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Precision Electronics
By Andrew Levido
Precision
Electronics
Part 8: Voltage References
Last month, we looked at sampling and aliasing in DACs (digital-to-analog converters)
and ADCs (analog-to-digital converters). Now we will describe how voltage references
work, as they are critical to the precision of both ADCs and DACs.
W
e have covered a fair bit of ground
so far in this Precision Electronics
series, but we have not looked
at the important topic of voltage references in detail. We mentioned them in
our discussion of analog-to-digital and
digital-to-analog conversion, but now
is the time for a deeper dive.
In the very first article in this series,
we discussed the difference between
precision and accuracy. We learned
that precision describes the repeatability or reliability of a measured
quantity and is all about understanding and quantifying sources of error.
On the other hand, accuracy is how
closely a measured value matches the
‘true’ value (or an accepted proxy) of
the quantity.
Most of what we have covered so
far has been concerned with precision
rather than accuracy. Voltage references
are one of the places where these two
come together. Put simply, voltage references provide a fixed voltage source
at some defined value. This sounds
simple, but there is a lot to unpack in
that statement.
Initial accuracy – or not
The “defined value” part is where
accuracy comes into play. A voltage
reference is expected to provide a
known output with some specific level
of accuracy; that is, some measure of
how close its output voltage is to the
‘true’ value.
This requirement implies the source
has been calibrated somehow against
a known standard. In many cases, this
calibration is done for you at the factory, but sometimes you are expected
to do it yourself.
An example of the former is the
MAX6225ACSA reference we used
in the DAC circuit in a previous instalment of this series. This is a 2.5V
reference with an initial accuracy of
±200ppm (±0.02%). This means the
voltage will be between 2.4995V and
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2.5005V at 25°C – a figure achieved by
calibration at manufacture.
Calibration would have been performed by trimming the chip’s output
voltage against a voltage standard with
even higher accuracy. This standard will have been itself calibrated
against an even better standard, and
so on, in a chain ultimately traceable
to the international standard definition of the volt.
Not all voltage references are trimmed
to a standard voltage at manufacture.
Some very expensive ultra-precision
voltage references have woeful initial
accuracy. The legendary LTZ1000,
which will cost you the best part of
$150, has an initial output voltage anywhere between 6.9V and 7.5V, but has
extraordinary precision.
The intention is that you will calibrate the device in which you use this
reference against some external standard, then take advantage of its incredible stability to ensure it stays that way.
Voltage reference errors
Whether you use a pre-calibrated
or post-calibrated voltage reference,
knowing its stability is critical. As you
would expect, changes in temperature
influence voltage references; achieving
a stable voltage over temperature has
been one of the driving forces behind
the development of voltage reference
technologies.
You will therefore see a figure for
temperature drift in the voltage reference data sheet in the familiar absolute (V/°C) or relative (ppm/°C) units.
In many cases, you will also see a
‘thermal hysteresis’ figure. This is the
maximum voltage change seen after
cycling the device over some fixed
temperature range – typically (but
not always) from 25°C to 50°C and
back. This is important because of
the focus on accuracy; if the voltage
changes slightly with temperature, we
at least want it to return to a consist-
ent value each time it reaches a given
temperature.
To add to the growing list of errors,
voltage references also drift with time,
even if the temperature is held constant. Most precision voltage reference data sheets therefore include a
figure for ‘long-term stability’ or similar. This can be expressed as an absolute or relative change in voltage per
root thousand hours (ppm/√kh) or per
thousand hours (ppm/kh).
The odd unit of root-kilohours is used
because long-term stability typically
has a logarithmic decay characteristic,
with more drift in the first 1000 hours
than in the second and so on.
Because of this early drift, some
manufacturers of precision equipment
using will ‘burn in’ the voltage references for a period before using them.
This improves the long-term accuracy of the references (since the higher
initial drift is done with), and allows
the manufacturer to weed out any that
show too much variation.
To put some context around these
errors, consider the MAX6225ACSA.
