Wouter van Ooijen <wouter@VOTI.NL> wrote:
> But I want to avoid needing a power source at the 4-20mA side.

So what's supplying the current to the sensor, then?

And if you already have a power source on the 4-20mA side, you can use some
of this to drive your isolated circuitry, including a low power ADC or I/F
converter.

Assuming you *do* need to supply the power to the sensor, here's a weird
idea I had. I don't know whether it would be accurate or linear enough, but
it might be worth breadboarding.

Take a low-power audio transformer as the isolation device. On the sensor
(secondary) side, connect a bridge rectifier and filter cap to supply the
DC current to the sensor. Drive the primary side with a square-wave of a
few kHz, using one of those small integrated H-bridge drivers that has a
load current sense output (e.g., half of an Allegro A3966SA with external
sense resistor). With a little signal conditioning, this output should be
proportional to the current in the isolated loop.

-- Dave Tweed

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