MCP47CMB02 datasheet(60/124 Pages) MICROCHIP

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MCP47CXBXX

DS20006089B-page 60

 2018-2019 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

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.

FIGURE 5-2:

Resistor Ladder Model

Block Diagram.

EQUATION 5-1:

RS CALCULATION

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.

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

RRL

256



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

RRL

1024



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

8-Bit Device

10-Bit Device

RRL

4096



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

12-Bit Device

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