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.
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?
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.
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 
Sounds fun. Would love to see how you make it work. Keep us posted, please.
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:
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
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?