Electronics > Projects, Designs, and Technical Stuff
HX711-based milliohm meter
Kalvin:
--- Quote from: dannyf on January 03, 2015, 01:56:40 pm ---I have to say that I cannot see a difference here, vs. the prior chart.
--- End quote ---
Do I understand this correctly that the temperature drift stays the same whether you heat only the ADC or only the bridge components?
dannyf:
I don't think so.
When the chip was heated, Ch B went from 2.1735MM to 2.1715MM.
When the resistors were heated, Ch B went from 2.175MM to 2.157MM, a much wider variation.
Now, I cannot be sure that the temperature changes are the same but the iron was at the same setting and the heating time is roughly the same. So I think the temperature changes are reasonably comparable.
Kalvin:
You are correct. I was just looking at the A channel drift, but I didn't look at the B channel drift carefully enough which is 10x more when heating the "bridge" components compared to heating only the ADC.
Kalvin:
I found this excellent article by late Jim Williams "Minimizing thermocouples
maintains 20-bit DAC precision": http://m.eet.com/media/1142215/82758.pdf
The article is applicable for the 24-bit ADC as well.
dannyf:
Just wanted to circle back on this.
Here is a test on a 1ohm (nominal) resistor, detected as 0.954645ohm by the meter
The readings fluctuates within about 65ppm of the mean, over a short period of time -> 50 readings x 0.5second / reading = 25 seconds.
Filtering can drastically reduce that ppm number, by the way, at the expense of slower responses.
I would conclude by saying that 1) the concept works; and 2) it doesn't work on this particular adc module -> too sensitive to temperature and the wrong way correlation between the A/B channels makes it impossible to hold calibration.
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