[MAGEEC] Accuracy of the power measurement shield

Jeremy Bennett jeremy.bennett at embecosm.com
Tue Mar 6 16:48:32 GMT 2018

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On 06/03/18 16:26, Mark Kettner wrote:
> Hello everyone,
> could someone please tell me the accuracy of the measurement
> shield. How can I assume really low power (milli or micro) are
> still valid measurements and not just noise? Or is this purely
> affected by the selected resistor?

Hi Mark,

Good to hear from you. James Pallister and Simon Hollis reckoned the
measurement is accurate to 1%.

I'm out of the office at the moment, so don't have a board to look at.
- From memory it uses a 10 bit ADC. To measure the current, the inserted
resistor aims to create a maximum 50mV drop. The largest resistor on
the board is 5R, which would therefore be for a peak current of 10mA.
There is an option to insert a larger resistor if you wish. Voltage
range is fixed I think at 0-6V.

The circuit was designed to sample at up to 2MHz on each sample point,
although at that frequency I would expect inductive and capacitive
affects to impact on the accuracy. The standard tool is written in
Python and in my experience struggles to sample at much more than
10kHz. However the design is fully open, so there is no problem with
modifying the tool and/or the firmware to sample at these higher
rates. You can find the software on GitHub.

> Unfortunately I couldn't find anything about that in the wiki.

Since the MAGEEC work finished some time ago, I've been less proactive
in updating the wiki. However I'm happy to add some notes about this.
I'll check the facts against the board when I get back.

Best wishes,

> -- Mit freundlichen Gruessen / Best Regards
> Mark Kettner _______________________________________________ mageec
> mailing list mageec at mageec.org 
> http://mageec.org/cgi-bin/mailman/listinfo/mageec

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