ICL7116 datasheet(6/14 Pages) INTERSIL | 31/2 Digit, LCD/LED Display, A/D Converter with Display Hold

6 page

Detailed Description

Analog Section

Figure 3 shows the Analog Section for the ICL7116 and

ICL7117. Each measurement cycle is divided into three

phases. They are (1) auto-zero (A-Z), (2) signal integrate

(INT) and (3) de-integrate (DE).

Auto-Zero Phase

During auto-zero three things happen. First, input high and low

are disconnected from the pins and internally shorted to analog

COMMON. Second, the reference capacitor is charged to the

reference voltage. Third, a feedback loop is closed around the

system to charge the auto-zero capacitor CAZ to compensate

for offset voltages in the buffer ampliﬁer, integrator, and compar-

ator. Since the comparator is included in the loop, the A-Z accu-

racy is limited only by the noise of the system. In any case, the

offset referred to the input is less than 10

µV.

Signal Integrate Phase

During signal integrate, the auto-zero loop is opened, the inter-

nal short is removed, and the internal input high and low are

connected to the external pins. The converter then integrates

the differential voltage between IN HI and IN LO for a ﬁxed time.

This differential voltage can be within a wide common mode

range: up to 1V from either supply. If, on the other hand, the

input signal has no return with respect to the converter power

supply, IN LO can be tied to analog COMMON to establish the

correct common mode voltage. At the end of this phase, the

polarity of the integrated signal is determined.

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 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. The time required for the

output to return to zero is proportional to the input signal.

Speciﬁcally the digital reading displayed is:

Differential Input

The input can accept differential voltages anywhere within the

common mode range of the input ampliﬁer, or speciﬁcally from

0.5V below the positive supply to 1V 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 2V full scale swing with little loss of

accuracy. The integrator output can swing to within 0.5V of

either supply without loss of linearity.

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

reference capacitor can gain charge (increase voltage) when

called up to de-integrate a positive signal but lose charge

(decrease voltage) when called up to de-integrate 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 such that it is 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.)

DISPLAY COUNT

1000

REF

-----------------







FIGURE 3. ANALOG SECTION OF ICL7116 AND ICL711

DE-

DE+

CINT

CAZ

RINT

BUFFER

A-Z

INT

A-Z

COMPARATOR

IN HI

COMMON

IN LO

DE-

DE+

INT

A-Z

CREF+

REF HI

CREF

A-Z

CREF-

DIGITAL

SECTION

A-Z AND DE

(±)

INTEGRATOR

INT

STRAY

V+

µA

INPUT

HIGH

2.8V

6.2V

V+

INPUT

LOW

A-Z

ICL7116, ICL7117

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