MCP4716 datasheet(37/70 Pages) MICROCHIP

37 / 70 page

 2012 Microchip Technology Inc.

DS25154A-page 37

MCP47A1

6.0

RESISTOR NETWORK

The Resistor Network is made up of an RAB resistor

ladder. The RAB resistor has a typical resistance of

20 k

. Figure 6-1 shows a block diagram for the

resistor network 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 6.3 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 RAB resistances. 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

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 6.2.2). The RAB (and RS)

resistance has small variations over voltage and

temperature. The typical RAB resistance is 10k .

6.2.1

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 reg-

ister value is derived from the Serial Shift Register

value (see

Section 6.3).

Any variation of the wiper resistance does not effect the

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

output buffer.

6.2.2

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.

VZS is the VOUT voltage when the wiper code is at

zero-scale.

RFS =

VREF

IVREF

RZS =

VZS

IVREF

VS =

( VFS - VZS )

Where:

RS =

IVREF

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