Differential Reference

The reference voltage can be generated anywhere within the

power supply voltage of the converter. The main source of com-

mon 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 volt-

age) 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.)

Analog COMMON

This pin is included primarily to set the common mode

voltage for battery operation

where the input signals are floating with respect to the power

supply. The COMMON pin sets a voltage that is approxi-

mately 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

which results from the LED drivers can cause some

degradation in performance. Due to their higher thermal resis-

tance, 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

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.

7106, 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

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.

FIGURE 4A.

FIGURE 4B.

FIGURE 4. USING AN EXTERNAL REFERENCE

V

REF LO

REF HI

V+

V-

6.8V

ZENER

V

REF HI

REF LO

COMMON

V+

ICL8069

1.2V

REFERENCE

6.8k

Ω

20k

Ω

V+

BP

TEST

21

37

TO LCD

BACKPLANE

TO LCD

DECIMAL

POINT

1M

Ω

FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT

WS7106 / WS7107

(WS7106) or for any system

WS7107, the internal heating

The WS

WS7106

WS7106

WS7107

WS7106

WS7107

WS7106 it is

6