MCP41XXX/42XXX
4.3
Calculating Resistances
1/2
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.
MCP42010
VB
A
B
VDD
(SIG -)
+
-IN
VA
VOUT
MCP601
A
B
(SIG +)
+IN
-
VSS
RB
RA
PA
PW
VOUT = (VA – VB)-------
VREF
Where:
PB
R
AB(256 – Dn)
RABDn
256
RA = -------------------------------------- RB = ------------------
256
(RAB)(256 – Dn)
R
R
WA(Dn) = ------------------------------------------- + RW
RAB = Total Resistance of pot
Dn = Wiper setting forDn = 0 to 255
256
(RAB)(Dn)
WB(Dn) = --------------------------- + RW
256
NOTE: Potentiometer values must be equal
Where:
FIGURE 4-7:
Single Supply
programmable differential amplifier using digital
potentiometers.
PA is the A terminal
PB is the B terminal
PW is the wiper terminal
R
R
R
R
D
is resistance between Terminal A and wiper
is resistance between Terminal B and Wiper
is overall resistance for pot (10 kΩ, 50 kΩ or 100 kΩ)
WA
WB
AB
W
n
4.2.3
PROGRAMMABLE OFFSET TRIM
is wiper resistance
is 8-bit value in data register for pot number n
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
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. R1 and R2 can
also be used to affect the boundary voltages, thereby
eliminating the use of these lower codes.
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.
Example:
PA
R = 10 kΩ
Code = C0h = 192d
PW
10 kΩ
PB
(RAB)(256 – Dn)
R
WA(Dn) = ------------------------------------------- + RW
VDD
256
VDD
(10kΩ)(256 – 192)
R1
R
R
WA(C0h) = -------------------------------------------------- + 52Ω
256
-
-IN
WA(C0h) = 2552Ω
MCP606
+
A
B
OUT
+IN
(RAB)(Dn)
R
WB(Dn) = --------------------------- + RW
VSS
256
0.1 uF
(10kΩ)(192)
R
R
WB(C0h) = ---------------------------------- + 52Ω
R2
256
VSS
WB(C0h) = 7552Ω
FIGURE 4-8:
By changing the values of
Note: All values shown are typical and
actual results will vary.
R and R , the voltage output resolution of this
1
2
programmable voltage reference circuit is
affected.
FIGURE 4-10:
calculations.
Example Resistance
DS11195C-page 16
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
