7

FN3086.6

July 21, 2005

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:

.

Zero Integrator Phase

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

analog COMMON. Second, the reference capacitor is charged

to the reference voltage. Finally, a feedback loop is closed

around the system to IN HI to cause the integrator output to

return to zero. Under normal conditions, this phase lasts for

between 11 to 140 clock pulses, but after a “heavy” overrange

conversion, it is extended to 740 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, 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 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 0.3V 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 READING = 1000

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







DE-

DE+

BUFFER

A-Z

INT

-

+

A-Z

COMPARATOR

IN HI

COMMON

IN LO

31

32

30

DE-

DE+

INT

A-Z

34

+

36

REF HI

REF LO

35

A-Z,

A-Z,

33

28

29

27

TO

DIGITAL

SECTION

A-Z AND DE

(±)

INTEGRATOR

INT

STRAY

STRAY

V+

10

µA

V-

N

INPUT

HIGH

2.8V

6.2V

V+

1

INPUT

LOW

-

+

-

+

-

+

ZI

ZI

AND ZI

ZI

26

FIGURE 2. ANALOG SECTION OF ICL7136

ICL7136