MP8820AS datasheet(5/12 Pages) EXAR

5 / 12 page

MP8820

Rev. 1.00

THEORY OF OPERATION

The defining feature of the MP8820 is that it digitizes a bipolar

input signal centered around a given voltage, VMID. The peak to

peak swing of AIN is defined by the two input reference voltages,

VREF(+) and VREF(–).

The MP8820 takes in the center voltage and the two refer-

ence voltages and moves the resistor ladder endpoints VRT and

VRB of the ADC around VMID by (VREF(+) – VREF(–)). In this

way, a unipolar to bipolar translation can take place without hav-

ing to use both a positive and negative supply. The center volt-

age acts as a bipolar zero and signals that moves below it are

considered negative and signals that exceed it are taken to be

positive. The block diagram is shown in

Figure 1.

AIN0-AIN7

8:1 MUX

T/H

A0-A2

DB0-DB7

AIN

VRT

VMID

VRB

8-Bit

ADC

Control

–

G=1

–

G=1

–

G=1

VMID

VREF(+)

VREF(–)

VDD

GND

Figure 1. MP8820 Block Diagram

Unlike a unipolar system where one end of the ADC’s resistor

ladder is modulated above an offset voltage, both ends of the

MP8820’s reference chain expand or contract around a fixed

VMID. The maximum positive full scale voltage is VRT = VMID +

(VREF(+) – VREF(–)). The maximum negative full scale voltage is

VRB = VMID – (VREF(+) – VREF(–)). This type of translation is par-

ticularly useful in single supply applications where the input is

centered about user specified VMID.

The ideal transfer characteristic of the MP8820 is shown in

Figure 2. An actual transfer characteristic with associated error

terms is shown in

Figure 3.

VIN

VRT = VMID + (VREF(+) – VREF(–))

VRB = VMID = (VREF(+) – VREF(–))

11111111

00000000

CODE

OUT

Figure 2. Ideal Transfer Characteristics

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