Re: *Tricky* current measurement.


"Fred Bartoli"
<fred._canxxxel_this_bartoli@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
message news:45e46c61$0$7501$426a74cc@xxxxxxxxxxxxxxx
I've some requirements for some challenging voltage measurement (I know,
subject said current, but see later). Namely:
- up to+/-4kV pk WRT ground
- % order accuracy up to 50kHz
- duty cycle max 10%

'til now, no pb.

Then the fun part:
- 80dB CMRR up to 100kHz
- 60dB CMRR up to 10MHz (yes!)
- the signal can go down to 4V and yet 1% accuracy is required (which is
an overall 16b accuracy)

No pb with power drawn by the probe but they don't want maintenance,
periodic cal and so on...

We are politely (well not so politely) asked to do this à la 2x
compensated dividers then diff amplifiers. Yeah...


OK. Now I have to backup that crap with a working solution, or as near as
possible to the specs.

My first thought was a load resistor (told you it was current
measurement:-) and two current shielded xfrmrs (two to reject the CM
currents from load to shield) for the BW, and a servoed DC-LF compensating
current, all that almost working like the Tek probe.

Unfortunately, looking closer, the LF-DC part isn't going to be that easy
(mainly Hall sensor noise, offset and low sensibility...) :
- the load can't go much below 100K (10W on average) giving a 40mA-ish pk
current (plus the needed HF zero).
- Hall sensors seems to be somewhat noisy
- can't use multi turn primary, due to the then uncontrollable parasitics
(still CMRR)

Thought about fluxgate but the switching frequency will be well into the
signal BW, so it's probably not OK. Plus I can't think of a simple way to
implement a gate suitable to the design (100, maybe 200 units made).


Any clever idea welcomed.


--
Thanks,
Fred.

Hello Fred,

This will only help a bit because these people make high current sensors:

www.danfysik.com Ultrastab 867-200I transducer.

200A full scale but errors and noise in the 10 ppm range .

If you wind more turns round it the sensitivity goes up but new errors may
come in. If it worked with 100 turns it would be 2 A full scale so errors in
the 20uA region which is 1% of 2mA which would be fine if you could afford
2A full scale current (only 8kW !! :-)
With your 40mA at 4kV max you are looking at an equivalent voltage error of
2V which is about 50 times outside your budget.

However you may get some ideas from what they can achieve and the way they
do it - you just need a 4A full scale device and you're there.

Good luck.

Michael Kellett

www.mkesc.co.uk


.

Relevant Pages

  • Re: *Tricky* current measurement.
    ... % order accuracy up to 50kHz ... easy (mainly Hall sensor noise, ... need less than 10fF parasitics imbalance to achieve the 60dB CMRR. ... Using FB to close a current compensating loop around the core takes care ...
    (sci.electronics.design)
  • Re: *Tricky* current measurement.
    ... % order accuracy up to 50kHz ... No pb with power drawn by the probe but they don't want maintenance, ... easy (mainly Hall sensor noise, ... Why not use an amplifier and ADC, all floating at the high voltage, and then ...
    (sci.electronics.design)
  • *Tricky* current measurement.
    ... % order accuracy up to 50kHz ... My first thought was a load resistor and two current shielded xfrmrs for the BW, and a servoed DC-LF compensating current, all that almost working like the Tek probe. ... Unfortunately, looking closer, the LF-DC part isn't going to be that easy (mainly Hall sensor noise, offset and low sensibility...): ...
    (sci.electronics.design)