3-198

TELCOM SEMICONDUCTOR, INC.

TC7106

TC7106A

TC7107

TC7107A

3-1/2 DIGIT A/D CONVERTERS

Decimal Point and Annunciator Drive

The TEST pin is connected to the internally-generated

digital logic supply ground through a 500

Ω resistor. The

TEST pin may be used as the negative supply for external

CMOS gate segment drivers. LCD display annunciators for

decimal points, low battery indication, or function indication

may be added without adding an additional supply. No more

than 1mA should be supplied by the TEST pin: its potential

R Unknown

R Standard

Figure 17. Low Parts Count Ratiometric Resistance

Measurement

Ratiometric Resistance Measurements

The true differential input and differential reference

make ratiometric reading possible. Typically in a ratiometric

operation, an unknown resistance is measured with respect

to a known standard resistance. No accurately defined

reference voltage is needed.

The unknown resistance is put in series with a known

standard and a current passed through the pair. The voltage

developed across the unknown is applied to the input and

the voltage across the known resistor is applied to the

reference input. If the unknown equals the standard, the

display will read 1000. The displayed reading can be deter-

mined from the following expression:

Displayed Reading =

x 1000

The display will overrange for R Unknown

≥ 2 x R

standard.

TC7106A

BP

TEST

37

21

V+

V+

GND

TO LCD

DECIMAL

POINT

TO LCD

BACK-

PLANE

4049

TC7106A

DECIMAL

POINT

SELECT

V

+

V

+

TEST

GND

4030

TO LCD

DECIMAL

POINTS

BP

Figure 16. Decimal Point Drive Using TEST as Logic Ground

+

–

+

–

ANALOG

COMMON

TC7106A

LCD DISPLAY

R

STANDARD

R

UNKNOWN

V

+