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TC811

3-1/2 Digit Analog-To-Digital Converter with Hold and Differential Reference Inputs

Microchip Technology 

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

Hold and Differential Reference Inputs

TC811

0.1µF

+

ANALOG

INPUT

–

1MΩ

0.01µF

180kΩ

0.068µF

35

CR+ EF

31 VI+N

33

C

34

REF

9–19

22–25

SEGMENT

DRIVE

30 VI–N TC811

32

ANALOG

COMMON

POL 20

BP 21 MINUS SIGN

V+ 38

LCD

BACKPLANE

28 VBUFF

0.47

µF

29 CAZ

27 VINT

OSC2

39

HLDR 1

VR+EF 36

VR–EF 33

V – 26

OSC1

40

240kΩ

10k Ω

+

9V

2 CONVERSION/SEC

22MΩ

TO ANALOG COMMON

(PIN 32)

20pF 470k

V+

10pF

V+

Figure 1. Typical Operating Circuit

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

(TINT), 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 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:

∫ 1

RINT CINT

tINT

VIN(t) dt =

0

VREF tDEINT

RINT CINT

where:

VREF = Reference voltage

tINT = Integration Time

tDEINT = Deintegration Time

ANALOG

INPUT

SIGNAL

C

INTEGRATOR

–

+

COMPARATOR

–

+

+/–

REF

VOLTAGE

SWITCH

DRIVER

PHASE

CONTROL

CONTROL

LOGIC

POLARITY CONTROL

CLOCK

DISPLAY

COUNTER

VIN ≈ VFULL SCALE

VIN ≈ 1.2 VFULL SCALE

FIXED

SIGNAL

INTEGRATE

TIME

VARIABLE

REFERENCE

INTEGRATE

TIME

Figure 2. Basic Dual Slope Converter

30

20

10

0

0.1/T

T = MEASUREMENT PERIOD

1/T

INPUT FREQUENCY

10/T

Figure 3. Normal-Mode Rejection of

Dual Slope Converter

For a constant VINT:

[ ] tDEINT

VIN = VREF tINT

© 2001 Microchip Technology Inc. DS21472A

5

TC811-7 11/5/96

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