Measure small current with high and changing common mode

[Wipffinder]

Hi folks,

For some time I am thinking how to design a measurement circuit and after learning that there a still great Forums in existence I wanted to ask your wisdom about it.

Measurement to do:
Measure small current signal (10µA – 100µA, precision of 5% is sufficient) at frequencies in the range of 10kHz – 400kHz with a common mode voltage that changes (20V/µs – 40V amplitude).
The common mode voltage is a trapezoid and measurement are taken when the common mode voltage is constant (but immediately after a flank).

How to output measurement:
The current should be converted to a voltage free of common mode voltage for reading with an ADC.

Restriction:
Not more than 100Ohm in series if a shunt is used (1mV – 10mV voltage drop)

What I fought about or tried:
1.   Using difference amplifier (INA157, LT6375 etc.) and a shunt. The problem is that this amplifiers are quite noisy (LT6375) or their common mode rejection at higher frequencies is not good enough (INA157)
2.   Making a differential amplifier with a matched resistor network and a low noise high CMRR Opamp. -> CMRR is a problem
3.   Hall sensors are slow or lack resolution, instrumentation amplifier may be restricted in common mode range

It is for a science project I want to do.
What general measurement method is best suited? Do you have any suggestions? For now I only tried the shunt – difference amplifier combination and it somehow works but noise is too big for my 5% requirement.

Have a nice day
nking how to design a measurment

[NiHaoMike]

Use an active I/V converter (on the order of 1-10V full scale voltage) with its ground connected to the source side, you'll need an isolated power supply to run it, such as a DC/DC converter or even batteries. It would also be a good idea to shield the entire I/V converter assembly with the shield connected to the source side. Then use a differential amplifier to isolate the output.

[Marco]

The common mode voltage is a trapezoid and measurement are taken when the common mode voltage is constant (but immediately after a flank).

How fast is the edge? If it's relatively slow you could just AC couple both sides of a shunt down.