ICL7106CPL datasheet(7/17 Pages) ARTSCHIP

7 / 17 page

ICL7106,ICL7107,ICL7016S,ISL7107S

31/2 Digit,

LCD/LED Display, A/D Converters

www.artschip.com

FIGURE 3. ANALOG SECTION OF ICL7106 AND ICL7107

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 temperature coefficient typically

less than 80ppm/x ℃.

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

reference

Temperature

Coefficient (TC), internal chip dissipation, and package

thermal resistance can increase noise near full scale from 25

µV to 80 µVp-p. Also the linearity in going from a high

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 IC, 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.

ICL7106CPL Datasheet(PDF) 7 Page - ARTSCHIP ELECTRONICS CO.,LMITED.

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