User manual

The burden voltage of the µCurrent is a fixed 10µV/µA and 10µV/nA on the lower ranges, and varies
on the mA range due to the switch resistance, but 70µV/mA is a nominal upper figure. These figures
are unmatched by almost any meter on the market.
So for example at a full scale of say 1000µA, that’s a maximum burden voltage of only 10mV. So
measuring the current rail of a 1.2V logic supply with full scale resolution would give you a worst case
drop of around 0.8%, a fairly insignificant figure.
The output voltage in mV is directly proportional to the input current, so you can simply read the
current value from your multimeter’s mV DC range.
The µCurrent thus effectively eliminates any issues to do with burden voltage by making it
insignificant in all but the most extreme applications.
How It Works
A current adapter is basically just a shunt resistor with an amplifier, and that is essentially all the
µCurrent is. But there are a few extra neat features to the design to make it as professional and
handy as possible, as we’ll see.
The heart of the design is U1, a Maxim MAX4239, a special “ultra-low offset/drift, low noise precision
amplifier”. Whew, what a mouthful!
As the name suggests, it’s a pretty high spec device. The key figure in this application is the near-
zero offset voltage. And not just “low offset” like many precision op-amps, this one has almost no
practical offset voltage at all. 0.1uV typical, with a maximum figure of 2.5uV over the entire
temperature range, if you want to know the actual numbers.
This class of op-amps are known as “auto-zero” (or “chopper”) amplifiers.
Maxim are a bit hush-hush on the actual internal workings of their particular device, saying only
“These characteristics are achieved through a patented autozeroing technique that samples and
cancels the input offset and noise of the amplifier. The pseudorandom clock frequency varies from
10kHz to 15kHz, reducing intermodulation distortion present in chopper-stabilized amplifiers.”
But we can get a good idea of how a basic auto-zero amplifier woks in the following diagram.