7

FN3082.7

Analog COMMON

This pin is included primarily to set the common mode

voltage for battery operation (ICL7106) or for any system

where the input signals are floating with respect to the power

supply. The COMMON pin sets a voltage that is

approximately 2.8V more negative than the positive supply.

This is selected to give a minimum end-of-life battery voltage

of about 6V. However, analog COMMON has some of the

attributes of a reference voltage. When the total supply

voltage is large enough to cause the zener to regulate (>7V),

the COMMON voltage will have a low voltage coefficient

(0.001%/V), low output impedance (

≅15Ω), and a

The limitations of the on chip reference should also be

recognized, however. With the ICL7107, the internal heating

which results from the LED drivers can cause some

degradation in performance. Due to their higher thermal

resistance, plastic parts are poorer in this respect than

ceramic. The combination of reference Temperature

Coefficient (TC), internal chip dissipation, and package

thermal resistance can increase noise near full scale from

25

dissipation count such as 1000 (20 segments on) to a low

dissipation count such as 1111(8 segments on) can suffer by

a count or more. Devices with a positive TC reference may

require several counts to pull out of an over-range condition.

This is because over-range is a low dissipation mode, with the

three least significant digits blanked. Similarly, units with a

negative TC may cycle between over-range and a non-over-

range count as the die alternately heats and cools. All these

problems are of course eliminated if an external reference is

used.

The ICL7106, with its negligible dissipation, suffers from

none of these problems. In either case, an external reference

can easily be added, as shown in Figure 4.

Analog COMMON is also 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 some 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 same holds true for the reference voltage. If

reference can be conveniently tied to analog COMMON, it

should be since this removes the common mode voltage

from the reference system.

Within the lC, analog COMMON is tied to an N-Channel FET

that can sink approximately 30mA of current to hold the

voltage 2.8V below the positive supply (when a load is trying

to pull the common line positive). However, there is only

10

µA of source current, so COMMON may easily be tied to a

more negative voltage thus overriding the internal reference.

TEST

The TEST pin serves two functions. On the ICL7106 it is

coupled to the internally generated digital supply through a

500

Ω resistor. Thus it can be used as the negative supply for

externally generated segment drivers such as decimal points

or any other presentation the user may want to include on

the LCD display. Figures 5 and 6 show such an application.

No more than a 1mA load should be applied.

The second function is a “lamp test”. When TEST is pulled

high (to V+) all segments will be turned on and the display

should read “1888”. The TEST pin will sink about 15mA

under these conditions.

CAUTION: In the lamp test mode, the segments have a constant DC

voltage (no square-wave). This may burn the LCD display if main-

tained for extended periods.

FIGURE 4A.

FIGURE 4B.

FIGURE 4. USING AN EXTERNAL REFERENCE

ICL7106

V

REF LO

ICL7107

REF HI

V+

V-

6.8V

ZENER

ICL7106

V

REF HI

REF LO

COMMON

V+

ICL8069

1.2V

REFERENCE

6.8k

Ω

20k

Ω

ICL7107

ICL7106

V+

BP

TEST

21

37

TO LCD

BACKPLANE

TO LCD

DECIMAL

POINT

1M

Ω

FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT

ICL7106, ICL7107, ICL7107S