Datasheet

© 2008 Microchip Technology Inc. DS22059B-page 35
MCP414X/416X/424X/426X
5.0 RESISTOR NETWORK
The Resistor Network has either 7-bit or 8-bit
resolution. Each Resistor Network allows zero scale to
full scale connections. Figure 5-1 shows a block
diagram for the resistive network of a device.
The Resistor Network is made up of several parts.
These include:
• Resistor Ladder
•Wiper
• Shutdown (Terminal Connections)
Devices have either one or two resistor networks,
These are referred to as Pot 0 and Pot 1.
FIGURE 5-1: Resistor Block Diagram.
5.1 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 5-1). The end points of the resistor ladder are
connected to analog switches which 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.
For an 8-bit device, there are 256 resistors in a string
between terminal A and terminal B. The wiper can be
set to tap onto any of these 256 resistors thus providing
257 possible settings (including terminal A and terminal
B).
For a 7-bit device, there are 128 resistors in a string
between terminal A and terminal B. The wiper can be
set to tap onto any of these 128 resistors thus providing
129 possible settings (including terminal A and terminal
B).
Equation 5-1 shows the calculation for the step
resistance.
EQUATION 5-1: R
S
CALCULATION
R
S
A
R
S
R
S
R
S
B
257
256
255
1
0
R
W
(1)
W
(01h)
Analog Mux
R
W
(1)
(00h)
R
W
(1)
(FEh)
R
W
(1)
(FFh)
R
W
(1)
(100h)
Note 1: The wiper resistance is dependent on
several factors including, wiper code,
device V
DD
, Terminal voltages (on A, B,
and W), and temperature.
Also for the same conditions, each tap
selection resistance has a small variation.
This R
W
variation has greater effects on
some specifications (such as INL) for the
smaller resistance devices (5.0 kΩ)
compared to larger resistance devices
(100.0 kΩ).
R
AB
8-Bit
N =
128
127
126
1
0
(01h)
(00h)
(7Eh)
(7Fh)
(80h)
7-Bit
N =
R
S
R
AB
256()
-------------=
R
S
R
AB
128()
--------------=
8-bit Device
7-bit Device