MCP41XXX/42XXX
4.1.2
POTENTIOMETER MODE
4.1
Modes of Operation
In the potentiometer mode, all three terminals of the
device are tied to different nodes in the circuit. This
allows the potentiometer to output a voltage propor-
tional to the input voltage. This mode 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 4-3. Note that reversing the polarity of the A and
B terminals will not affect operation.
Digital potentiometer applications can be divided into
two categories: rheostat mode and potentiometer, or
voltage divider, mode.
4.1.1
RHEOSTAT MODE
In the rheostat mode, the potentiometer is used as a
two-terminal resistive element. The unused terminal
should be tied to the wiper, as shown in Figure 4-2.
Note that reversing the polarity of the A and B terminals
will not affect operation.
V1
A
A
V2
W
W
B
B
MCP4XXXX
MCP4XXXX
Resistor
FIGURE 4-3:
Three terminal or voltage
divider mode.
FIGURE 4-2: Two-terminal or rheostat
In this configuration, the ratio of the internal resistance
defines the temperature coefficient of the device. The
resistor matching of the RWB resistor to the RAB resistor
performs with a typical temperature coefficient of
1 ppm/°C (measured at code 80h). At lower codes, the
wiper resistance temperature coefficient will dominate.
Figure 2-3 shows the effect of the wiper. Above the
lower codes, this figure shows that 70% of the states
will typically have a temperature coefficient of less than
5 ppm/°C. 30% of the states will typically have a
ppm/°C of less than 1.
configuration for the digital potentiometer. Acting
as a resistive element in the circuit, resistance is
controlled by changing the wiper setting.
Using the device in this mode allows control of the total
resistance between the two nodes. The total measured
resistance would be the least at code 00h, where the
wiper is tied to the B terminal. The resistance at this
code is equal to the wiper resistance, typically 52Ω for
the 10 kΩ MCP4X010 devices, 125Ω for the 50 kΩ
(MCP4X050), and 100 kΩ (MCP4X100) devices. For
the 10 kΩ device, the LSB size would be 39.0625Ω
(assuming 10 kΩ total resistance). The resistance
would then increase with this LSB size until the total
measured resistance at code FFh would be 9985.94Ω.
The wiper will never directly connect to the A terminal
of the resistor stack.
In the 00h state, the total resistance is the wiper resis-
tance. To avoid damage to the internal wiper circuitry in
this configuration, care should be taken to ensure the
current flow never exceeds 1 mA.
For dual devices, the variation of channel-to-channel
matching of the total resistance from A to B is less than
1%. The device-to-device matching, however, can vary
up to 30%. In the rheostat mode, the resistance has a
positive temperature coefficient. The change in wiper-
to-end terminal resistance over temperature is shown
in Figure 2-8. The most variation over temperature will
occur in the first 6% of codes (code 00h to 0Fh) due to
the wiper resistance coefficient affecting the total resis-
tance. The remaining codes are dominated by the total
resistance tempco RAB, typically 800 ppm/°C.
DS11195C-page 14
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
