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I'm trying to recover a signal that is encoded and then modulated as 1mA current load pulses on the supply line. What's the best way to do this?

  • 250us long 1mA current increase on each edge of (followed by 250us normal current state)
  • Original signal is 2kHz (cannot be changed)
  • Supply current is 0~30mA (so for 5mA sys current the modulation would be 6mA for 250us)
  • Need to filter out false pulses less than ~200us wide
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  • \$\begingroup\$ A waveform picture would help. \$\endgroup\$ – Andy aka Jul 16 '14 at 21:44
  • \$\begingroup\$ Do you know the clock, or do you have to recover the clock from the data. If the latter, be prepared for some computation. \$\endgroup\$ – Olin Lathrop Jul 16 '14 at 22:25
  • \$\begingroup\$ Can you use a current transformer of some ilk? \$\endgroup\$ – EM Fields Jul 16 '14 at 23:15
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I would look at what is known as a current loop receiver. There are standards that operate from 4 mA to 20 mA, one example is the BB (now TI) RCV420. It's likely you could adapt a similar IC since that one might be EOL (End of Life)

Because it is it a current loop signal having larger voltages is OK so having larger sense resistors makes your life way easier, rather than a \$ 1 \Omega\$ resistor.

enter image description here

Here is a snip from that datasheet to give you some ideas if you want to implement it in op-amps.

Looking at that datasheet it says that the maximum current is 30 mA, which I suspect means that you design criteria probably match that standard.

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Use a current shunt resistor (1Ω) and an opamp as a differential amplifier or application specific IC to measure the voltage drop across the resistor. If you can't find any good tolerance (0.1%) 1Ω resistors, connect ten 10Ω resistors in parallel. If you don't want to use an opamp, connect two probes across the resistor and use thw math A-B function of an oscilloscope.

Have a look at this application example from LT. current sense example

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  • \$\begingroup\$ I have a 10 ohm resistor there now so it's generating 10mV pulses across the resistor but I'm not sure how to connect an op amp in order to be able to detect them without a consistent reference voltage. \$\endgroup\$ – spizzak Jul 17 '14 at 14:12
  • \$\begingroup\$ @spizzak: I updated the answer with a wikipedia reference for the differential amplifier and a link to a application specific IC. \$\endgroup\$ – Evan Jul 17 '14 at 14:51
  • \$\begingroup\$ The problem is that I have a semi-constant system current on top of which the 1mA is modulated. As far as I understand these current detectors wont be able to differentiate between 1mA constant current with 1mA modulation or 2mA current \$\endgroup\$ – spizzak Jul 17 '14 at 15:11
  • \$\begingroup\$ @spizzak: You are correct. These detectors are there just to detect the current. To demodulate it you'll have to perform whatever signal processing is necessary for your system. \$\endgroup\$ – Evan Jul 17 '14 at 15:26
  • \$\begingroup\$ It's not just the demodulation but how do I isolate the AC current pulses from the DC current draw? \$\endgroup\$ – spizzak Jul 17 '14 at 19:36
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Well I'd start by looking at the differential voltage signal across a series resistor. Are you allowed to put (say) 10 ohms in series? An instrument amp maybe. If you had access to the original clock signal, you could do a lockin type thing.

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