Precise current measurement (0.1mA to 150.0mA) at open circuit voltages upto 1.5kV

Hello,

I hope someone clever can help me out with this problem as I've spent a lot of times investigated many methods with no success.

I need to accurately measure current to at least 0.1mA and up-to around 150mA in an open circuit with voltages of up-to 1.5kV. To explain this is for measuring electron beam and so the current to be measured is the current induced by the electrons.

Things I've looked into are:

- Usual shunt resistor with comparator although obviously voltages are just too high

- Allegro ACS712 hall sensor chips but their sensitivity is too low (even using their 5A version) and I think the voltages are also too high.

- Using Optocouplers with ADC but the max current I've found on commercial chips are around 60mA

I'm hoping to measure this signal through the 5V 12bit ADC on my micro-controller and so 0.1mA needs to be converted to at less that 1mV.

I would be so grateful if anyone knows how this is done, I've spent so many hours researching this but can't manage to work out how this is done in industry and I cant find any clever IC's around. Unfortunately my analogue design is really rather too amateur to deal with the problem. Thanks

One method I would try is to let the current flow through an opto isolator with a shunt resistor to allow the LED to shine into its transistor. Get a second one with a similar shunt resistor and use both the transistors in a long tail pair. Now the long tailed pair will only balance when both the transistors are subject to the same amount of light i.e. the same current. Use the error signal to power an amplifier to drive the current through the balancing OPTO. You now have the same current flowing at a more satisfactory earth bound voltage.
Frank

All precise isolated current measurement methods require an isolated power supply for the primary side circuit, except for magnetic current transducers with hall sensors. Open magnetic circuit sensors like the Allegro ACS types aren't sensitive enough, but closed core sensors can be equipped with a multi-turn primary windings and achieve the required sensitivity.

Look at the architecture of the MAX4172. Maybe you could build something like it from an opamp and a very high voltage transistor. I used to do it at 300V with a MPSA92. You might run a low power opamp from a coin cell for years, or try putting a couple of diodes in series with your HV lead to steal a little power.

use shunt type of sensor on high side, then high accurate sigma-delta ADC with serial 1-2-3 wire interface transduced data through optocouplers. Supply can be organized as shunt regulator with measuring system ground (low point of shunt, ADC ground, around circuitry ground) connected to external ground through high ohmic current limiting/protection resistor. So your measuring system ground will be at ~1.5KV above external ground. Optocoupler should witstand at least 2x1.5KV and PCB have to have thin air gape between In/Out to avoide breakdown.