Datasheet
MCP41XXX/42XXX
DS11195C-page 16 2003 Microchip Technology Inc.
FIGURE 4-7: Single Supply
programmable differential amplifier using digital
potentiometers.
4.2.3 PROGRAMMABLE OFFSET TRIM
For applications requiring only a programmable voltage
reference, the circuit in Figure 4-8 can be used. This
circuit shows the device used in the potentiometer
mode along with two resistors and a buffered output.
This creates a circuit with a linear relationship between
voltage-out and programmed code. Resistors R
1
and
R
2
can be used to increase or decrease the output volt-
age step size. The potentiometer in this mode is stable
over temperature. The operation of this circuit over
temperature is shown in Figure 2-3. The worst perfor-
mance over temperature will occur at the lower codes
due to the dominating wiper resistance. R
1
and R
2
can
also be used to affect the boundary voltages, thereby
eliminating the use of these lower codes.
FIGURE 4-8: By changing the values of
R
1
and R
2
, the voltage output resolution of this
programmable voltage reference circuit is
affected.
4.3 Calculating Resistances
When programming the digital potentiometer settings,
the following equations can be used to calculate the
resistances. Programming code 00h effectively brings
the wiper to the B terminal, leaving only the wiper resis-
tance. Programming higher codes will bring the wiper
closer to the A terminal of the potentiometer. The equa-
tions in Figure 4-9 can be used to calculate the terminal
resistances. Figure 4-10 shows an example calculation
using a 10 kΩ potentiometer.
FIGURE 4-9: Potentiometer resistances
are a function of code. It should be noted that,
when using these equations for most feedback
amplifier circuits (see Figure 4-4 and Figure 4-5),
the wiper resistance can be omitted due to the
high impedance input of the amplifier.
FIGURE 4-10: Example Resistance
calculations.
MCP601
V
B
V
SS
V
DD
-IN
+IN
V
OUT
AB
A
B
(SIG -)
MCP42010
MCP42010
1/2
1/2
V
REF
NOTE: Potentiometer values must be equal
+
-
Where:
R
A
R
AB
256 D
n
–()
256
---------------------------------------= R
B
R
AB
D
n
256
------------------=
R
AB
Total Resistance of pot=
D
n
Wiper setting forD
n
0 to 255==
V
OUT
V
A
V
B
–()
R
B
R
A
-------=
V
A
(SIG +)
MCP606
OUT
V
SS
V
DD
-IN
+IN
V
DD
V
SS
R
1
R
2
B
A
MCP41010
0.1 uF
+
-
PA
PB
PW
Where:
PA is the A terminal
PB is the B terminal
PW is the wiper terminal
R
WA
is resistance between Terminal A and wiper
R
WB
is resistance between Terminal B and Wiper
R
AB
is overall resistance for pot (10 kΩ, 50 kΩ or 100 kΩ)
R
W
is wiper resistance
D
n
is 8-bit value in data register for pot number n
R
WA
D
n
()
R
AB
()256 D
n
–()
256
-------------------------------------------- R
W
+=
R
WB
D
n
()
R
AB
()D
n
()
256
---------------------------- R
W
+=
Example:
Code = C0h = 192d
R = 10 kΩ
Note: All values shown are typical and
actual results will vary.
PA
PB
PW
10 kΩ
R
WA
D
n
()
R
AB
()256 D
n
–()
256
-------------------------------------------- R
W
+=
R
WB
C0h()
10k
Ω
()192()
256
----------------------------------- 52
Ω
+=
R
WA
C0h()
10k
Ω
()256 192–()
256
--------------------------------------------------- 52
Ω
+=
R
WA
C0h()2552
Ω
=
R
WB
D
n
()
R
AB
()D
n
()
256
---------------------------- R
W
+=
R
WB
C0h()7552
Ω
=