MCP47FXB2XT-EST datasheet(66/124 Pages) MICROCHIP

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MCP47FXBX4/8

DS20006368A-page 66

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5.1

Resistor Ladder

The resistor ladder is a digital potentiometer with the B

Terminal internally grounded and the A Terminal

connected to the selected reference voltage (see

Figure 5-2). The volatile DAC register controls the

wiper position. The wiper voltage (VW) is proportional to

the DAC register value divided by the number of resis-

tor elements (RS) in the ladder (256, 1024 or 4096)

related to the VRL voltage.

The output of the resistor network will drive the input of

an output buffer.

FIGURE 5-2:

Resistor Ladder Model.

The resistor network is made of these three parts:

• Resistor ladder (string of RS elements)

• Wiper switches

• DAC register decode

The resistor ladder (RRL) has a typical impedance of

approximately 120 k

. This resistance may vary from

device to device by up to ±20%. Since this is a voltage

divider configuration, the actual RRL resistance does

not affect the output given a fixed voltage at VRL.

Equation 5-1 shows the calculation for the step

resistance:

EQUATION 5-1:

RS CALCULATION

If the unbuffered VREF pin is used as the VRL voltage

source, this voltage source should have a low output

impedance.

When the DAC is powered down, the resistor ladder is

disconnected from the selected reference voltage.

5.2

Voltage Reference Selection

The resistor ladder has up to four sources for the

reference voltage. Two user control bits (VRnB:VRnA)

are used to control the selection, with the selection con-

nected to the VRL node (see Figure 5-3 and Figure 5-4).

FIGURE 5-3:

Resistor Ladder Reference

Voltage Selection Block Diagram.

The four voltage source options for the resistor ladder

are:

VDD pin voltage

Internal voltage reference (VBG)

VREF pin voltage unbuffered

VREF pin voltage internally buffered

Analog Mux

Note 1: The analog switch resistance (RW)

does not affect performance due to the

voltage divider configuration.

RS(2)

DAC

RS(1)

RS(2n - 1)

RS(2n - 2)

RS(2n - 3)

RS(2n)

VRL

RRL

RW (1)

2n - 1

2n - 2

2n - 3

PDnB:PDnA

Note:

The maximum wiper position is 2n –1,

while the number of resistors in the

resistor ladder is 2n. This means that

when the DAC register is at full scale,

there is one resistor element (RS)

between the wiper and the VRL voltage.

RRL

256



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

RRL

1024



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

8-bit Device

10-bit Device

RRL

4096



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

12-bit Device

VRL

VDD

Buffer

VREF

Band Gap

VRnB:VRnA

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