Electronics > Projects, Designs, and Technical Stuff

HX711-based milliohm meter

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dannyf:
Don't get me wrong - the millimeter will still work. The drifting we are talking about are quite low and once you have "degained" the adc readings, it is practically gone.

Kalvin:

--- Quote ---However, the killer is the chip's long-term drift. The blue lines drifted 400 lbs, about 0.15%. Not acceptable for a 24bit adc in my view.

--- End quote ---

This small drift 0.15% just made me wonder if there could be some other reason other than ADC itself behind the drift. Anyway, finding out whether it is the ADC or the resistor bridge causing the drift can be found out by carefully heating/cooling the ADC chip or bridge components a little. Yet, the 0.15% error is quite low anyhow :)

wraper:
Likely internal bandgap reference is not very stable. As I understand, when using internal voltage regulator (+external transistor), that should cancel out measurement drift as bridge supply voltage should drift by the same ratio as reference. You could check how much reference voltage drifts on pin 6 and if bridge supply voltage drifts by the same ratio.

dannyf:
I will experiment more when I get more time.

dannyf:

--- Quote ---I will experiment more when I get more time.
--- End quote ---

As promised, I did.

Set-up: HX711 module controlled by a LM4F120, data out via serial to the PC, reading CH_A and CH_B alternately.

The experiment started at room temperature. At about count 300, I put a hot iron on the chip, separated by four layers of paper towl. The iron is at least 275c and through the paper towl, it is too hot to hold onto the tip with my finger.

Put the tip on the chip  - the rest of the circuit not impacted by the hot iron.

At about count 400, the paper towl had a burned mark on it and started to smoke. I took the tip off the hx711 - at that point, the hx711 was hot to the touch - 85C at least.

Throughout the whole process, the reading is uninterrupted.

The experiment stop'd at count 750.

The adc readings are then charted below.

You can calculate the ppm for both channels.

Two things surprised me:

1) the two channels are negative correlated - I kind of observed that in earlier charts but this is the most obvious. This would make using one of them as a reference to measure the other difficult.

2) the two channels have quite different tempco.

Interesting, in a bad way.

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