Current measurement circuit

@TPS
Hi man...really miss the forum...but can't help it..am all tied up.

@MPU_Beginner
With the 1k shunt and 1uA current you have a drop of 1mV.
So your differencing amp,the INA,will have to select 1mV from ~5000mv(the uP Vcc). Now if you need a 0.1% accuracy the INA will have to provide a CMRR of ~133dB,which is beyond it's range.So for practical purposes the ckt would not work very well.

btw what is R2 for?

have you looked at log amps from TI?

Hi Sashijoseph,
thx a lot for ur detailed answer. I know that the CMRR is 20 log (diff gain/CM gain), so how did you calculate the 133dB?

The log amplifier needs a current as an input. So how shall I connect it to my shunt? I think it does'nt fit to my case, as well as the transimpedance amlifier, because I'm sensing the current in the supply line of the uP!

Yes I think R2 is not needed.

think of it this way...
the INA sees a small differential voltage 1mV riding on a big common mode voltage 5000mV.To extract that 1mV,it has to reject 5000mV.For it to do so with an accuracy of 0.1%(Precision measurement techniques call for 0.05% accuracy),would require rejecting 5000mV for 0.1% x 1mV ie 0.001mV.
in dB ==> 20 log (5000/0.001) = 133dB.

Moreover in your case the full scale shunt voltage handled by the INA is 10mV.So you need to scale your shunt drop of 1mV to atleast 10mV.Try simulating with a 10k resistor.

Thx Sashijoseph for your patience and for your etailed answer.

I'm looking for other sense amplifiers. TI has the INA110 it seems to be much better than the INA213 especially for small currents...

Hi SashiJoseph its good to see you back again

Hi MPU....

see this book from page 13 to onwards.... it will help you...

http://www.8051projects.net/forum-t47374-last.html