TC7136
TC7136A
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 = RUNKNOWN × 1000.
RSTANDARD
The display will overrange for RUNKNOWN ≥
2 ×RSTANDARD.
Simple Inverter for Fixed Decimal Point
or Display Annunciator
V+
TC7136
TC7136A 21
BP
37
TEST
V+
4049
GND
Multiple Decimal Point or
Annunciator Driver
TO LCD
DECIMAL
POINT
TO LCD
BLACK
PLANE
V+
V+ BP
TC7136
TC7136A
DECIMAL
POINT
SELECT
TO LCD
DECIMAL
POINTS
TEST
4030
GND
3-258
Figure 11. Decimal Point and Annunciator Drives
LOW POWER, 3-1/2 DIGIT
ANALOG-TO-DIGITAL CONVERTERS
RSTANDARD
RUNKNOWN
VR+EF V+
VR–EF
VI+N
TC7136
TC7136A
VI–N
ANALOG
COMMON
LCD
Figure 12. Low Parts Count Ratiometric Resistance
Measurement
+ 9V
160 kΩ
1N4148
SENSOR
R2
50 kΩ
300 kΩ
R1
50 kΩ
300 kΩ
V+
V–
VI–N
VI+N
TC7136
TC7136A
VR+ EF
VR– EF
COMMON
Figure 13. Temperature Sensor
+ 9V
5.6 kΩ
160 kΩ
1N4148 R1
20 kΩ
0.7%/°C
PTC
R3
R2
20 kΩ
V+
V–
VI–N
VI+N
TC7136
TC7136A
VR+ EF
VR– EF
COMMON
Figure 14. Positive Temperature Coefficient Resistor
Temperature Sensor
TELCOM SEMICONDUCTOR, INC.