3-221

TELCOM SEMICONDUCTOR, INC.

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3-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTERS

TC7126

TC7126A

PIN DESCRIPTION (Cont.)

40-Pin PDIP

Pin Number

Normal

(Reverse)

Name

Description

34

(7)

C

+

REF

A 0.1

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

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.

40

(1)

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

39per 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.

Figure 1. Basic Dual-Slope Converter

where:

GENERAL THEORY OF OPERATION

(All Pin Designations Refer to the 40-Pin DIP)

Dual-Slope Conversion Principles

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

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

version technique will aid in following detailed TC7126A

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

An opposite polarity constant reference voltage is then

integrated until the integrator output voltage returns to

zero. The reference integration time is directly proportional

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:

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

COMPARATOR

RC

RC

,

0