Author Topic: OLED display accuracy  (Read 220 times)

dalbert

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OLED display accuracy
« on: April 25, 2020, 10:12:34 AM »
First, thanks for a great product!

I've been testing the CR using 1% resistors and observing measured currents 3 ways:
  • CR OLED display
  • CR output voltage (using multimeter)
  • Advantest R6552 bench multimeter (my control)

Using a 3v33 source and a 470K (469.63K) resistor, which should yield 7.09uA I observe the following:
  • CR OLED: 6.0-6.4uA
  • CR Output: 7.09-7.10mV (perfect!)
  • R6552: 7.08uA

So the voltage output of the CR is excellent and matches my (much more expensive) R6552; it even has very slightly less burden voltage than the more expensive meter!

The OLED display (readings through uC ADC) seems to have a fixed offset of roughly 1uA.  This is not a big deal for my application, but it held true across a range of resistor (fixed load) values (i.e. it seems to be a fixed offset rather than a percentage error).  Is this expected?

Felix

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Re: OLED display accuracy
« Reply #1 on: April 26, 2020, 06:36:32 PM »
The ADC is non linear and especially in at the range extremes you should not expect to match accuracy of the raw output. And each individual MCU is different in ADC response, hence I have to manually try to calibrate each to the best of my ability. Still, very likely that 2 CRs will show slightly different OLED readings.

Take the OLED as a cherry on top of the cake, its very useful in most cases and its very convenient, even though if you have a CR you most likely have a basic DMM to read the raw output which is far faster and more precise.

If you have the latest firmware, then the ADC range reference is split in half as described in the latest mk4 release, this is increasing OLED readings resolution as well as overall accuracy.
Recent batches do come with this firmware.

dalbert

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Re: OLED display accuracy
« Reply #2 on: April 30, 2020, 02:04:25 PM »
Thanks Felix, the accuracy is indeed quite good with the voltage output and as I'd mentioned, the OLED is good enough for my needs and saves having more wires and/or a large bench meter on my workspace; I just wasn't sure what the expectations  were.  Thanks again for a great replacement for the uCurrent!