4-1/2 DIGIT ANALOG-TO-DIGITAL
CONVERTER WITH ON-CHIP LCD DRIVERS
1
2
3
4
5
6
7
8
TC7129
C
C
R
REF
INT
INT
REF HI
REF LO
DE
DE
INTE-
GRATOR
X10
–
+
10
pF
COMPARATOR 1
INT
1
–
IN HI
+
–
TO DIGITAL
SECTION
BUFFER
DE–
DE+
DE+
DE–
+
100 pF
COMPARATOR 2
ZI, X10
COMMON
IN LO
INT
REST
INT INT
1,
2
–
+
500 kΩ
+
–
TC7129
V
CONTINUITY
COMPARATOR
200 mV
CONTINUITY
TO DISPLAY DRIVER
Figure 13. Integrator Block Diagram
Table I. Switch Legends
Label
Meaning
DE
Open during all deintegrate phases.
DE–
Closed during all deintegrate phases when input
voltage is negative.
–
+
DE+
INT1
INT2
Closed during all deintegrate phases when input
voltage is positive.
IN HI
COM
BUFFER
Closed during the first integrate phase
(measurement of the input voltage).
Closed during the second integrate phase
(measurement of the amplifier offset).
INT
REST
ZI
Open during both integrate phases.
Closed during the rest phase.
TC7129
Closed during the zero-integrate phase.
Closed during the X10 phase.
Open during the X10 phase.
X10
X10
IN LO
CONT
–
+
500 kΩ
200 mV
V
TO DISPLAY
DRIVER
(NOT LATCHED)
The buffer amplifier has a common-mode input voltage
range from 1.5V above V– to 1V below V+. The integrator
amplifier can swing to within 0.3V of the rails, although for
best linearity the swing is usually limited to within 1V. Both
amplifiers can supply up to 80 µA of output current, but
should be limited to 20 µA for good linearity.
Figure 14. Continuity Indicator Circuit
CONTINUITY output (pin 27) will be pulled HIGH, activating
the continuity annunciator on the display. The continuity
pin can also be used as an input to drive the continuity
annunciator directly from an external source. A schematic
of the input/output nature of this pin is shown in Figure 15.
Continuity Indicator
A comparator with a 200 mV threshold is connected
between IN HI (pin 33) and IN LO (pin 32). Whenever the
voltage between inputs is less than 200 mV, the
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3-243
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