Poor Men's Source Measurement Unit (SMU)

Posted on zo 03 januari 2021 in NLNet

SMU ?

In my previous professional life doing semiconductor process development I used so called source measure units (SMUs) for measuring out transistors. Although I don't remember the exact type number, I guess I had access to a HP 4145A semiconductor parametric analyzer.

Nice thing about these units is that these allow 4-quadrant operation, e.g. allow all the 4 combinations of positive or negative voltage with positive or negative current. It allows to force a voltage and measure current or the other way around.

In my new professional life working on open source semiconductors I don't have that luxury anymore. I did look on-line but even second hand version of an old HP 4145A costs too much for the numbers of times I will need such equipment. From the other side switching to single-quadrant power supplies and loads was not really the way I wanted to go. So I was on the look-out for a better solution.

AD5522

After much searching around I found the Analog Devices AD5522 chip and it's companion development board. Although this chip is not in the same range as the HP 4145A regarding the specifications it should still allow to fulfill my measurement needs at a much lower cost than that high precision equipment.

First test

So I went ahead and bought the development board together with a Mean Well PT-65C power supply board. To house the Mean Well power supply I repurposed the case and the switch of a failed Amiga 1200 power supply as can be seen from the next photo:

The Analog Devices support software runs only under Windows and I don't have a Windows computer set up anymore. So I needed to set up a virtual machine with the software installed. Luckily USB port forwarding to the virtual machine did work to access the development board with the support software through a USB connection from my Mint 20 Linux machine.

I connected a 6.2KΩ resistor to the output of channel 3. In the next plot one can see that a voltage sweep from -6V to 6V correctly results in a measured current going from -1mA to 1mA:

Currently I am playing with a Raspberry Pi to use one of it's SPI interfaces on it's 40 pin header to setup and drive the AD5522. First results seem to be encouraging and the plan is to use this setup then to perform the measurement tests on the NLNet018TV.

Story (hopefully) to be continued...