Measuring small current signal on top of DC offset

[Picketfence]

I like to measure a large current (200 A) with small fluctuations (10 mA 10 Hz - 400Hz) in a cable.
For this I have a Fluxgate sensor (LEM ultrastab200) with a conversion ratio of 1/1000 and roughly 1 ppm noise.
Therefore I receive a signal of 200 mA and fluctuations in the 10 µA regime with noise in the 10 nA range.
I am interested in the 10 µA fluctuations. I need to remove/filter the DC offset and amplify the 10 µA signal to at least some hundreds of mV .
All of this I need to do while adding as little noise as possible.
Has Anyone already done something similar or have hints how to accomplish this?

(The Background of this is that I then want to feed the signal into a Feedback controller and with a to stabilize the current in the cable with a transistor.)

Thank you for your help in advance

 

[erikl]

With a secondary current load resistor of (max.) 30Ω you receive about 0.3mV of fluctuations ac voltage. Separate it from the (max.) 6V DC voltage via a capacitor and amplify it.

 

[jiripolivka]

I like to measure a large current (200 A) with small fluctuations (10 mA 10 Hz - 400Hz) in a cable.
For this I have a Fluxgate sensor (LEM ultrastab200) with a conversion ratio of 1/1000 and roughly 1 ppm noise.
Therefore I receive a signal of 200 mA and fluctuations in the 10 µA regime with noise in the 10 nA range.
I am interested in the 10 µA fluctuations. I need to remove/filter the DC offset and amplify the 10 µA signal to at least some hundreds of mV .
All of this I need to do while adding as little noise as possible.
Has Anyone already done something similar or have hints how to accomplish this?

(The Background of this is that I then want to feed the signal into a Feedback controller and with a to stabilize the current in the cable with a transistor.)

Thank you for your help in advance

IF i understand well, you have a large DC current (and) voltage, plus low-frequency fluctuations on the line.
I would suggest to use a current transformer (like used in clamp AC ammeters) which will isolate the large DC component and on its secondary (more than one turn) you can get the desired signal to observe.
If you use the fluxgate sensor, it works like such transformer. Then you can use an opamp for the desired signal, with an offset voltage set to compensate the DC component.You can also connect the opamp as a high-pass filter to cover the frequency band of interest.