54 September/October 2018
Of course, I’m just being picky because I
seldom need more than three or four digits of
accuracy, but it sure would be nice to know
the answer. Fortunately these days, there are
a number of very accurate voltage reference
circuits that you can build or purchase for a few
dollars. A ready-to-use unit made by Agilent
is shown in Figure 1. It puts out 2.500, 5.000,
7.500, and 10.00 volts and costs less than
$20 on eBay. The actual precisely-measured
voltages are recorded on the side label, good
to six digits. The heart of it is a laser-trimmed
Analog Devices AD584 voltage reference IC.
BUILD YOUR OWN
Or … you can build your own reference out of
parts in your junk box. Figures 2 and
3 show a simple
breadboard I made in an hour or so and the schematic.
The only component I didn’t have on hand was the
There are two grades of AD584s for hobbyists — J
and K — which specify the accuracy of the outputs. J is
+ 30 mV and K is + 10 mV for their 10.00 volt outputs.
The IC in the breadboard is the K version and the spec
sheet lists a max error of + 3. 5 mV for the 2.500 volt
output. You can see that the measured error is a lot less
— only +1.0 mV. That’s one part in 2,500. Good enough
for most measurements!
FIGURE 1. This $20 voltage reference module switches
between 2.500, 5.000, 7.500, and 10.00 volts, and has a built-in USB rechargeable battery.
FIGURE 2. My DIY breadboard uses a laser-trimmed AD584
DIP; 2.500V output is accurate to 1 mV.
FIGURE 3. The schematic of the DIY breadboard can use two
versions of AD584s, depending on the accuracy desired.
Sometimes I wonder which of my portable
digital voltmeters I can trust — the B&K,
Fluke, or Amprobe. Usually, they’re
pretty close but it bugs me not knowing
whether they are right on the nose.
By David Goodsell