Datasheet

© 2009 Microchip Technology Inc. DS22152B-page 41
MCP40D17/18/19
6.0 RESISTOR NETWORK
The Resistor Network is made up of two parts. These
are:
• Resistor Ladder
•Wiper
Figure 6-1 shows a block diagram for the resistive
network.
Digital potentiometer applications can be divided into
two resistor network categories:
• Rheostat configuration
• Potentiometer (or voltage divider) configuration
The MCP40D17 is a true rheostat, with terminal B and
the wiper (W) of the variable resistor available on pins.
The MCP40D18 device offers a voltage divider
(potentiometer) with terminal B internally connected to
ground.
The MCP40D19 device is a Rheostat device with
terminal A of the resistor floating, terminal B internally
connected to ground, and the wiper (W) available on
pin.
6.1 Resistor Ladder Module
The resistor ladder is a series of equal value resistors
(R
S
) with a connection point (tap) between the two
resistors. The total number of resistors in the
series (ladder) determines the R
AB
resistance
(see Figure 6-1). The end points of the resistor ladder
are connected to the device Terminal A and Terminal B
pins. The R
AB
(and R
S
) resistance has small variations
over voltage and temperature.
The Resistor Network has 127 resistors in a string
between terminal A and terminal B. This gives 7-bits of
resolution.
The wiper can be set to tap onto any of these 127
resistors thus providing 128 possible settings
(including terminal A and terminal B). This allows zero
scale to full scale connections.
A wiper setting of 00h connects the Terminal W (wiper)
to Terminal B (Zero Scale). A wiper setting of 3Fh is the
Mid scale setting. A wiper setting of 7Fh connects
the Terminal W (wiper) to Terminal A (Full Scale).
Table 6-1 illustrates the full wiper setting map.
Terminal A and B as well as the wiper W do not have a
polarity. These terminals can support both positive and
negative current.
FIGURE 6-1: Resistor Network Block
Diagram.
TABLE 6-1: WIPER SETTING MAP
Wiper Setting Properties
07Fh Full Scale (W = A)
07Eh - 040h W = N
03Fh W = N (Mid Scale)
03Eh - 001h W = N
000h Zero Scale (W = B)
R
S
A
R
S
R
S
R
S
B
N = 127
N = 126
N = 125
N = 1
N = 0
RW
(1)
W
01h
Analog
Mux
RW
(1)
00h
RW
(1)
7Dh
RW
(1)
7Eh
RW
(1)
7Fh
Note 1: The wiper resistance is tap dependent.
That is, each tap selection resistance
has a small variation. This variation has
more effect on devices with smaller R
AB
resistance (5.0 kΩ).