
12
TC7136
TC7136A
Low Power 3-1/2 Digit Analog-To-
Digital Converters
TC7136-6
10/18/96
2001 Microchip Technology Inc.
DS21461A
Figure 11. Decimal Point and Annunciator Drives
Figure 13. Temperature Sensor
Figure 12. Low Parts Count Ratiometric Resistance
Measurement
TC7136
TC7136A
V
+
V
–
VIN
–
VIN
+
VREF
+
VREF
–
COMMON
5.6 k
160 k
R2
20 k
1N4148
9V
R1
20 k
+
R3
0.7%/
°C
PTC
TC7136
TC7136A
V
+
V
–
VIN
–
VIN
+
VREF
+
VREF
–
COMMON
50 k
R2
160 k
300 k
300 k
R1
50 k
1N4148
SENSOR
9V
+
Ratiometric Resistance Measurements
The TC7136A's true differential input and differential
reference make ratiometric readings possible. In 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 applied to the refer-
ence input. If the unknown equals the standard, the display
will read 1000. The displayed reading can be determined
from the following expression:
Displayed reading =
× 1000.
The display will overrange for RUNKNOWN ≥ 2 × RSTANDARD.
RUNKNOWN
RSTANDARD
VREF
+
VREF
–
VIN
+
VIN
–
ANALOG
COMMON
TC7136
TC7136A
LCD
RSTANDARD
RUNKNOWN
V
+
V+
TC7136
TC7136A
V+
TC7136
TC7136A
4049
4030
BP
TEST
BP
TEST
GND
TO LCD
DECIMAL
POINT
TO LCD
BLACK
PLANE
TO LCD
DECIMAL
POINTS
DECIMAL
POINT
SELECT
21
37
Multiple Decimal Point or
Annunciator Driver
Simple Inverter for Fixed Decimal Point
or Display Annunciator
Figure 14. Positive Temperature Coefficient Resistor
Temperature Sensor