5

FN3093.4

Detailed Description

Analog Section

Figure 3 shows the Block Diagram of the Analog Section for

the ICL7135. Each measurement cycle is divided into four

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

(INT), (3) de-integrate (DE) and (4) zero-integrator (Zl).

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

for offset voltages in the buffer amplifier, integrator, and

comparator. Since the comparator is included in the loop, the

AZ accuracy 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

internal 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; within one volt of 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 latched into

the polarity F/F.

De-Integrate Phase

The third 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 inte-

grator output to return to zero. The time required for the out-

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

Specifically the digital reading displayed is:

.

Zero Integrator Phase

The final phase is zero integrator. First, input low is shorted

to analog COMMON. Second, a feedback loop is closed

around the system to input high to cause the integrator

output to return to zero. Under normal condition, this phase

lasts from 100 to 200 clock pulses, but after an overrange

conversion, it is extended to 6200 clock pulses.

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

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

However, since the integrator also swings with the common

mode voltage, care must be exercised to assure the integrator

output 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 applications the integrator swing can be reduced to

less than the recommended 4V full scale swing with some

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

of either supply without loss of linearity.

Analog COMMON

Analog COMMON is used as the input low return during auto-

zero and de-integrate. If IN LO is different from analog

COMMON, a common mode voltage exists in the system and

is taken care of by the excellent CMRR of the converter.

However, in most applications IN LO will be set at a fixed

known voltage (power supply common for instance). In this

application, analog COMMON should be tied to the same

point, thus removing the common mode voltage from the

converter. The reference voltage is referenced to analog

COMMON.

Reference

The reference input must be generated as a positive voltage

with respect to COMMON, as shown in Figure 4.

OUTPUT COUNT

10,000

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

⎝⎠

⎜⎟

⎛⎞

=

FIGURE 4A.

FIGURE 4B.

FIGURE 4. USING AN EXTERNAL REFERENCE

6.8V

V-

ZENER

REF HI

ICL7135

COMMON

V+

6.8k

Ω

ICL8069

REF HI

ICL7135

COMMON

V+

V+

20k

Ω

1.2V

REFERENCE

ICL7135