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HX711-based milliohm meter
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dannyf:
That makes sense.

I haven't seen any spec for the ADC input resistance myself but I haven't found loading to be an issue here.

So an analog switch that multiplexs the input pins would work- 17 bits have a range of 128k so if you can get the switch resistance to 8 ohm or lower, 1meg ohm ADC resistance would work. Alternatively you can buffer.
JohnnyBerg:

--- Quote from: Kalvin on February 28, 2015, 01:10:26 pm ---I bought two modules for fun, inspired by your experimenting.

--- End quote ---

Me too  ;D


--- Quote ---
For example, the LTC2440 costs around $5 in 1K volumes, which is not much. It needs also a good few dollar reference to be accurate.

--- End quote ---

The refernce will be much more .. for a real 24 bit ADC  :o

Remember 24 bit = 16,777,216 count
Kalvin:

--- Quote from: JohnnyBerg on February 28, 2015, 01:40:35 pm ---
--- Quote from: Kalvin on February 28, 2015, 01:10:26 pm ---I bought two modules for fun, inspired by your experimenting.

--- End quote ---

Me too  ;D


--- Quote ---
For example, the LTC2440 costs around $5 in 1K volumes, which is not much. It needs also a good few dollar reference to be accurate.

--- End quote ---

The refernce will be much more .. for a real 24 bit ADC  :o

Remember 24 bit = 16,777,216 count

--- End quote ---

The LTC2440 datasheet doesn't directly state its accuracy, but the nonlinearities and full scale errors shown on pages 5 and 6 are in range of 1 - 2.5 ppm and the figure "INL vs Output Rate" on page 7 shows linearity to be 17 to 18 bits*. The offset error looks negligible and the RMS noise figures are close to 2uV. Although the device is 24-bit ADC, the actual accuracy may be in range of 18 - 20 bits. Averaging will reduce the noise**, but it would not reduce nonlinearity error.

Creating a circuit which will exhibit 120dB dynamic range and 1:1000000 accuracy requires very careful design, high quality components, and knowledge (that I do not have). Creating a design with the dynamic range of 90 - 100dB may be within my capabilities.

* Edit: The figure is for the conversion rates of 2000 conversions/seconds and faster giving reduced 17 - 18 bits linearity. The figure "integral Nonlinearity vs Conversion Rate"  on page 6 shows that using the slower conversion rates the nonlinearity stays quite close to 2 ppm giving the "0.0005% INL, No Missing Codes" stated in the datasheet.

** Edit: On page 14 and 25 there are tables showing the effect of conversion rate vs. the effective number of bits (ENOB) available, from 17 bits up to 24 bits.
Kalvin:
I decided to take a look at this dannyf's project. When I started working with the board I found following:

- The AGND was not connected to the GND. Just added a jumper between AGND and GND make proper connection.
- I changed the 8k2 ohm resistor in the VFB node to 10khm so that the AVDD is less than 4V in order to have some headroom to my Arduino board's 4.5V VDD.
- I found out that the solderless breadboards are uselelss with this project, so the measurement signal wirings and connections are better to be soldered properly.
- I changed the output rate from "0" to "1" giving approx 8 measurements per second.

After that, I was able to achieve following using a reference resistor of 1.000 ohm and simple null-calibration:

- Using 4-wire Kelvin-type measurement
- Measurement time 8 hours at room temperature epprox. 21C
- Initial drift was 3 milliohms
- After initial drift, the variance was +/- 0.5 milliohms
- Measurement current is in range 5mA - 10mA.

The accuracy and resolution is quite close to a 12-bit ADC. I used the HX711's own internal reference and the power supply was derived directly from Arduino's voltage regulator without any specific filtering. I didn't test temperature sensitivity, but warming the chip with a finger did produce few milliohms drift (not too bad).

Br,
/Kalvin

Edit: Here is the plot data collected today for a period of 9 hours. The resistance is quite nicely within +/- 0.25 milliohms after initial transition. Taken the initial transition into account, the results are within +/- 0.5 milliohms.

dannyf:
I have done more work recently on this that you may find interesting: https://dannyelectronics.wordpress.com/2015/10/25/a-hx711-based-milliohm-meter/

Running the circuit at 2ma and utilizing a 10R reference resistor, the resistance readings are stable to the 4th digit (after the chip has reached thermal stability).

I am starting to appreciate this little bugger now.
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