Datasheet

© 2006 Microchip Technology Inc. DS21945E-page 43
MCP4021/2/3/4
6.1 Resistor Configurations
6.1.1 RHEOSTAT CONFIGURATION
When used as a rheostat, two of the three digital
potentiometer’s terminals are used as a resistive
element in the circuit. With terminal W (wiper) and
either terminal A or terminal B, a variable resistor is
created. The resistance will depend on the tap setting
of the wiper and the wiper’s resistance. The resistance
is controlled by changing the wiper setting.
The unused terminal (B or A) should be left floating.
Figure 6-2 shows the two possible resistors that can be
used. Reversing the polarity of the A and B terminals
will not affect operation.
FIGURE 6-2: Rheostat Configuration.
This allows the control of the total resistance between
the two nodes. The total resistance depends on the
“starting” terminal to the wiper terminal. At the code
00h, the R
BW
resistance is minimal (R
W
), but the R
AW
resistance in maximized (R
AB
+ R
W
). Conversely, at the
code 3Fh, the R
AW
resistance is minimal (R
W
), but the
R
BW
resistance in maximized (R
AB
+ R
W
).
The resistance step size (R
S
) equates to one LSb of the
resistor.
The change in wiper-to-end terminal resistance over
temperature is shown in Figure 2-6, Figure 2-17,
Figure 2-27 and Figure 2-37. The most variation over
temperature will occur in the first few codes due to the
wiper resistance coefficient affecting the total
resistance. The remaining codes are dominated by the
total resistance tempco R
AB
.
6.1.2 POTENTIOMETER
CONFIGURATION
When used as a potentiometer, all three terminals are
tied to different nodes in the circuit. This allows the
potentiometer to output a voltage proportional to the
input voltage. This configuration is sometimes called
voltage divider mode. The potentiometer is used to
provide a variable voltage by adjusting the wiper
position between the two endpoints as shown in
Figure 6-3. Reversing the polarity of the A and B
terminals will not affect operation.
FIGURE 6-3: Potentiometer Configuration.
The temperature coefficient of the R
AB
resistors is
minimal by design. In this configuration, the resistors all
change uniformally, so minimal variation should be
seen.
The wiper resistor temperature coefficient is different
from the R
AB
temperature coefficient. The voltage at
node V
3
(Figure 6-3) is not dependent on this wiper
resistance, just the ratio of the R
AB
resistors, so this
temperature coefficient in most cases can be
ignored.
Note: To avoid damage to the internal wiper
circuitry in this configuration, care should
be taken to insure the current flow never
exceeds 2.5 mA.
A
B
W
Resistor
R
AW
R
BW
or
Note: To avoid damage to the internal wiper
circuitry in this configuration, care should
be taken to insure the current flow never
exceeds 2.5 mA.
A
B
W
V
1
V
3
V
2