MCP47DA1 datasheet(41/76 Pages) MICROCHIP

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DS25118D-page 41

MCP47DA1

6.0

RESISTOR NETWORK

The Resistor Network is made up of an R1 resistor, an

RAB resistor ladder, and an R2 resistor connected

together. These three resistors are equal (R1 = RAB =

R2) each with a typical resistance of 10k. The R1

resistor is also connected to the external VREF pin while

the R2 resistor is also internally connected to ground.

Figure 6-1 shows a block diagram for the resistor net-

work and output buffer. The resistance from the VREF

pin to ground is referred to as RVREF.

The 7-bit I2C data byte (00h-7Fh) is decoded to the 6-bit

wiper value (00h-40h). Section 6.4 describes the serial

shift buffer to wiper register decoding.

6.1

RVREF Resistance

RVREF resistance is the resistance from the VREF pin to

ground and is the sum of the R1, RAB, and R2 resis-

tances. Equation 6-1 shows how to calculate RVREF.

6.1.1

VREF PIN CURRENT (IVREF)

The current into the VREF pin is dependent on the volt-

age on the VREF pin (VREF) and the RVREF resistance.

The VREF pin’s voltage source current capability should

support a resistive load that is the minimum RVREF

resistance.

EQUATION 6-1:

CALCULATING RVREF

6.2

R1 and R2 Fixed Resistors

The R1 and R2 resistors are implemented so that based

on temperature and process variations, these resistors

track the RAB resistor ladder. The typical R1 and R2

resistances are 10k

 .

6.3

RAB Resistor Ladder

The RAB resistor ladder is a digital potentiometer in a

voltage divider configuration. The RAB resistor ladder

has 64 RS resistors in series. This resistor ladder has 65

wiper taps which allow wiper connectivity to the bottom

(terminal B), Zero Scale, and the top (terminal A), Full

Scale, of the resistor ladder (see Figure 6-1). With an

even number of RS resistors in the RAB ladder, when the

wiper is at the mid-scale value, VOUT equals VREF/2.

The RAB resistance also includes the RFS and RZS

resistances (see Section 6.3.2). The RAB (and RS)

resistance has small variations over voltage and

temperature. The typical RAB resistance is 10k.

6.3.1

THE WIPER

The value in the volatile wiper register selects which

analog switch to close, connecting the W terminal to

the selected node of the resistor ladder. The wiper

Section 6.4).

Any variation of the wiper resistance does not effect the

voltage at the W terminal, and therefore the input of the

output buffer.

6.3.2

RFS AND RZS RESISTORS

The RFS and RZS resistances are artifacts of the RAB

resistor implementation. These resistors are included

in the block diagram to help better model the actual

device operation. Equation 6-2 shows how to estimate

the RS, RFS, and RZS resistances, based on the

measured voltages of VREF, VFS, and VZS and the

measured current IVREF.

EQUATION 6-2:

ESTIMATING RS, RFS,

AND RZS

RVREF =

(VREF)

(IVREF)

VREF is the voltage on the VREF pin.

IVREF is the current into the VREF pin.

VFS is the VOUT voltage when the wiper code is at

full scale (SSR = 60h through 7Fh).

VZS is the VOUT voltage when the wiper code is at

zero scale (SSR = 00h through 20h).

RFS =

( (VREF - (64 * VS) ) - VFS )

(IVREF)

RZS =

( VZS - (64 * VS) )

(IVREF)

VS =

( VFS - VZS )

Where:

RS =

IVREF

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