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 R1 and
R2 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
mance over temperature will occur at the lower codes
due to the dominating wiper resistance. R1 and R2 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
R1 and R2, 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
using a 10 k
potentiometer.
FIGURE 4-9:
Potentiometer resistances
are a function of code. It should be noted that,
when using these equations for most feedback
the wiper resistance can be omitted due to the
high impedance input of the amplifier.
FIGURE 4-10:
Example Resistance
calculations.
MCP601
VB
VSS
VDD
-IN
+IN
VOUT
AB
A
B
(SIG -)
MCP42010
1/2
VREF
NOTE: Potentiometer values must be equal
+
-
Where:
RA
RAB 256 Dn
–
()
256
---------------------------------------
=
RB
RABDn
256
------------------
=
RAB
Total Resistance of pot
=
Dn
Wiper setting forDn 0 to 255
=
VOUT
VA VB
–
()
RB
RA
-------
=
VA
(SIG +)
MCP606
OUT
VSS
VDD
-IN
+IN
VDD
VSS
R1
R2
B
A
MCP41
010
0.1 uF
+
-
PA
PB
PW
Where:
PA is the A terminal
PB is the B terminal
PW is the wiper terminal
RWA is resistance between Terminal A and wiper
RWB is resistance between Terminal B and Wiper
RAB is overall resistance for pot (10 k, 50 k or 100 k)
RW is wiper resistance
Dn is 8-bit value in data register for pot number n
RWA Dn
()
RAB
() 256 Dn
–
()
256
-------------------------------------------- RW
+
=
RWB Dn
()
RAB
() Dn
()
256
---------------------------- RW
+
=
Example:
Code = C0h = 192d
R = 10 k
Note: All values shown are typical and
actual results will vary.
PA
PB
PW
10 k
RWA Dn
()
RAB
() 256 Dn
–
()
256
-------------------------------------------- RW
+
=
RWB C0h
()
10k
() 192
()
256
----------------------------------- 52
+
=
RWA C0h
()
10k
() 256 192
–
()
256
--------------------------------------------------- 52
+
=
RWA C0h
()
2552
=
RWB Dn
()
RAB
() Dn
()
256
---------------------------- RW
+
=
RWB C0h
()
7552
=