Okay, you’ve wired up
the latest whiz-bang
sensor to your
project. The sensor has
been connected to the
12-bit A/D and now the
moment of truth. You
apply power and look at
the data ... Hmmmm,
the numbers don’t look
quite right. After much
head scratching, you
measure the A/D
voltage reference with
your trusty DMM and
get to thinking. A 12-bit
A/D means the input
signal is going to be
represented by one of
4,096 possible codes;
one part in 4,096 is
In other words, if you
want to get 12 bits of
absolute accuracy, the
reference voltage must
be within .024% of the
desired value. This puts
a heavy burden on the
test equipment used to
measure the reference
So, just how accurate
is your DMM?
BUILD A .01%
If you had a separate accurate voltage reference available, you could
determine if your DMM is “telling
the truth.” This is but one example
of why you might want an accurate
voltage reference. Other possible
reasons include using one to
calibrate your DMM, or to construct
a precision voltage-to-frequency
converter or current source.
The applications for a precision
reference fall into two general categories: instrument (DMM, DVM)
calibration/accuracy verification, or
as a circuit component, such as the
reference for an A/D converter.
The construction part of this article is geared towards voltmeter calibration and/or accuracy verification.
However, the following discussion of
various reference voltage parameters
applies equally well to using a voltage reference as a circuit element.
I’ll quickly go through several
voltage reference options, starting
with inexpensive and fairly crude
■ FIGURE 1.
devices and progress up to a precision lab-quality circuit.
On the low end of the
reference accuracy continuum, we
have the lowly zener diode. It is
readily available and 5% accurate
devices are inexpensive. A 5.1V,
1N751A zener can be purchased
for about a dime and is a good
starting point for our discussion.
Zeners provide their stated
breakdown voltage only with a
particular test current, Iz, flowing
through them. This means that the
series-limiting resistor shown in
Figure 1 must be sized to guarantee
that this current will, indeed, flow.
For instance, with a 9V supply
and a 1N751A which has a Iz of 20
mA, the resistor value is ( 9-5.1)/20
mA = 195 ohms. Note that the value
of R1 will be different if anything
other than a high impedance is
placed across the zener.
I don’t want to dwell too much
on the lowly zener but there is one
point worth emphasizing. You may
be asking, what is the advantage of
using a zener if an accurate supply
voltage and a precision resistor are
needed to get the precise desired
PCB patterns and artwork are
available on the Nuts & Volts
website at www.nutsvolts.com.