| Electronics > Projects, Designs, and Technical Stuff |
| HX711-based milliohm meter |
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| dannyf:
As this is a weight scale adc, I suspect that you can buy a weight scale, with its lcd display and everything, and hack it to be a milliohm or millivolt meter - same principle. That would be interesting to attempt. |
| Kalvin:
Averaging ie. oversampling is a method to reduce the random noise in measurements in order to gain back those lost bits. In principle, if you take four measurements and compute the average, you will gain one extra bit. If you take 16 measurements and compute the average, you will gain two extra bits etc. Atmel has a nice application note "AVR121: Enhancing ADC resolution by oversampling" which discusses this in more detail http://www.atmel.com/images/doc8003.pdf Br, Kalvin |
| dannyf:
Here is the data sample, straight out of the chip (the only processing is 2's complement, not applicable here). The blue lines are for 64x gain, and the pink lines are for 32x gain. The ratio is 43x (43.3x) over about 500 samples. Deviation is fairly small, looks to be 50 lsbs short term. Reasonably respectable. 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. |
| Kalvin:
This is just guessing: Could the bridge and its components cause this drifting. I mean that resistors temperature and resistance might change as you pass current through the components. One other thing that came to my mind is possibility of thermocouple ie. Seebeck-effect due to different metals affecting each other causing voltage difference due to temperature change. Of course, the ADC itself might cause the drifting, too. |
| dannyf:
The current is less than 1ma (0.2ma to be exact). It would be hard that self-heating is the issue here. I think the adc is the problem. |
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