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.

Digital Section

Figures 7 and 8 show the digital section for the

, an internal digital

ground is generated from a 6V Zener diode and a large

P-Channel source follower. This supply is made stiff to

absorb the relative large capacitive currents when the back

plane (BP) voltage is switched. The BP frequency is the

clock frequency divided by 800. For three readings/sec., this

is a 60Hz square wave with a nominal amplitude of 5V. The

segments are driven at the same frequency and amplitude

and are in phase with BP when OFF, but out of phase when

ON. In all cases negligible DC voltage exists across the

segments.

Figure 8 is the Digital Section of the

7106 except that the regulated supply and back

plane drive have been eliminated and the segment drive has

been increased from 2mA to 8mA, typical for instrument size

common anode LED displays. Since the 1000 output (pin 19)

must sink current from two LED segments, it has twice the

drive capability or 16mA.

In both devices, the polarity indication is “on” for negative

analog inputs. If IN LO and IN HI are reversed, this indication

can be reversed also, if desired.

V+

BP

TEST

DECIMAL

POINT

SELECT

CD4030

GND

V+

TO LCD

DECIMAL

POINTS

FIGURE 6. EXCLUSIVE ‘OR’ GATE FOR DECIMAL POINT DRIVE

7

SEGMENT

DECODE

SEGMENT

OUTPUT

0.5mA

2mA

INTERNAL DIGITAL GROUND

TYPICAL SEGMENT OUTPUT

V+

LCD PHASE DRIVER

LATCH

7

SEGMENT

DECODE

LOGIC CONTROL

INTERNAL

7

SEGMENT

DECODE

1000’s

100’s

10’s

1’s

TO SWITCH DRIVERS

FROM COMPARATOR OUTPUT

DIGITAL

GROUND

CLOCK

40

39

38

OSC 1

OSC 2

OSC 3

BACKPLANE

21

V+

TEST

V-

500

Ω

37

26

6.2V

COUNTER

COUNTER

COUNTER

COUNTER

1

c

a

b

c

d

f

g

e

a

b

a

b

c

d

f

g

e

a

b

c

d

f

g

e

†

ONE INVERTER SHOWN FOR CLARITY

7106 DIGITAL SECTION

WS7106 / WS7107

WS7106

FIGURE 7. WS

WS7106 and

WS7107, respectively. In the WS7106

WS7107. It is identical

to the WS

7