Keithley DMM6500: Verify a µCurrent Manually

The µCurrent is a test instrument that translates current to volt. You can think of it as a shunt resistor with an amplifier.

It's specifically designed to be used for low current measurements.

In this blog, I want to compare its measurements with the DMM6500, in mA range.

Not for a specific reason. Just to have a valid test setup that I can automate in the next post.

The Test Setup

• a 2 V power supply, loaded by a dc load.in the 2 to 90 mA range
• µCurrent and DMM6500 in series to measure the current
• 121GW DMM to measure the output of the µCurrent. 1mA measured results in 1 mV output.
• pen and paper

Test Procedure

The 121GW DMM, set to mV range, measures the out put of the µCurrent.

Because the 1 mV = 1 mA factor, I just have to write down the measured value and give it the mA unit.

The DMM3500 set to DCI, in slow with high precision mode. I don't need the precision for this example, but the slow reading helps me get a stabler screen.

• Set to DCI
• Press Menu -> Measure -> Quickset -> Activate Control.
• Slide the control to the very left, for high resolution at low speed.

• Press Home

The DC load (I'm using my homegrown one) is set to sink current from the supply. I'm not asking it for a precise current because I'm not speccing the load.

As long as the same current runs through the µCurrent and the DMM6500 it's OK. And they are in series, so that requirement is fulfilled.

The load current is controlled by a DAC. I'm stepping it from 120 to 960, roughly 2 to 90 mA.

At each increment, I write down the readings of both meters.

Results

In my setup, the µCurrent measures just a tad higher than the DMM6500.

The measurements can be influenced by the 121GW's precision in the mV range. No worries here, its virtually in sync with the DMM6500 in this range.

The graph isn't very revealing. The lines fully overlap. That's actually a good thing.

The purpose of this post wasn't to make a very precise specification of the µCurrent. I want to set the scene for the automation in the next blog.

This exercise allows to look back and have a comparison point.

I'll have to find a way in the automated setup to switch the DMM3500 between current and volt measurement.

Switching the instrument's mode is easy, but I'll have to alter the circuit, because the ground input is shared between volt and current mode, and the µCurrent input ground is connected to its output ground.

If you look at the test setup, you can see that I can't get away with simply wiring the DMM6500 in for current and measure the µCurrent output in the same circuit.

Maybe I'll use an oscilloscope to measure the µCurrent output ...