ICL7109CPL datasheet(9/25 Pages) INTERSIL

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De-Integrate Phase

The final phase is de-integrate, or reference integrate. Input

low is internally connected to analog COMMON and input

high is connected across the previously charged (during

auto-zero) reference capacitor. Circuitry within the chip

ensures that the capacitor will be connected with the correct

polarity to cause the integrator output to return to zero cross-

ing (established in Auto-Zero) with a fixed slope. The time

required for the output to return to zero is proportional to the

input signal.

Differential Input

The input can accept differential voltages anywhere within the

common mode range of the input amplifier, or specifically from

1V below the positive supply to 1.5V above the negative sup-

ply. In this range, the system has a CMRR of 86dB typical.

However, care must be exercised to assure the integrator out-

put does not saturate. A worst case condition would be a large

positive common mode voltage with a near full-scale negative

differential input voltage. The negative input signal drives the

integrator positive when most of its swing has been used up

by the positive common mode voltage. For these critical appli-

cations the integrator output swing can be reduced to less

than the recommended 4V full scale swing with little loss of

accuracy. The integrator output can swing to within 0.3V of

either supply without loss of linearity.

The ICL7109 has, however, been optimized for operation

with analog common near digital ground. With power sup-

plies of +5V and -5V, this allows a 4V full scale integrator

swing positive or negative thus maximizing the performance

of the analog section.

Differential Reference

The reference voltage can be generated anywhere within the

power supply voltage of the converter. The main source of

common mode error is a roll-over voltage caused by the

reference capacitor losing or gaining charge to stray capacity

on its nodes. If there is a large common mode voltage, the ref-

erence capacitor can gain charge (increase voltage) when

called up to deintegrate a positive signal but lose charge

(decrease voltage) when called up to deintegrate a negative

input signal. This difference in reference for positive or

negative input voltage will give a roll-over error. However, by

selecting the reference capacitor large enough in comparison

to the stray capacitance, this error can be held to less than 0.5

count worst case. (See Component Value Selection.)

The roll-over error from these sources is minimized by having

the reference common mode voltage near or at analog

COMMON.

Component Value Selection

For optimum performance of the analog section, care must

be taken in the selection of values for the integrator capaci-

tor and resistor, auto-zero capacitor, reference voltage, and

conversion rate. These values must be chosen to suit the

particular application.

The most important consideration is that the integrator out-

put swing (for full-scale input) be as large as possible. For

example, with

±5V supplies and COMMON connected to

DE-

DE+

CINT

CAZ

RINT

BUFFER

A-Z

INT

A-Z

IN HI

COMMON

IN LO

DE-

DE+

INT

A-Z

CREF+

REF IN+

CREF

REF IN-

A-Z

CREF-

TO ZERO CROSS

DETECTOR

DIGITAL SECTION

A-Z

INTEGRATOR

INT

DE(

±)

BUFFER

COMPARATOR

REF OUT

6.2V

µA

V-

V+

INT

DE+

DE-

FROM CONTROL

LOGIC

DIGITAL SECTION

FIGURE 2. ANALOG SECTION OF ICL7109

ICL7109

Teflon

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