The initial accuracy was ±200ppm with
a tempco (temperature coefficient) of
±2ppm/°C and a thermal hysteresis of
20ppm. The worst-case long-term drift
is ±20ppm/kh. These are all pretty
good figures, but the LTZ1000 is in a
different class.
This latter chip includes an on-chip
heater to keep the reference at a stable
temperature, resulting in temperature
drift in the ±0.05ppm/°C range. No
thermal hysteresis figure is given, since
the reference is always held at a constant temperature. Long-term stability
is quoted as 0.28ppm/√kh.
I should note that building a device
using the LTZ1000 or similar ultra-
precision references is no trivial task.
To get the most out of the chip, you
must employ some ridiculously expensive high-precision, low-tempco resistor dividers and worry about all sorts
Practical Electronics | August | 2025
Part 8 – voltage references
of crazy details. Take this quote from
the data sheet as an example:
The Kovar input leads of the TO-5
package form thermocouples when
connected to copper PC boards. These
thermocouples generate outputs of
35µV/°C. It is mandatory to keep
the ... leads at the same temperature, otherwise 1ppm to 5ppm shifts
in the output voltage can easily be
expected from these thermocouples.
Air currents blowing across the leads
can also cause small temperature
variations...
There is a whole online community
dedicated to getting the best possible
performance out of these and similar
devices. Just search for “voltnuts”.
Series and shunt references
Voltage references fall into two major
categories: three-terminal (series) or
two-terminal (shunt) devices, both of
which are shown in Fig.1. Series references ‘regulate’ an input voltage to
produce the reference output.
As you might imagine, the output
is influenced by changes in the input
voltage and by the load current, so for
series regulators you will see figures in
the data sheet for line and load regulation (ie, how stable the output is despite variations in input voltage and
output current, respectively).
In the case of the MAX6225ACSA,
the line regulation is ±7ppm/V for a
Vin above 10V and the load regulation
is ±6ppm/mA. You should therefore
ensure the input to a series voltage reference is well-regulated and keep the
load current low and stable.
Achieving the latter can be a bit of
a juggling act: you can add an op amp
buffer to minimise the load current,
but this will itself introduce errors
that might outweigh those produced
by the reference’s load regulation. You
have to crunch the numbers to work
out what is optimal.
On the other hand, shunt references maintain a constant voltage drop
while current is flowing through them.
The voltage drop is influenced by the
device current, and you can see from
Fig.1 that this current is determined by
both the source and the load currents.
Shunt references therefore usually
include a single figure for output voltage change with current that encompasses both line and load regulation.
You should therefore aim to keep
the device current constant if you
want to achieve the best results with
Practical Electronics | August | 2025
shunt-type references, most likely by
employing an active current source
and by keeping the load current fixed.
Changes in shunt current with temperature will add to the shunt element’s
inherent temperature drift, so your
current source needs to be relatively
stable with temperature.
Zener references
The zener diode, with its well-
defined reverse breakdown voltage,
is the simplest form of shunt reference.
This breakdown characteristic occurs
because of two different mechanisms:
the zener effect at low voltages, and
the avalanche effect at higher voltages. The zener effect has a negative
tempco, while the avalanche effect has
a positive tempco.
As both these effects are present in
zener diodes with breakdown voltages around 5-7V, it is possible to have
these temperature effects more-orless cancel each other out by careful
selection of the diode and its operating conditions.
For example, a BZX55C5V1 5.1V,
400mW zener diode has a tempco of between +0.02% and -0.02% (±1mV/°C).
By contrast, a BZX55C12 (12V) zener
has a tempco of +0.11% (+13mV/°C).
The initial accuracy of the 5.1V zener
won’t be great, but fed with a constant
current, its voltage stability will be surprisingly good.
In fact, the LTZ1000 and MAX6225ACSA both use internal zener
diodes as the basis of their reference.
