TC7106A datasheet(8/32 Pages) MICROCHIP

8 / 32 page

TC7106/A/TC7107/A

DS21455C-page 8

3.0

DETAILED DESCRIPTION

(All Pin designations refer to 40-Pin PDIP.)

3.1

Dual Slope Conversion Principles

The TC7106A and TC7107A are dual slope, integrating

Analog-to-Digital Converters. An understanding of the

dual slope conversion technique will aid in following the

detailed 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). See

Figure 3-1.

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 signal, 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. An inher-

ent benefit is noise immunity. Noise spikes are

integrated or averaged to zero during the integration

periods. Integrating ADCs are immune to the large

conversion errors that plague successive approxima-

tion converters in high noise environments. Interfering

signals with frequency 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 50/60Hz 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

+/–

VRTRI

TSI

VIN(t)dt =

∫

Where:

VR = Reference voltage

TSI = Signal integration time (fixed)

TRI = Reference voltage integration time (variable).

VIN = VR

TRI

TSI

0.1/T

1/T

10/T

Input Frequency

T = Measured Period

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