Using external shunt resistors

We were just asked if Joulescope can support an external shunt resistor. I am posting the reply here to share the knowledge!

Joulescopes normally use their internal shunt resistors, which allows the Joulescope to perform its fast autoranging for amazing dynamic range. You can also specify a fixed internal shunt resistor. The 10A and 2A ranges use an internal 10 mΩ shunt resistor.

Using an external shunt is not something we have considered during Joulescope’s design process. However, we see how it could be useful for some applications where you want higher current measurements or have an existing design with a shunt resistor. So, I took a look. It turns out, you can, with some caveats:

  1. The voltage across the shunt resistor is between -3.6 mV and 20 mV (otherwise the reading will saturate)
  2. The shunt resistor value is much less than 1111 Ω

Here’s how:

  1. Connect the + side of your shunt resistor to IN+
  2. Connect the - side of your shunt resistor to OUT+
  3. Start the Joulescope UI, and select “Current Range” 18 µA. This places an internal 1111 Ω shunt resistor in parallel with your external shunt resistor.
  4. Capture data! You will need to manually convert the scale. The UI will display 20 mV across the shunt resistor as approximately 18 µA.

I just tried this out on the bench with some parts I had available: a 1 Ω shunt and a 1 kΩ load with a bench supply. At 1V, I expect 1 mA. Our scale to convert from the UI reading to truth is:

internal shunt resistor / external shunt resistor = 1111 Ω / 1 Ω = 1111

The Joulescope UI displays 0.91 µA. Let’s apply the scale to convert to truth:

0.91 µA * 1111 = 1.011 mA

which is accurate to 1.01%, great considering that I used a 1% resistor and that we bypassed the Joulescope gain calibration.

At 2V, I measure 1.83 µA, as expected. While you may need to compute the gain factor for your application, the offset and linearity should still be great.

Remember to use Kelvin-style 4 wire connections to your shunt resistor, especially for large currents.

1 Like