You can temperature-compensate
a zener diode by putting it in series
with a forward-biased standard diode,
as long as you choose a zener with a
tempco of about +2mV/°C. That’s because the tempco of a regular diode’s
forward drop is about -2.1mV/°C.
Otherwise, you can buy temperature-
compensated zener references like the
LM329, or even the LM399, which includes an on-chip heater.
We mentioned above that you need
to maintain a constant current through
a zener reference. For example, the
BZX55C5V1 has a dynamic resistance
of up to 35W, so a change in bias current of just 1mA will shift the output
voltage by 35mV – as much as a temperature rise of 35°C.
Fig.2 shows a clever circuit that uses
the zener itself to provide a stable bias
current and allows the output voltage
to be adjusted to boot. The zener voltage is amplified by the non-inverting
Fig.1: voltage references are
available as either three-terminal
series pass devices or twoterminal shunt devices. In either
case, keeping the input voltage or
current (and the output current)
constant is critical to getting the
best accuracy.
Fig.2: in this circuit, the zener
bias current is derived from its
own stable output. R1 and R2
allow the output voltage to be
amplified if necessary.
amplifier to produce an output voltage,
Vout = Vz (1 + R1 ÷ R2). The output voltage is then used to establish the zener
bias current via R3, Iz = (Vout – Vz) ÷ R3.
You must use a single supply with
this circuit – with a split supply, it
could settle into a second stable state
with the zener forward-biased. If you
are worried about start-up, you can add
a 1MW or greater resistor from the zener’s cathode to the positive supply.
You will often see the term ‘buried
zener’ to describe precision zener
references. This just means that the
diode junction (where the reverse
breakdown occurs) is formed below
the surface of the semiconductor and
covered with a layer of diffusion material, resulting in a more stable device
with lower noise.
Band-gap references
Band-gap voltage references are
73
Precision Electronics
much more common than zener references these days, especially inside
integrated circuits. Pretty much every
voltage regulator, linear or switching,
uses one.
Developing a semiconductor voltage
reference with a low tempco was no
trivial task, but it came down to adding
a voltage with a positive tempco to another voltage with a negative tempco
just like the compensated zener.
The idea was first used commercially by Bob Widlar in 1971. Bob
Widlar was an erratic genius who pioneered the first commercially successful op amps, comparators and three
terminal-voltage regulators, among
many others.
Part of his success was to recognise and work with the strengths of
the IC production process. He understood that it was very hard to manufacture components of precise absolute
value in silicon, but was relatively easy
to make components with precisely
matched values.
The Ebers-Moll large-signal BJT
model (a really useful model I strongly recommend you study) tells us that
the base-emitter voltage (Vbe) of a transistor is related to its collector current
(Ic) by the relationship Vbe = Vt loge(Ic
÷ Is). Vt is the thermal voltage (proportional to absolute temperature) and Is
is the reverse saturation current (also
highly temperature-dependant).
With a fixed collector current, the
base-emitter voltage has a negative
tempco of -2.01mV/°C, since the tempco
of the Is term dominates. However, the
difference between the base-emitter
voltages of two transistors with dif-
Fig.3: the simplest band gap
reference consists of just three
transistors. The voltage across R3
(Vbe1 – Vbe2), and therefore that
across R2, has a positive tempco,
offsetting the negative tempco of
Q3’s base-emitter voltage.
74
ferent collector current densities has
a positive tempco.
This can be achieved by using two
identical transistors with different collector currents. If you are mathematically inclined, it is pretty easy to see
why by simplifying the expression Vbe1
– Vbe2 = Vt(loge[Ic1 ÷ Is] – loge[Ic2 ÷ Is]).
This becomes Vbe1 – Vbe2 = Vt × loge(Ic1
÷ Ic2). Note that Is has disappeared,
leaving Vt with its positive tempco
the only temperature-dependent term.
Fig.3 shows how this phenomenon
can be used in practice. Identical
transistors Q1 and Q2 have collector currents in a 10:1 ratio because
of the values of R1 and R2. Both
transistors have the same base voltage, so the voltage at the emitter of
Q2 must be V be1 – V be2. The 10:1 I c
ratio means this voltage will be 2.3Vt
(around 60mV).
