AN219
Comparing Digital Potentiometers to Mechanical
Potentiometers
Author:
Bonnie C. Baker,
Microchip Technology Inc.
Earlier mechanical potentiometers were built by wrap-
ping a resistive wire around a cylinder. With this con-
struction, the wiper moves from one winding to the
next. As the wiper is moved across the element, there
are discrete steps in resistance. Following this style of
fabrication, the mechanical potentiometer was built
using a resistive thick film that was screened onto a
ceramic substrate. With this construction, the change in
resistance across the element is continuous.
There are a variety of resistive materials that are used
by mechanical potentiometer manufacturers. They
include molded conductive plastic, conductive plastic
film, screened conductive plastic, and cermet. Each
resistive material has its own set of performance char-
acteristics. In this application note the digital potentiom-
eter will only be compared to the more popular cermet
potentiometer. Cermet is a thick film resistive material
that is a mixture of fine particles of ceramic or glass and
precision metals such as silver, platinum, rhodium, or
gold. The wiper of the mechanical potentiometer slides
along the distance on the resistive material providing
an analog resistive output that has an infinite number of
positions across the span of the element.
INTRODUCTION
Resistor potentiometers can be found in electronic cir-
cuits across a wide spectrum of applications. Most typ-
ically, they function in a voltage divider configuration in
order to execute various types of tasks, such as offset
or gain adjust. The two types of potentiometers com-
pared in this application note are the mechanical poten-
tiometer (also called a trimmer potentiometer) and the
digital potentiometer. The physical descriptions and cir-
cuit models of these two devices are shown in Figure 1.
Basics of Mechanical Potentiometers
The first type of potentiometer on the market was
mechanical in nature. This type of potentiometer is still
available and adjustments of the wiper are imple-
mented by twisting a knob, moving a slider, or using a
screw driver. Although this method seems awkward,
given the advent of the digital potentiometer, mechani-
cal potentiometers still find their way into various elec-
tronic circuits.
PA
PW
PB
PA
PW
PB
contact
resistance
wiper
resistance
MCP41010
Digital
Potentiometer
Model
An example of PCB
mountable Mechanical
potentiometers
Mechanical
Potentiometer
Model
MCP42010
Figure 1:
The mechanical potentiometer is constructed so that the user can easily adjust the position of
the wiper (PW) by hand or with a screw driver. The digital potentiometer is manufactured so that the
position of the wiper is adjusted by means of a serial digital code. The circuit representation of the digital
potentiometer and the mechanical potentiometer is fundamentally the same.
2000 Microchip Technology Inc.
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DS00219A-page 1
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The metal contacts of the mechanical potentiometer
can affect the performance and reliability of the device.
Higher cost potentiometers use multi-fingers made
from precious metals in order to promote longer life as
well as improve electrical performance in all environ-
ments. These higher quality potentiometers are not
included in the discussions in this application note.
Beyond the basic differences in fabrication and func-
tionality of these two styles of potentiometers, there are
several specifications that describe the difference and
similarities of these devices further.
Changes of Resistive Element Due to
Environmental Cycling
Environmental changes such as temperature or humid-
ity can have an adverse effect on an application circuit
where a mechanical potentiometer is used. Since
mechanical potentiometers have moving parts, they
can be more sensitive to these types of environmental
changes. The reaction of a typical mechanical potenti-
ometer to these types of environmental changes is
shown in Table 1.
Environmental Event
(per Mil-R-94 standard)
Temperature Cycling
High Temperature
Exposure
Humidity excursions
Maximum Allowable
Resistance Change of
Mechanical
Potentiometer
±1%
to
±10%
±2%
@ 125
°C
for 250
hours
±15%
Basics of Digital Potentiometers
Digital potentiometers (Figure 2) were introduced in the
market after the mechanical potentiometer. The digital
potentiometer is fabricated using the same silicon tech-
nology used in active analog and digital integrated cir-
cuits use. This device comprises a combination of
segmented resistive elements and on-chip switches.
