TC7126ACKW datasheet(9/22 Pages) MICROCHIP

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DS21458B-page 9

TC7126/A

3.0

DETAILED DESCRIPTION

(All Pin Designations Refer to 40-Pin PDIP.)

3.1

Dual Slope Conversion Principles

The TC7126A is a dual slope, integrating analog-to-

digital converter. An understanding of the dual slope

conversion technique will aid in following the detailed

TC7126/A operation theory.

The conventional dual slope converter measurement

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) (seeFigure3-1).

FIGURE 3-1:

BASIC DUAL SLOPE

CONVERTER

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 sig-

nal, reference voltage and integration time:

EQUATION 3-1:

For a constant VIN:

EQUATION 3-2:

The dual slope converter accuracy is unrelated to the

integrating resistor and capacitor values, as long as

they are stable during a measurement cycle. Noise

immunity is an inherent benefit. Noise spikes are inte-

grated or averaged to zero during integration periods.

Integrating ADCs are immune to the large conversion

errors that plague successive approximation converters

in high noise environments. Interfering signals with fre-

quency components at multiples of the averaging period

will be attenuated. Integrating ADCs commonly operate

with the signal integration period set to a multiple of the

50Hz/60Hz power line period (see Figure 3-2).

FIGURE 3-2:

NORMAL MODE

REJECTION OF DUAL

SLOPE CONVERTER

REF

Voltage

Analog

Input

Signal

Display

Switch

Driver

Control

Logic

Clock

Counter

Polarity Control

Phase

Control

VIN ≈ VREF

VIN ≈ 1.2 VREF

Variable

Reference

Integrate

Time

Fixed

Signal

Integrate

Time

Integrator

Comparator

Where:

VR = Reference voltage

TSI = Signal integration time (fixed)

TRI = Reference voltage integration time (variable)

TSI

VIN(t)dt =

∫

VRTRI

-------

0.1/t

1/t

10/t

Input Frequency

t = Measurement Period

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