TC820CPL datasheet(8/22 Pages) TELCOM

8 / 22 page

3-156

TELCOM SEMICONDUCTOR, INC.

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

AND LOGIC PROBE

TC820

GENERAL THEORY OF OPERATION

Dual-Slope Conversion Principles

The TC820 analog-to-digital converter operates on the

principle of dual-slope integration. An understanding of the

dual-slope conversion technique will aid the user in follow-

ing the detailed TC820 theory of operation following this

section. A conventional dual-slope converter measurement

cycle has two distinct phases:

(1) Input Signal Integration

(2) Reference Voltage Integration (Deintegration)

Referring to Figure 2, the unknown input signal to be

converted is integrated from zero for a fixed time period

(t INT), measured by counting clock pulses. A constant

reference voltage of the opposite polarity is then integrated

until the integrator output voltage returns to zero. The

reference integration (deintegration) time (TDEINT) is then

directly proportional to the unknown input voltage (VIN).

In a simple dual-slope converter, a complete conver-

sion requires the integrator output to "ramp-up" from zero

and "ramp-down" back to zero. A simple mathematical

equation relates the input signal, reference voltage, and

integration time:

10 11 12 13 14 15

LOGIC HIGH

LOGIC LOW

OVERRANGE

PKHOLD

LOW BATT

FREQ/

VOLTS

LOGIC

DP0/LO

DP1/HI

DGND

COM

BUZ OUT

BUZ IN

PK HOLD

OSC1

OSC2 OSC3 VBUFF CAZ

VINT

VDD

VSS

REF

–

REF

TC820

REF C

–

REF

–

40kHz

470

Ω

100

Ω

0.47

µF

0.2

µF

0.1

µF

22M

Ω

NOTE:

Pin numbers are for

40-pin package.

PIEZO

BUZZER

DGND

VDD

µF

VDD

22k

Ω

VREF = 200mV

CHANGE

RANGE

SWITCH

S1a

NO DP

DP3

DP2

DP1

DGND

S1e

S1d

LOGIC

FREQ

GND

VIN

VDD

S1c

S1b

S1a

TO SWITCH

100k

Ω

0.01

µF

VDD

COM

RANGE/FREQ

–

REF

VOLTAGE

ANALOG

INPUT

SIGNAL

DISPLAY

SWITCH

DRIVER

CONTROL

LOGIC

CLOCK

COUNTER

POLARITY CONTROL

PHASE

CONTROL

VIN

VFULL SCALE

1.2 VFULL SCALE

VARIABLE

REFERENCE

INTEGRATE

TIME

FIXED

SIGNAL

INTEGRATE

TIME

INTEGRATOR

COMPARATOR

Figure 2. Basic Dual-Slope Converter

Figure 1. Typical Operating Circuit

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