The voltage across R2 will therefore
be 23Vt or about 600mV. The output
voltage, Vout, will be this voltage plus
the Vbe drop in transistor Q3 for an
output of around 1.2V.
Importantly, the tempco of Vout will
be that of Vbe3, around -2.01mV/°C, plus
23 times that of Vt (23 × +86.2µV/°C
= +1.98mV/°C), resulting in an overall tempco of -27.4µV/°C, which is
around 10ppm per °C. Not bad for
three transistors.
This type of reference is called a
bandgap reference because its output
voltage for zero tempco corresponds
with the theoretical bandgap voltage
of the semiconductor material (~1.14V
for silicon).
Paul Brokaw developed an improved
circuit in 1974, overcoming some of
the limitations of the Widlar circuit,
which could only produce a 1.2V
output and required a fairly constant
supply current. Brokaw’s circuit is
shown in Fig.4.
Brokaw’s circuit uses transistors
with identical collector currents (due
to identical R3s), but with differing
physical on-chip areas to achieve different current densities. The voltage
across R1 is the difference in baseemitter voltages, Vt × loge(N) where
N is the ratio of transistor areas. The
voltage across R2 is therefore V t ×
log e (N) × 2(R2 ÷ R1), which has a
positive tempco due to Vt.
The reference output voltage will be
this voltage plus the base-emitter voltage of Q1 with its negative tempco. By
choosing the right values for R1, R2
and N, you can cancel the temperature
dependencies as we did before. Fig.5
shows the same circuit configured to
provide output voltages higher than
the nominal 1.2V bandgap voltage.
You can buy off-the-shelf bandgap
references for around 60¢ each. The
Microchip LM404x series, for example, are packaged as shunt references
and available with output voltages
from 1.225V to 5.000V. The 2.5V version in C grade has an absolute accuracy of ±0.5%.
The versatile and very popular TL431
is even cheaper. This is also a 2.5V
reference with a ±0.5% initial accuracy (C grade) with a typical tempco of
±10ppm/°C. You frequently see these
devices in the voltage regulation circuits of low-cost flyback switch-mode
power supplies.
Many of our projects have used them
too. For example, the DC Supply Protectors project from the May 2025 issue
used one to set the over-voltage threshold. The 500W Power Amplifier published in the April, May & June 2023
issues of PE also used two, as part of
the load-line protection circuitry.
Exotic references
Another more exotic voltage reference technology is the ‘JFET pinch-off’
reference. These work on a similar principle to the bandgap reference in that
the difference in pinch-off voltage of
two JFETs has a negative tempco that
offsets the positive tempco of a current source.
The ADR420 reference uses this technique to achieve an initial accuracy of
±400ppm (B grade) with a tempco of
±3ppm/°C. Its long-term stability is
50ppm/kh. The big advantage of this
type of reference over bandgap references is their very low noise.
Another interesting reference technology is the floating gate reference.
These rely on a Mosfet (actually an
array of Mosfets) with a well-insulated
‘floating’ gate. At manufacture, charge
is applied to the gate which, like a capacitor, charges to a particular voltage.
The Mosfet then acts as a high-
impedance voltage follower to read
out this voltage. The voltage will remain
stable as long as the gate charge does
not change.
The only commercial examples I
am aware of are in the ISL2090 series.
The 2.50V B-grade version has an initial accuracy of ±0.02%, a tempco of
±7ppm/°C and long-term stability of
20ppm/kh. By my calculations (asPractical Electronics | August | 2025
Part 8 – voltage references
suming a gate capacitance of 100pF
charged to 1V), the gate leakage must
be something less than 12 electrons
per hour! Amazing.
Voltage reference noise
Depending on your application, you
may have to take voltage reference noise
into account. Unlike op amps, there is
little consistency in how manufacturers specify the noise in their voltage
references, especially at very low frequencies (say, below 10Hz).
