TC826 datasheet(6/18 Pages) MICROCHIP

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TC826

DS21477B-page 6

3.0

DETAILED DESCRIPTION

3.1

Dual Slope Conversion Principles

The TC826 is a dual slope, integrating analog-to-digital

converter. The conventional dual slope converter mea-

surement cycle has two distinct phases:

• Input Signal Integration

• Reference Voltage Integration (De-integration)

The input signal being converted is integrated for a

fixed time period (TSI). Time is measured by counting

clock pulses. An opposite polarity constant reference

voltage is then integrated until the integrator output

voltage returns to zero. The reference integration time

is directly proportional to the input signal (TRI)

(Figure 3-1).

In a simple dual slope converter, a complete conver-

sion requires the integrator output to ‘ramp-up’ and

‘ramp-down’.

A simple mathematical equation relates the input signal

reference voltage and integration time:

EQUATION 3-1:

FIGURE 3-1:

BASIC DUAL SLOPE CONVERTER

tINT

VIN(t)dt =

VRTRI

∫

Where:

VR = Reference Voltage

VSI = Signal Integration Time (Fixed)

TRI = Reference Voltage Integration Time

(Variable)

–

Integrator

REF

Voltage

Switch Driver

–

Phase Control

Polarity Control

Control

Logic

Comparator

Clock

Counter

Display

Analog Input

Signal

+/–

Fixed Signal

Integrate

Time

Variable

Reference

Integrate

Time

VIN ≈ 1/2 VFULL SCALE

VIN ≈ 1/4 VFULL SCALE

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