uCurrent vs zero-resistance ammeter

[Aleksolder]

In all the discussion about the uCurrent, I find no mention of zero-resistance ammeters. What is it about zero-resistance ammeters that seemingly precludes them from this application?

I've been using my uCurrent almost daily since it arrived a few weeks ago, and this thought only just ocurred to me, despite having installed thousands of ZRAs (evidently without much comprehension).

[ConKbot]

I havent heard the term ZRA before, however there is a current measuring device with zero effective resistance on the inputs.  A transimpedance amplifier http://en.wikipedia.org/wiki/Transimpedance_amplifier appears to be the circuit level equivalent of the ZRA devices you mention.  A voltage inverse of the burden voltage is applied to the resistor, keeping both input terminals at the same voltage, eliminating any burden voltage. 

Now nothing is perfect, and the GBP of the op-amp and the value of the gain resistor will end up appearing inductive as your frequency increases. ( http://www.edn.com/electronics-blogs/the-signal/4398048/TIA-Input-Z--Infinite--or-Zero--What-is-it--really- ) 

The main reason I see against TIA's in something like the uCurrent, is that forcing current across the feedback resistor will be current hungry, drawing just as much current as your circuit.  So if youre measuring 10 mA, then your op-amp is burning 10mA x whatever your supply rail is in power.  Not ideal for something coin cell powered.

I havent used one, but a SMU may be able to perform a zero resistance current measurement if its compliance voltage was set at 0V?  I'm not sure, perhaps someone who's used one can chime in.

[robrenz]

I havent used one, but a SMU may be able to perform a zero resistance current measurement if its compliance voltage was set at 0V?  I'm not sure, perhaps someone who's used one can chime in.

The 2450 SMU has <100µV burden voltage on current measurements

[Aleksolder]

To give some scale to the topic: Measured values with a ZRA are typically 5 to 25 uA DC. The ZRAs I used would run for about 90 minutes on 2 AA batteries (that's three months at six-hour awakening intervals), so yes, they are power-pigs.

ZRAs are used in corrosion, and I can't see why they aren't used in micro-current DC electronics. DMMs inject current and voltage for other measurements, why not inject current to measure current?

I don't recall the term TIA (I did read the references provided above), and I don't recall the circuit of the devices I installed.

[Aleksolder]

Here's an example ZRA circuit, simple enough even for me to mostly understand:

http://kaktuscircuits.blogspot.hk/2014/05/zero-resistance-ammeter.html