TC7135CBU View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

TC7135CBU

4-1/2 digit A/D converter

Microchip Technology 

TC7135

3.0 DETAILED DESCRIPTION

All pin designations refer to the 28-pin PDIP package.

3.1 Dual-Slope Conversion Principles

The TC7135 is a dual-slope, integrating A/D converter.

An understanding of the dual-slope conversion

technique will aid in following the detailed TC7135

operational theory.

The conventional dual-slope converter measurement

cycle has two distinct phases:

1. Input signal integration.

2. Reference voltage integration (de-integration).

The input signal being converted is integrated for a

fixed time period. 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.

In a simple dual-slope converter, a complete

conversion 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:

∫ -----------1------------

RINTCINT

TI

0

N

T

VI

N

(

T

)

D

T

=

V----R----E---F---T----D---E---I--N----T-

RINTCINT

Where:

VREF = Reference voltage

TINT = Signal integration time (fixed)

TDEINT = Reference voltage integration time

(variable)

For a constant VIN:

EQUATION 3-2:

VIN

=

V----R----E---F---T----D---E----I--N---T-

TINT

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

inherent benefit is noise immunity. Noise spikes are

integrated, or averaged, to zero during the integration

periods.

Integrated ADCs are immune to the large conversion

errors that plague successive approximation converters

in high-noise environments (see Figure 3-1).

Analog Input

Signal

Integrator

-

+

Comparator

-

+

REF

Voltage

Switch

Drive

Phase

Control

Polarity Control

Control

Logic

Clock

Display

≈ VIN VREF

≈ VIN 1/2 VREF

Counter

Fixed

Signal

Integrate

Time

Variable

Reference

Integrate

Time

FIGURE 3-1:

Basic Dual-Slope Converter.

3.2 TC7135 Operational Theory

The TC7135 incorporates a system zero phase and

integrator output voltage zero phase to the normal two-

phase dual-slope measurement cycle. Reduced

system errors, fewer calibration steps and a shorter

overrange recovery time result.

The TC7135 measurement cycle contains four phases:

1. System zero.

2. Analog input signal integration.

3. Reference voltage integration.

4. Integrator output zero.

Internal analog gate status for each phase is shown in

Figure 3-1.

TABLE 3-1: INTERNAL ANALOG GATE STATUS

Conversion Cycle Phase

SWI SWRI+ SWRI- SWZ

SWR

SW1

System Zero

—

—

— Closed Closed Closed

Input Signal Integration

Closed

—

—

—

—

—

Reference Voltage Integration

— Closed* —

—

—

Closed

Integrator Output Zero

—

—

—

—

—

Closed

* Assumes a positive polarity input signal. SWRI would be closed for a negative input signal.

SWIZ

—

—

—

Closed

Reference Figures

Figure 3-2

Figure 3-3

Figure 3-4

Figure 3-5

DS21460C-page 6

 2004 Microchip Technology Inc.

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