MCP48CMB21 datasheet(56/106 Pages) MICROCHIP

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MCP48CXBXX

DS20006160A-page 56

 2019 Microchip Technology Inc.

5.1

Resistor Ladder

The resistor ladder is a digital potentiometer with the

A Terminal connected to the selected reference voltage

and

the

B Terminal

internally

grounded

(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.

The Resistor Network is made up of three parts:

• Resistor Ladder (string of RS elements)

• Wiper switches

• DAC register decode

The resistor ladder has a typical impedance (RRL) of

approximately 71 k

. This resistor ladder resistance

(RRL) may vary from device to device up to ±10%.

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.

If the unbuffered VREF pin is used as the VRL voltage

source, the external voltage source should have a low

output impedance.

When the DAC is powered down, the resistor ladder is

disconnected from the selected reference voltage.

EQUATION 5-1:

RS CALCULATION

FIGURE 5-2:

Resistor Ladder Model

Block Diagram.

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

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

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