TC7106/A/TC7107/A
DS21455D-page 20 © 2008 Microchip Technology Inc.
10.0 TYPICAL APPLICATIONS
10.1 Decimal Point and Annunciator
Drive
The TEST pin is connected to the internally generated
digital logic supply ground through a 500Ω 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 1 mA should be
supplied by the TEST pin; its potential is approximately
5V below V+ (see Figure 10-1
).
FIGURE 10-1: Decimal Point Drive Using
Test as Logic Ground.
10.2 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 determined from the
following expression:
EQUATION 10-1:
The display will over range for:
R
UNKNOWN
≥
2 x R
STANDARD
FIGURE 10-2: Low Parts Count
Ratiometric Resistance Measurement.
FIGURE 10-3: Temperature Sensor.
TC7106A
BP
TEST
37
21
V+
V+
GND
To LCD
Decimal
Point
To LCD
Decimal
Point
To LCD
Backplane
4049
TC7106A
Decimal
Point
Select
V+
V+
TEST
GND
4030
BP
Displayed (Reading)
R
UNKNOWN
R
STANDARD
-----------------------------
1000
×
=
V
REF
+
V
REF
-
V
IN
+
V
IN
-
Analog
Common
TC7106A
LCD Display
R
STANDARD
R
UNKNOWN
V+
V+
V-
V
IN
-
V
IN
+
V
REF
+
V
REF
-
Common
50 k
Ω
R
2
160 k
Ω
300 k
Ω
300k
Ω
R
1
50 k
Ω
1N4148
Sensor
9V
+
TC7106A
V
FS
= 2V
