Microchip Technology TSSOP20EV Hoja De Datos
© 2009 Microchip Technology Inc.
DS22233A-page 39
MCP434X/436X
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.
resolution. Each Resistor Network allows zero scale to
full scale connections.
shows a block
diagram for the resistive network of a device.
The Resistor Network is made up of several parts.
These include:
These include:
• Resistor Ladder
• Wiper
• Shutdown (Terminal Connections)
• Wiper
• Shutdown (Terminal Connections)
Devices have either four resistor networks. These are
referred to as Pot 0, Pot 1 Pot 2, and Pot 3.
referred to as Pot 0, Pot 1 Pot 2, and Pot 3.
FIGURE 5-1:
Resistor Block Diagram.
5.1
Resistor Ladder Module
The resistor ladder is a series of equal value resistors
(R
(R
S
) with a connection point (tap) between the two
resistors. The total number of resistors in the series
(ladder) determines the R
(ladder) determines the R
AB
resistance (see
). 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
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).
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).
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).
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
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
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 k
smaller resistance devices (5.0 k
Ω)
compared to larger resistance devices
(100.0 k
(100.0 k
Ω).
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