How to measure high impedance current source

Technical Discussion
1

Hi team,

We are using the high impedance current source circuit mentioned below that can be controlled by external controller to vary the output currents. we need to measure the output current from the below mentioned circuit validate. Kindly advise us how we can proceed the measurement with JS110 in our case.
XTR111

And kindly advise us, in user manual its mentioned as Up to ±48V common-mode voltage between USB ground and IN-, IN+, OUT+, and OUT-

2

Hi @mathankumar, and welcome to the forum!

As you noted, your Joulescope’s isolation is rated to ±48V between USB ground and the sensor-side inputs and outputs. It is also only rated for +15V between IN+ and IN-. It looks like your current source might approach +24V, which is beyond the JS110 ±20V absolute maximum. If you only need current and charge (no voltage, power, energy), you can use your Joulescope as an ammeter only. Here is a connection diagram:

js110_connect
js110_connect2000×1125 196 KB

Does this work for what you want to measure? Does this answer your question?

3

Hi Mli,

No, this solution is not working for us. we provided 20mA current output from our circuit, but we are not getting the measurement in JS110, we are getting only in uA

20201005_132341_current measurement
20201005_132341_current measurement1019×379 40.5 KB
. kindly check the attached waveform.

4

Hi @mathankumar,

I confirm that the waveform you sent shows -1.6 µA, not the 20 mA you anticipate.

Just to double-check:

  • IN+ is connected to the output of your supply
  • OUT+ is connected to the load
  • The load is not connected to the output of the supply or IN+

As with all systems, I recommend simplifying the problem to build confidence in your system and your instruments. For example, here is one approach:

  1. Disconnect your load and your Joulescope from the supply.
  2. Select a resistor with approximately the same impedance as your target load.
  3. Connect a simple resistor between your supply out and ground.
  4. Turn on the supply and use a multimeter to measure the voltage across the resistor.
    Using Ohm’s law, you expect I = V / R.
  5. Insert your Joulescope between the supply and the resistor. Check that your Joulescope current measurement matches the multimeter current measurement.
  6. Replace the resistor with your target load.

Let me know what you find!

5

Hi Mli,

The solution as we can use as the Ammeter in the previous reply is working fine.
Thanks for your support.

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