The resistive elements are manufactured using stan-
dard p-type silicon diffusions. Each resistive element
can be switched from one side to the other side of the
wiper using a serial digital command.
The digital potentiometer exhibits the same fundamen-
tal operation as the mechanical potentiometer with one
primary exception. The wiper position is digitally pro-
grammed with a microcontroller. This style of adjust-
ment allows the designer to adjust circuit performance
dynamically using a digital controller. The additional
programmability provides a solution where human
intervention is not required. With this “hands-off” pro-
grammability, the digital potentiometer offers signifi-
cant flexibility for a variety of applications.
Because this system is digital, the number of wiper
positions is no longer infinite. For example, Microchip’s
MCP41XXX and MCP42XXX family of potentiometers
are all 8-bit and have 256 unique linear positions along
the total resistive element.
PA0 PW0 PB0
Table 1:
The environment can have an
adverse effect on the reliability of the mechanical
potentiometer. The specifications in this table
were taken from data sheets of higher quality
mechanical potentiometers.
PA1 PW1 PB1
RDAC1
Data Register 0
D7
D0
RDAC2
Data Register 1
D7
D0
RS
Decode
Logic
CS
D7
D0
16 Bit Shift Register
SCK
SI
SO
SHDN
Figure 2:
This is an example of a dual digital potentiometer. The digital potentiometer is programmed via
a serial interface.
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2000 Microchip Technology Inc.
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Since digital potentiometers are manufactured using a
standard CMOS process with no moving parts, the
reaction to these environmental changes are signifi-
cantly reduced.
1kΩ through 1MΩ. This range of potentiometers are
available in both the digital and mechanical potentiom-
eters.
Total Resistance Tolerance
The total resistance tolerance of the element between
terminal PA and terminal PB varies from part to part.
With digital potentiometers that variance is dependent
on processing variance of the resistive material and
switches. Typical digital potentiometer total resistance
tolerances are between
±20%
to
±30%.
On the other
hand, variance of the cermet material in mechanical
potentiometers range from
±10
to
±25%.
Although there seems to be a degree of difference
between the digital potentiometer and mechanical
potentiometer, the variability of the nominal resistance
of both devices is considerably larger than standard 1%
discrete resistors. In some applications, these toler-
ance values can cause errors that are too large. For
additional design help, refer to the numerous circuit
ideas in Microchip’s application note, AN-691,”Optimiz-
ing Digital Potentiometer Circuits to Reduce Absolute
and Temperature Variations”.
Vibration or Shock
Vibration or shock can also have an effect on an appli-
cation circuit by causing physical movement. All
devices that are soldered on a PCB can have failures
due to vibration or shock, but the moving mechanism of
mechanical potentiometers may also move.
A typical specification for a mechanical potentiometer
would be a
±2%
change due to vibrations that span
from 10Hz to 2kHz. Another way of describing the
effects of movement on the mechanical potentiometer
is force. Typically 20Gs of force on a higher quality
mechanical potentiometer would cause a maximum of
±1%
resistive change.
Since there are no moving parts in digital potentiome-
ters, the element will remain unchanged with vibration
or shock tests unless discontinuities occur in the PCB
construction.
Mean Time to Failure Life
One type of failure that is quantified with mechanical
potentiometers is the mean time to failure life of the
wiper adjustment capability. A typical specification for
this type of failure would be that the device could sur-
vive several hundred cycles without discontinuity. A
cycle is defined as changing the wiper position across
full scale once. With thin film mechanical potentiome-
ters, such as those constructed of cermet, a failure
resulting from repeated cycles manifests itself as
reduced performance.
Since the wiper of the digital potentiometer is controlled
by electrical switches, the resistive elements are not
effected by repeated cycles. Consequently, the digital
potentiometer is a more robust solution.
Temperature Coefficient
Mechanical potentiometers and digital potentiometers
drift with temperature. The range of typical drift specifi-
cations for the total resistance of the mechanical poten-
tiometer is from
±100ppm/°C
to
±300ppm/°C.
