el cheapo micro-ammeter

[Kalvin]

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?

[dannyf]

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.

[Kalvin]

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

[dannyf]

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

[dannyf]

I'm happy to report that for the 10ua/v setting (100K feedback resistor), a NE5532 works as well.

I have not tried the 1ua/v or 0.1ua/v settings but given the significantly higher Ibias, I would suspect that offset would overwhelm a bjt opamp like NE5532 at those settings. So you may have to put in an Ibias generator there. But I will try later.

[dannyf]

I would expect errors caused by bias current drift to be a real issue, particularly for a jfet based design. So to get the meter to work over a wide temperature range requires a lot of work that's not here yet.

However, if you are just trying to perform a quick measurement, even if repeated over a wide temperature range, it's still ok.

[JohnnyBerg]

btw ever since i read those volt nut threads i become tempco concious ... at those nano currents ... if you put finger to some components wont the drift be very noticeable? even if blow on something ... wont the 80+ nano amps fly around?

I have a whole bunch of voltage standards on my bench. From super accurate to the most el cheapo you can think (TL431 and LM385)
The cheapo ones even can "detect" if the door of my lab opens. Or waving my hand at 2 meter distance.
Not very good as a voltage standard, but the ultimate motion detector. 

[Kalvin]

btw ever since i read those volt nut threads i become tempco concious ... at those nano currents ... if you put finger to some components wont the drift be very noticeable? even if blow on something ... wont the 80+ nano amps fly around?

I have a whole bunch of voltage standards on my bench. From super accurate to the most el cheapo you can think (TL431 and LM385)
The cheapo ones even can "detect" if the door of my lab opens. Or waving my hand at 2 meter distance.
Not very good as a voltage standard, but the ultimate motion detector. 

Have you observed how many accurate digits these el cheapo style reference will provide in room temperature range after they have been calibrated? And, do you have any guesstimate what will happen if the el cheapo reference is placed inside a styrofoam or any other good insulator ie. how well the insulated reference would then perform? Or does the insulation increase the accuracy at all?

[JohnnyBerg]

Have you observed how many accurate digits these el cheapo style reference will provide in room temperature range after they have been calibrated?

Actually, I have. 
They are "usable" within 2 digits. A LM385 at 1.23V and a TL431 at 2.49V

And, do you have any guesstimate what will happen if the el cheapo reference is placed inside a styrofoam or any other good insulator ie. how well the insulated reference would then perform?

Actually, I did some test 
When well insulated, thermal effects like the draught from opening a door are gone. But that was to be expected.

Or does the insulation increase the accuracy at all?

No, it does not. Those el cheapo "references" suffer from many other effects, like thermal hysteresis, aging instability etc.

Actually they perform very reasonable in line and load regulation, but I suppose that is what they are made for 

[dannyf]

I have not tried the 1ua/v or 0.1ua/v settings but given the significantly higher Ibias, I would suspect that offset would overwhelm a bjt opamp like NE5532 at those settings.

At 1ua/v (1Mohm feedback resistor), offset is 0.3v; At 0.1ua/v (10Mohm resistor), offset is 3.5v -> so you need 12v or so to get it to have a reasonable range.

[Kalvin]

I have not tried the 1ua/v or 0.1ua/v settings but given the significantly higher Ibias, I would suspect that offset would overwhelm a bjt opamp like NE5532 at those settings.

At 1ua/v (1Mohm feedback resistor), offset is 0.3v; At 0.1ua/v (10Mohm resistor), offset is 3.5v -> so you need 12v or so to get it to have a reasonable range.

Just trim it to zero (nb. option B doesn't affect the gain):
http://wiki.analog.com/university/courses/electronics/text/chapter-3#offset_adjustment_external_methods

[dannyf]

Or to use relative measurement on your DMM.

[Kalvin]

Or to use relative measurement on your DMM.

Or use an analog voltmeter / panel meter which typically have a knob for zero adjustment.

[Vgkid]

^^^ wont remove the input offset.nI was thinking that you could add another potentiometer coupled to a fraction of the
+/- rails to null out the offset.
MT-037, analog.com

[Smith]

I have made quite some low current meters. Some basic tips:

1. Get an instrumentation amp (low bias one)
2. Make a basic  I/V converter with good resistors
3. Give it a low noise output buffer with offset compensation
4. Make a (low noise) dual supply
5. Use switches/relays and multiple resostors to create multuple feedback ranges.

When going to nA range and lower, cleanup well after soldering.

Keithley has a free low level measurements handbook in PDF. Its awesome!

[dannyf]

Some highly unscientific tests on tempco:

TL082, TL074, NE5532, TDA1308: readings (relative) barely budged when a heater (=my thumb @ 30c) is applied to the IC, on all resetings (10ua/v to 0.1ua/v).

LM358: readings barely budged for anything other than 0.1ua/v.

So maybe Ibias' impact isn't as significant as I thought, at least for this type of measurements (relative) and range (=nanoamp).

[dannyf]

Got some time and made a little progress.

the input terminals are mounted, as is the power switch, into a repurposed chocolate case.

Next is to solder in the TL074 and resistors. I have some mil-spec teflon wires to go from the input terminal to the pins - total overkill for this level of current.