This is the area we usually operate in, and it is the territory where
1 ∕ noise tends to dominate, meanf
ing we can’t just extrapolate from a
wideband noise voltage to a noise
density figure.
Because we are dealing with DC signals, we can almost always add some
filtering to reduce the voltage noise.
Many references come with a ‘noise reduction’ pin that you bypass to ground
with a small capacitor to improve the
noise performance.
For example, the MAX6225ACSA
has such a pin which, if bypassed with
a 1µF capacitor, will reduce the noise
density above 100Hz from around
40nV/√Hz to under 15nV/√Hz.
If we add an external filter, we have
to make sure it does not adversely influence the reference voltage. Fig.6
shows one example of how we could
do this. R1 and C1 form a low-pass filter
with a -3dB frequency of 0.016Hz. The
bottom end of C1 is bootstrapped by
R2/C2 so that the voltage across C1 is
zero in the steady state.
If we did not do this, the leakage
current through the capacitors would
cause a voltage error as it is dropped
across R1. You should also use a lownoise precision op amp buffer so that
you don’t add new errors.
Miscellany
Precision voltage references can be
expensive, so it is worth treating them
with respect. Below are a few considerations you may need to be aware of,
depending on your application.
Solder shift: the worst thermal shock
a voltage reference is likely to experience is during the assembly process,
particularly if it is reflowed onto the
printed circuit board (PCB). Like thermal hysteresis, this can cause a permanent change in the output voltage,
known as ‘solder shift’.
You generally won’t find information
on this in the data sheets, but there are
Practical Electronics | August | 2025
a few app notes out there that discuss
it. It is only going to be of concern for
very high precision applications but is
worth knowing about. You might want
to hand-solder the reference if your application falls into this category.
Start-up time: many references use
their own output to provide stabilised bias conditions (like the circuit
in Fig.2), so they include start-up circuitry to ensure everything comes up
in an orderly fashion. This means that
the output voltage might not reach its
final stable value for some time.
It is not unusual for this time to be
tens or even hundreds of milliseconds.
Your application should be aware of
this and not use the reference until it
is stable.
Board flex: flexing a PCB that contains a precision reference can produce
a measurable change in the output.
One manufacturer suggests this can
be as much as a 60ppm peak-to-peak
change for a reference mounted on
a standard 1.6mm FR4 board that is
100mm wide board and flexed up and
down by 1.8mm.
You can minimise such mechanical
stress by fixing boards down firmly
and/or by using slots in the board to
mechanically isolate the reference.
Leakage: leakage currents across the
surface of a printed circuit board can
cause errors in precision references.
References with noise-filtering pins
(eg, the MAX6225ACSA) can be especially vulnerable, since these usually
expose a high-impedance summing
node to the outside world.
A few tens of nano amps flowing
into or out of one of these nodes can
shift the output voltage by hundreds
of ppm. Flux contamination or skin
oils are more than enough to allow this
level of current leakage, so it pays to
clean your precision boards thoroughly and to keep your fingers off them
once you have.
Fig.4: the Brokaw band gap
reference uses two transistors with
the same collector currents, but of
differing areas to produce a voltage
across R2 with a positive tempco to
offset the negative tempco of Q1’s
base-emitter voltage.
Fig.5: the Brokaw band gap
reference can easily be adapted to
produce higher output voltages.
Conclusion
Precision voltage references are
unique in that they are one component that combines both precision
and accuracy, allowing the device
they are used in to deal in absolute
quantities.
In the next and final article of this
series, we will zoom out and look at
the big picture – how one might go
about the high-level design of a precision electronics device from a wholesystem perspective.
PE
Fig.6: this RC filter reduces the
noise voltage produced by a voltage
reference by limiting the bandwidth
to 0.016Hz. R1 and C1 are the filter,
while R2 and C2 bootstrap the
bottom of C1 to eliminate its leakage
current, which would be otherwise
be dropped across R1, causing the
reference voltage to drop.
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