accurate measurements at uA range using 9227

[johnsold]

Thanos,

The circuit Henrik posted takes advantage of a concept called "virtual ground." Using your circuit for discussion, the actual current through the load (which you want to measure) flows through Rf, through the output of the op amp, through the op amp power supply to ground, then through the battery to the load. The virtual ground effect is due to the feedback  around the op amp. The output voltage of the op amp is such that the voltage difference between the two inputs is approximately zero. Since the non-inverting (+) input is connected to ground, the voltage at the inverting input is very close to zero also. This is true even though essentially no current flows into the op amp inputs. From the point of view of the load it appears and operates exactly as if the load is connected directly to ground, so long as the amplifier remains in linear operation. 

If the value of Rf is 1E6 ohms and the load current is 1 uA, then the voltage across Rf will be 1 V. Because the amplifier is in an inverting configuration the output voltage is -1 V with respect to ground.

If it is possible for the load to pass current larger than a few microamps, even under transient conditions, then additional protection or an alternative circuit would be required. 

Lynn

[thanvg]

Thanks a lot for the assitance,

Lynn, thanks for making Henrik' s advice easier to digest (without having to search the shelf for an old electronics book). It is totally clear now.

I also loved the uCurrent solution, a nice implementation of the TIA concept with added features as range selection etc. Will try and source one of those.

Many thanks,

Best regards,

Thanos