5% Crude uCurrent Mockup is shockingly accurate!

[BiOzZ]

ok so im at home and i needed a way to measure the current draw of a current project for work and i wanted to do it in the most basic of ways possible so i just stuck my cheap mastech in to uA mode and measured it on that ... the impedance was way to high for the 3.3v logic and got way too much voltage drop

i do not own a uCurrent as im broke (tho id love one when i get the cash!)

so i scoured the parts bin and i had a left over ICL7650SCPA (datasheet http://www.intersil.com/content/dam/Intersil/documents/fn29/fn2920.pdf ) 2mhz 'super' chopper amplifier ... massive overkill but ok!

i made this simple circuit based entirely off the uCurrnet (thanks dave!)



And with all off the shelf 5% resistors and cheap jumpers on a breadboard with only 1 decoupling cap i got these results!

Tested with a voltagestandard.com calabrated voltage and current standard thats only like 3 monts in cal ... for the 1ma test i used the 1ma 2ppm source on it, for the 50 and 500 uA tests i used the 2ppm resistors (the 10k and 100k) and the calibrated 5v out


^
1mA




^
500uA


^
50uA

 
IM NOT SAYING THIS IS SOMETHING TO MAKE FOR PRECISION I SIMPLY GOT LUCKY

Why does this work? Undeniable proof that there is a god?

no like everything else it can be explained with science!

for one i got an AMAZING opamp in there ... there about $6 a peace and in this setup has an offset at the lowest voltage i can measure ... 0.0mv and yes my meter was calibrated (manually) to have the voltage as dead on (tested with the voltagestandard.com standard but it was calabrated in my unis cal lab)

Secondly all the resistors are from one lot and have about the same resistance deviation

Third i got lucky with my 100k and 1k having a ratio of 99,100 (not exactly but with my rough math) 

Forth i have central air so the temperature remains stable

Fifth its powered off batteries and my house has very low ambient noise

AGAIN DONT TAKE THIS AS AN EXCUSE TO SWAP OUT YOUR 0.5% RESISTORS FOR 5% ... i was LUCKY

and yes thats an end table ... dont ask XP

[babysitter]

Congratulations for this result!

Its way better to have this tool than to not having it, especially when being broke.
Seems that your parts just fit together very well and a lot of errors somewhat canceled out. I hope they keep to be like this after soldering to a more permanent thingie.

The only enemies you might have are changing ambient or aging, it might be further off in some years but you can start worring about this when it happens.

If you use 5% resistors for this then you will probably also use some for your projects, so when measuring power consumption, the DUT might well change more than your measurement rig, without being a problem.

BR

Hendrik