Measure Low Current With Large Jumps

[adamey]

I want to be able to measure and log low currents (from 10-200ma) with accuracy to within 5ma.

However the problem is the circuit can occasionally have current spikes anywhere from 1-30A.

I thought of using a microcontroller to measure the voltage across a shunt (amplified to give me 0-5V), but if I set it so that 5V equals 30A (my max current) then I will get poor resolution at the current levels I want to log (below 200ma).

I guess what I'd like is a circuit that outputs 0-4V for 0- 200ma and 4-5V for 200ma-30A. This would give me precision for low currents while still being able to measure larger current. Or am I approaching this the wrong way?

 

[NorthGuy]

You can go with two separate amplifiers/ADCs. One for 0-200mA ragnge, and one for 0-30A range. Then if 0-30A ADC gives you results below 200mA, you use the number from 0-200mA ADC.

 

[adamey]

^ That makes sense. I guess all I would need is some sort of limiter on the low current amplifier so it never goes above 5V when the current is over 200ma.

I had it stuck in my head to use a single ADC. Sometimes the answers are so simple they elude you.

 

[KeepItSimpleStupid]

Also take a look at log amplifiers.

 

[Nigel Goodwin]

Also take a look at log amplifiers.

That would make life FAR more complicated

NorthGuy's solution is easy and simple, a fine answer to the question - and thinking a little 'outside the box'.

 

[dr pepper]

Design your circuit to operate at 200ma, then protect the i/p's of the op amp amps with diodes reverse biased to the supply, and use an op amp thats ok with large voltage differences across its i/p's.
Doesnt matter then if you over range the amp.

 

[simonbramble]

I would go with Dr Pepper's solution, but use an LTC6102. This is a chopper stabilized high side current sense amplifier with very low input offset voltage. it is the input offset voltage that kills the accuracy at low currents (where the drop across the sense resistor is small).

Design for the 200mA setting, but protect this with back to back diodes across the sense resistor

https://www.linear.com/product/LTC6102