# Wide range current sensing at +5kVDC

I am working on a control and interface board for high voltage supplies, which requires some current measurement to be implemented. The general terms of the question are similar to this one. The device through which I want to sense the current is at up to +5kV DC but the ground is not guaranteed to be isolated so implementing low-side current sensing is non-trivial. In writing this question up, I have also found this answer which is for a very similar application, but I am expecting the current drawn to range from nano-amps to maximum of about half a milliamp in my case.

I wish to measure output current from the high voltage power module across a few decades, ideally from sub-microamps up to milliamps. The ultimate "display" of the data will be handled by a microcontroller, but in order to access the required dynamic range I've been thinking of using a LOG112 to directly sense the current relative to a reference. To get hold of the current from the high voltage circuit in the first place, I've been told that an optocoupler will work down to a few microamps and I've been recommended the Vishay CNY66, which in a simplified sense I intend to implement as below:

simulate this circuit – Schematic created using CircuitLab

In practice I'm using a LOG2112 since this whole system will have two high voltage channels. As a current reference I'm taking the logarithmic amplifier's onboard 2.5V reference and putting it through a sense resistor.

Does this sort of architecture seem sensible? I'm not entirely sure how I should choose the value of R lim (R2) or what sort of sensitivity I should expect.

Merry Christmas.

EDIT: Changed the diagram to fix an error with the high voltage load's connection.

• You've begun the specification process, with dynamic-range ideas (0.1uA to 1milliAmp, for example). Other requirements: what resolution (# bits), what accuracy (10%, 0.01%), what speed (3 measurements/second), how fast to settle to accuracy measurement after the current has a STEP change, what temperature stability, how often to calibrate, what volume will you allocate (1 cubic foot, 1 cubit inch) for the measurement, what power, how many power voltages, floating power? what interferers are nearby, on and on. Dec 19, 2017 at 17:15
• Thanks for your reply. On the digital side all the questions of resolution are mostly solved in that I will use the microcontroller's onboard ADC to access the measurement. Accuracy of a few percent would be fine. Settling within a second would be fine. Temperature stability of less than 1% per degree C is desirable. Calibration should not be needed. The measurement subsystem's footprint should be less than 45 x 14 mm. Power draw less than a few tens of milliwatts would be fine. Power voltages already available are +15, -15 and +5. Any more questions? Dec 19, 2017 at 17:21
• You don't really want to connect the HV (even through an LED/5M resistor) directly to the LOG, do you? Don't you mean connect the LOG to the opto output? Dec 19, 2017 at 19:44
• @WhatRoughBeast You're right. Sorry, I'll try and edit that now. The "5M resistor" is really just the load for our high voltage system. As you say, the LOG will be connected to the opto output in this scheme. Dec 20, 2017 at 11:35