5

TC811

TC811-7 11/5/96

© 2001 Microchip Technology Inc.

DS21472A

3-1/2 Digit Analog-To-Digital Converter with

Hold and Differential Reference Inputs

where:

GENERAL THEORY OF OPERATION

Dual-Slope Conversion Principles

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

The TC811 is a dual slope, integrating analog-to-digital

converter. An understanding of the dual slope conversion

technique will aid the user in following the detailed TC811

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

reference voltage of the opposite polarity is then integrated

until the integrator output voltage returns to zero. The

In a simple dual slope converter, a complete conversion

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:

30

20

10

0

0.1/T

1/T

10/T

INPUT FREQUENCY

T = MEASUREMENT PERIOD

Figure 2. 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

C

COMPARATOR

≈

≈

VREF

+

TC811

33

35

240k

Ω

10k

Ω

31

29

39

40

VREF

–

0.47

µF

0.1

µF

V

–

1

OSC

2

OSC

TO ANALOG COMMON

(PIN 32)

2 CONVERSION/SEC

180k

Ω

0.068

µF

0.01

µF

ANALOG

INPUT

+

–

C

REF

34

C

REF

+

VIN

+

VIN

–

ANALOG

COMMON

VINT

VBUFF

CAZ

20

21

38

SEGMENT

DRIVE

9–19

22–25

POL

BP

V

+

MINUS SIGN

BACKPLANE

28

LCD

1M

Ω

27

30

32

33

36

9V

+

26

20pF

10pF

V +

V +

22M

Ω

470k

1

HLDR

0

∫

1

[ ]

Figure 3. Normal-Mode Rejection of

Dual Slope Converter

Figure 1. Typical Operating Circuit