MCP433X datasheet(43/88 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS22242A-page 43

MCP433X/435X

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 four resistor networks. These are

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

(RS) with a connection point (tap) between the two

resistors. The total number of resistors in the series

(ladder)

determines

the

RAB 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 RAB

(and RS) 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:

RS CALCULATION

257

256

255

RW (1)

(01h)

Analog Mux

RW (1)

(00h)

RW (1)

(FEh)

RW (1)

(FFh)

RW (1)

(100h)

Note 1: The wiper resistance is dependent on

several factors including, wiper code,

device VDD, Terminal voltages (on A, B

and W), and temperature.

Also for the same conditions, each tap

selection resistance has a small variation.

This RW variation has greater effects on

some specifications (such as INL) for the

smaller

resistance

devices

(5.0 k

)

compared to larger resistance devices

(100.0 k

).

RAB

8-Bit

N =

128

127

126

(01h)

(00h)

(7Eh)

(7Fh)

(80h)

7-Bit

N =

RAB

256



-------------

RAB

128



--------------

8-bit Device

7-bit Device

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