are correct though that the resolution achievable with the embedded ADC (taking into account the front end gain of 0.1392) is 329 uV per LSB.

Thanks,
JColvin

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Hey,

As you mentioned the analog input range of OpenLogger is +/-10V and the ADC is 16-bits and your math is correct that theoretically that would result in ~0.3mV as the smallest voltage difference we can measure.  In reality every non-ideal system component starts to chip away at that precision.

For OpenLogger our tests show that we can consistently measure to about 3mV (effectively 12 bits of resolution for our ±10V range) and assuming the OpenLogger is appropriately calibrated our accuracy is typically within about 10mV due to noise and distortion.  We can improve these values by oversampling and averaging to get within about 1.5mV which results in an effective number of bits (ENOB) of about 14.

The Digilent Instrumentation protocol was originally designed to support OpenScope MZ and returned values in units of 1mV.  We decided to use the same units in our communication with OpenLogger to leverage some of the existing commands.  What you are seeing is not an error with the export, but a side effect of our decision to return values from the hardware in 1mV increments since anything beyond that resolution is noise.

Let us know if you have any questions about this.

Thanks!

-Kristoff

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Thanks for your reply. Whilst I appreciate that noise will mean you can't really realize 16bit resolution, the ADC is still going to pump out readings to a resolution of 0.3mV and that resolution could have been preserved for the file export. I would have preferred it if I could see the real numbers, not have them artificially limited by the file-export resolution of 1mV. I hoped it might be possible to simply change the export format specifiers to produce an output write resolution of 0.1mV...

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