Electronics > Projects, Designs, and Technical Stuff
HX711-based milliohm meter
Kalvin:
I didn't want to hijack the original el cheapo micro-ammeter thread so I will post my comments here instead as they are related to the HX711.
--- Quote from: Kalvin on February 27, 2015, 10:09:50 am ---dannyf, what do you think whether the HX711, the inexpensive 24-bit ADC you were evaluating few months ago would be usable with this micro-ammeter project?
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--- Quote from: dannyf on February 27, 2015, 11:34:58 am ---I abandoned that project. The drifting, particularly the opposite tempco for the A/B channels, is a killer for my intended application (milliohm meter). I think it would have worked if the two channels had similar or at least the same sign of their tempcos. Without temperature compensation, you are better to just treat it as a 12bit adc.
The only solution I could think of was an oven and that wasn't doable in a hand-held device.
I was told its sister, hx712, has solved the tempco problem but I am not about to try it. I think a good 20 - 24bit adc (external ref + 1 channel, or two differential channels) would have worked better for the milliohm meter application.
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--- Quote from: Kalvin on February 27, 2015, 08:28:02 pm ---Thanks, dannyf. I have two of those boards here and I thought to give a try. Plan is to use analog switches to route the A and B inputs to the known voltage reference and known zero voltage. The measured values would be then used for calibration and removing the offset. Here's been quite good discussion about different voltage references, so maybe I can find something usable. And need to find suitable analog switches. Of course, all this in the spirit of cheapo :)
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--- Quote from: dannyf on February 27, 2015, 10:41:28 pm ---Sounds fun. Would love to see how you make it work. Keep us posted, please.
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The initial plan it to device an analog multiplexer controlled by the microcontroller so that the HX711's A and B inputs could be interchanged, optionally driven by a known reference voltage and known zero voltage. I could not find any information about the input impedance of the A and B inputs, so I must assume it to be less than 100kohms, so the resistance of the analog switches must be taken into account. Of course adding a low-offset quad op amp with high input impedance as a buffer for A and B inputs would be an option, which would make the resistance of the analog switches insignificant.
Here I will now assume that selecting an analog switch device having resistance close to 1 ohms would eliminate the need for any buffering op amps. However, I am not quite sure how cheaply one can find these low resistance analog switches.
The analog switches comes in variety configurations: from single switches to multiplexers. The multiplexers come typically in single and dual channel configurations. A dual-channel multiplexer has typically a good match between the switch resistance within a device.
Selecting two dual 4-to-1 analog MUXes would allow quite flexible calibration operations:
--- Code: ---MUX# A+ A- B+ B-
0 A B Normal measurement
1 B A Channels swapped
2 0 0 Channels connected to zero reference
3 Ref Ref Channels connected to known reference voltage
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Do we really need the reference voltage for the calibration, or would it suffice only to swap A and B channels for the measurement signal from time to time for calibration. If the channels should have different tempcos, one should be able to calculate the error between the channels as we switch the MUXes back and forth. Of course, using a good reference voltage for the calibration, we would get more accurate absolute measurement values.
In the ratiometric measurement application like this milliohm meter, the absolute accuracy is not a concern, but the relative accuracy between the channels A and B is important.
What do you think? Any suggestions?
JohnnyBerg:
Is this the correct datasheet?
Unless I'm missing something, there is no spec for accuracy and linearity?
Thanks @dannyf for the measurements. As I see it, this is a pretty useless device for doing some serious ADC?
Kalvin:
--- Quote from: JohnnyBerg on February 28, 2015, 11:53:41 am ---Is this the correct datasheet?
Unless I'm missing something, there is no spec for accuracy and linearity?
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Here is a chinese datasheet with a bit more information: http://www.hobos.com.cn/upload/datasheet/HX711.pdf
My chinese is not that good that I could decipher what the datasheet really says, but luckily there are some numeric information available which is comprehensible.
dannyf:
Kalvin: what's the application and what's your approach? Sounds like it is to use a multiplexer to calibrate the adc and then ...?
JohnnyBerg: yeah. The one Kalvin linked to is better however. I basically couldn't figure out a way to deal with the opposite tempcos.
Kalvin:
--- Quote from: dannyf on February 28, 2015, 12:09:50 pm ---Kalvin: what's the application and what's your approach? Sounds like it is to use a multiplexer to calibrate the adc and then ...?
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I have no particular application in my mind. I bought two modules for fun, inspired by your experimenting. Only reason to pursuit this a bit further is the mental and practical challenge whether one can push the part into its limits (say, 17-bits of accuracy) as stated in the datasheet, compensate the tempco effects using cheap compensation method, and whether one can get 17-bit accuracy using dollar range parts (in room temperature ie. in limited temperature range). Of course the easiest and most sensible way would be to buy a good 24-bit ADC and high precision voltage reference. 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.
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