Typical
drift versus temperature specification for the digital
potentiometer is around
±800ppm/°C.
With both types
of potentiometers, the temperature coefficient differ-
ence between the A element (resistance between PA
and PW minus the wiper resistance) and B element
(resistance between PB and PW minus the wiper resis-
tance) is very low.
The magnitude of these specifications may or may not
affect the performance of the circuit. If it is found that
they do, numerous circuit ideas are available in Micro-
chip’s application note, AN-691,”Optimizing Digital
Potentiometer Circuits to Reduce Absolute and Tem-
perature Variations”.
Nominal Total Resistance
The nominal total resistance of a potentiometer is the
typical specified resistance (in ohms) that can be mea-
sured between terminal PA and terminal PB per
Figure 1. Typical values for digital potentiometers are
10kΩ, 50kΩ, and 100kΩ. Nominal resistance values
below 10kΩ become difficult to implement in silicon
because of the switch resistances. Values higher than
100kΩ are possible but require more silicon, which
increases the cost of the device.
The range of the selection of the mechanical potenti-
ometer is considerably wider with values such as 10Ω,
20Ω, 50Ω, 100Ω, 200Ω, 500Ω, 1kΩ, 2kΩ, 5kΩ, 10kΩ,
20kΩ, 25kΩ, 50kΩ, 100kΩ, 250kΩ, 500kΩ, 1MΩ, and
2MΩ.
The mechanical potentiometer might be considered
attractive because of the wide range of nominal resis-
tance offerings. However, the most common nominal
resistance ranges used in adjustment type circuits are
Power Rating
Mechanical potentiometers can sustain more power
dissipation than the digital potentiometers. It is not
unusual to have a mechanical potentiometer that is
capable of dissipating 0.5W @ 70°C (usually specified
for 1000 hours). However, the wiper of the mechanical
potentiometer usually can only conduct up to 1mA of
current. This becomes a limitation if the potentiometer
is configured so that the wiper is directly connected to
terminal A or terminal B.
The digital potentiometer is capable of conducting
power up to 0.0055W @ 70°C. It also has a 1mA max-
imum wiper current restriction.
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Preliminary
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Temperature Range
Both the mechanical potentiometer and digital potenti-
ometer are specified to be able to operate over indus-
trial temperature range of -40°C to 85°C. Most typically,
the mechanical potentiometer is specified to operate
over the military range of -55°C to 125°C.
REFERENCES:
Baker, Bonnie C.,
“Optimizing Digital Potentiometer
Circuits to Reduce Absolute and Temperature Varia-
tions”,
AN-691, Microchip Technology Inc.
Todd, Carl David,
“The Potentiometer Handbook:
Users’ Guide to Cost-effective Applications”,
McGraw-
Hill, 1975.
Baker, Bonnie C.,
“Using a Digital Potentiometer to
Optimize a Precision Single Supply Photo Detection
Circuit”,
AN-692, Microchip Technology Inc.
Baker, Bonnie C.,
“Using Digital Potentiometers to
Design Low Pass Adjustable Filters”,
AN-737,
Microchip Technology Inc.
CONCLUSION
Mechanical potentiometers have advantages in terms
of having a wide variety of values available and tighter
specifications such as nominal resistance, tolerance,
temperature coefficient, power rating and temperature
range specifications. But in many applications the over-
riding factors are related to environmental and reliabil-
ity issues. These characteristics are not necessarily
specified by the mechanical potentiometer vendor.
Digital potentiometers go hand in hand with the drive
towards digital system control. This type of potentiom-
eter is considerably more robust that its predecessor,
the mechanical potentiometer, in terms of environmen-
tal exposure issues and longevity with repeated use of
the wiper. But beyond the reliability issues, the digital
potentiometer offers hands-off programmability. This
programmability also allows the user to repeatedly and
reliably return to the same wiper position.
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NOTES:
2000 Microchip Technology Inc.
Preliminary
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