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Manxome, you interpreted the graphs correctly. Each red dot is the result of capturing 100, 40 or 21 samples and crunching the readings into one single result.
I start at zero load, take a series of readings, save the result after filtering and increase the load by about 100mA, then repeat.
The blue dots are single readings from the 34401A multimeter. it contains its own filter so there is no need to do any processing on those readings. But this is a $700 instrument so one would expect accuracy.
The measurements are done by Excel. I wrote a VBA macro that controls the instruments and the Espruino.
The reason why the red line is above the blue line is that there is a small gain error. I need to adjust the program to correct. There is also a small offset error.
The reason why the voltage is drooping as load is increased is that there is a long cable from the bench power supply to the test object where I am measuring the input voltage. In this measurement, the output voltage is set at 15V, so the output power at 4A load is 60W. The test object is a buck-boost converter that handles input voltages to 60V (80V max). The output voltage of this particular setup is 12V to 60V but it can be much wider. It is also possible to control power flow in the reverse direction. I am using the Espruino in my test setup to generate signals that control the power converter. It also measures input and output current and voltage plus a number of other signals. And that is where I needed to filter out noise, as the power converter can handle 500W or more and this means a lot of noise on measurement signals.
I will see if I can capture some data without filtering so we can see the noise. I did not have time today. The reason why the last curve shows three samples with same value, then the next sample is 16.6mV lower, then comes four more samples that are 2*16.6mV lower is that the 12 bit resolution shows up. when you only have one sample and no averaging the value must always be n * 16.6mV.
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tage
@Manxome
@tage, I am trying to understand your graphs. Referring to just the top graph for a sec, does each red dot (reading) represent 100 samples, winnowed down to 80 as you described, then averaged? And you took one such reading, then increased the load by 100mA, and then took another, and so on?
Why do you think the readings of the top two graphs are almost exclusively above the blue line?
Almost seems like there is some accumulated positive error which drives the readings upwards...
Also, why do you think the readings on the bottom graph are often repeated two, three, even four times in a row for successively higher loads? I mean, you are throwing away 20 points, yet the remaining point just happens to be the same for successive readings? My guess is that you are only getting a few distinct readings (maybe 3-4 or fewer?), ie, the readings are pretty tightly grouped. Oh, and those successive readings...are almost exclusively above the blue line!
Thanks!
Oh, btw, I would be interested in a side-by-side comparison graph where no data was thrown out for each of the 3 sample sizes :)