TC7136ACPI datasheet(9/22 Pages) MICROCHIP

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DS21461B-page 9

TC7136/TC7136A

3.0

DETAILED DESCRIPTION

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

3.1

Dual Slope Conversion Principles

The TC7136/A is a dual slope, integrating analog-to-

digital converter. An understanding of the dual slope

conversion technique will aid in following detailed

TC7136/A operational theory.

The conventional dual slope converter measurement

cycle has two distinct phases (see Figure 3-1).

Input signal integration

Reference voltage integration (de-integration)

The input signal being converted is integrated for a

fixed time period (tSI), 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).

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:

FIGURE 3-1:

BASIC DUAL SLOPE

CONVERTER

FIGURE 3-2:

NORMAL MODE

REJECTION OF DUAL

SLOPE CONVERTER

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 convert-

ers 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 50Hz/60Hz power line period.

---------

() t

∫

------------

Where:

VR = Reference voltage

tSI = Signal integration time (fixed)

tRI = Reference voltage integration time

(variable)

--------

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

CINT

0.1/t

1/t

10/t

Input Frequency

t = Measured Period

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