3-251

TELCOM SEMICONDUCTOR, INC.

7

6

5

4

3

1

2

8

GENERAL THEORY OF OPERATION

(All Pin designations refer to 40-Pin Dip)

Dual-Slope Conversion Principles

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

digital converter. An understanding of the dual-slope con-

version technique will aid in following detailed TC7136/A

operational theory.

The conventional dual-slope converter measurement

cycle has two distinct phases:

(1) Input signal integration

(2) Reference voltage integration (deintegration)

The input signal being converted is integrated for a fixed

opposite polarity constant reference voltage is then inte-

grated until the integrator output voltage returns to zero. The

reference integration time is directly proportional to the input

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:

TC7136/A PIN DESCRIPTION (Cont.)

Pin No.

40-Pin PDIP

Normal

(Reverse)

Name

Description

33

(8)

C

–

REF

See pin 34.

34

(7)

C

+

REF

A 0.1

µF capacitor is used in most applications. If a large common-mode

mV scale is used, a 1

µF capacitor is recommended and will hold the roll-over

error to 0.5 count.

35

(6)

V

–

REF

See pin 36.

(5)

V

–

REF

The analog input required to generate a full-scale output (1999 counts). Place

100 mV between pins 35 and 36 for 199.9 mV full scale. Place 1V between

pins 35 and 36 for 2V full scale. See paragraph on Reference Voltage.

36

(4)

TEST

display should read –1888. It may also be used as a negative supply for exter-

nally-generated decimal points. See paragraph under Test for additional informa-

tion.

37

(3)

See pin 40.

38

(2)

See pin 40.

39

(1)

Pins 40, 39 and 38 make up the oscillator section. For a 48 kHz clock (3 readings per

second) connect pin 40 to the junction of a 180 k

Ω resistor and a 50 pF capacitor. The

180 k

Ω resistor is tied to pin 39 and the 50 pF capacitor is tied to pin 38.

.

RC

RC

,

where:

Figure 1. Basic Dual-Slope Converter

REF

VOLTAGE

ANALOG

INPUT

SIGNAL

DISPLAY

SWITCH

DRIVER

CONTROL

LOGIC

CLOCK

COUNTER

POLARITY CONTROL

PHASE

CONTROL

VIN

VIN

VFULL SCALE

1.2 VFULL SCALE

VARIABLE

REFERENCE

INTEGRATE

TIME

FIXED

SIGNAL

INTEGRATE

TIME

INTEGRATOR

CINT

COMPARATOR

[ ]

0

∫

LOW POWER, 3-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTERS

TC7136

TC7136A