3-1/2 DIGIT A/D CONVERTERS
TC7106
TC7106A
TC7107
TC7107A
Ratiometric Resistance Measurements
Decimal Point and Annunciator Drive
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:
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
is approximately 5V below V+.
+
V
+
V
4049
R Unknown
Displayed Reading =
x 1000
TC7106A
R Standard
TO LCD
DECIMAL
POINT
21
BP
The display will overrange for R Unknown ≥ 2 x R
standard.
GND
37
TEST
TO LCD
BACK-
PLANE
+
+
V
V
REF
–
V
R
REF
STANDARD
+
LCD DISPLAY
+
V
V
IN
+
R
V
UNKNOWN
TC7106A
BP
–
V
IN
TO LCD
DECIMAL
POINTS
DECIMAL
ANALOG
COMMON
POINT
TC7106A
SELECT
Figure 17. Low Parts Count Ratiometric Resistance
Measurement
4030
GND
TEST
Figure 16. Decimal Point Drive Using TEST as Logic Ground
3-198
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
