MCP48FVB21 datasheet(38/84 Pages) MICROCHIP

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MCP48FVBXX

DS20005466A-page 38

 2015 Microchip Technology Inc.

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

resistor 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 these three parts:

• Resistor Ladder (string of RS elements)

• Wiper switches

• DAC Register decode

The resistor ladder (RRL) has a typical impedance of

approximately 140 k

. This resistor ladder resistance

(RRL) 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.

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.

256



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

1024



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

8-bit device

10-bit device

4096



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

12-bit device

RS(2n)

RS(2n - 1)

RS(2n - 2)

RS(1)

2n - 1

2n - 2

RRL

VRL

DAC

PD1:PD0

Analog Mux

Where:

# of Resistors in

Resistor Ladder

256 (MCP48FVB0X)

= 1024 (MCP48FVB1X)

= 4096 (MCP48FVB2X)

DAC Register Value

# of Resistors in Resistor Ladder

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



Note 1:

The analog switch resistance (RW) does

not affect performance due to the voltage

divider configuration.

(1)

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