Datasheet

MCP4021/2/3/4
DS21945E-page 48 © 2006 Microchip Technology Inc.
8.0 APPLICATIONS EXAMPLES
Non-volatile digital potentiometers have a multitude of
practical uses in modern electronic circuits. The most
popular uses include precision calibration of set point
thresholds, sensor trimming, LCD bias trimming, audio
attenuation, adjustable power supplies, motor control
overcurrent trip setting, adjustable gain amplifiers and
offset trimming. The MCP4021/2/3/4 devices can be
used to replace the common mechanical trim pot in
applications where the operating and terminal voltages
are within CMOS process limitations (V
DD
= 2.7V to
5.5V).
8.1 Set Point Threshold Trimming
Applications that need accurate detection of an input
threshold event often need several sources of error
eliminated. Use of comparators and operational
amplifiers (op amps) with low offset and gain error can
help achieve the desired accuracy, but in many applica-
tions, the input source variation is beyond the
designer’s control. If the entire system can be
calibrated after assembly in a controlled environment
(like factory test), these sources of error are minimized,
if not entirely eliminated.
Figure 8-1 illustrates a common digital potentiometer
configuration. This configuration is often referred to as
a “windowed voltage divider”. Note that R
1
and R
2
are
not necessary to create the voltage divider, but their
presence is useful when the desired threshold has
limited range. It is “windowed” because R
1
and R
2
can
narrow the adjustable range of V
TRIP
to a value much
less than V
DD
– V
SS
. If the output range is reduced, the
magnitude of each output step is reduced. This
effectively increases the trimming resolution for a fixed
digital potentiometer resolution. This technique may
allow a lower-cost digital potentiometer to be utilized
(64 steps instead of 256 steps).
The MCP4021’s and MCP4023’s low DNL
performance is critical to meeting calibration accuracy
in production without having to use a higher precision
digital potentiometer.
EQUATION 8-1: CALCULATING THE
WIPER SETTING FROM
THE DESIRED V
TRIP
FIGURE 8-1: Using the Digital
Potentiometer to Set a Precise Output Voltage.
8.1.1 TRIMMING A THRESHOLD FOR AN
OPTICAL SENSOR
If the application has to calibrate the threshold of a
diode, transistor or resistor, a variation range of 0.1V is
common. Often, the desired resolution of 2 mV or
better is adequate to accurately detect the presence of
a precise signal. A “windowed” voltage divider, utilizing
the MCP4021 or MCP4023, would be a potential
solution as shown in Figure 8-2.
FIGURE 8-2: Set Point or Threshold
Calibration.
V
TRIP
V
DD
R
2
R
WB
+
R
1
R
AB
R
2
++
---------------------------------- -
⎝⎠
⎛⎞
=
R
AB
R
Nominal
=
R
WB
R
AB
D
63
----- -
⎝⎠
⎛⎞
•=
D
V
TRIP
V
DD
--------------
⎝⎠
⎛⎞
R
1
R
AB
R
2
++()R
2
–()•
⎝⎠
⎛⎞
63•=
Where:
D = Digital Potentiometer Wiper Setting (0-63)
V
DD
V
OUT
R
2
A
R
1
W
B
MCP4021
CS
U/D
V
TRIP
0.1 µF
Comparator
V
CC+
V
CC–
V
DD
R
sense
R
1
R
2
B
A
V
DD
W
MCP4021
CS
U/D
MCP6021