MCP436X datasheet(39/80 Pages) MICROCHIP

39 / 80 page

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. 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

(1)

(01h)

Analog Mux

(1)

(00h)

(1)

(FEh)

(1)

(FFh)

(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

Ω)

compared to larger resistance devices

(100.0 k

Ω).

RAB

8-Bit

N =

128

127

126

(01h)

(00h)

(7Eh)

(7Fh)

(80h)

7-Bit

N =

256

()

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

128

()

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

8-bit Device

7-bit Device

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