TC7129
FIGURE 4-13:
CONTINUITY INDICATOR
CIRCUIT
4.14 Integrator Section
The integrator section includes the integrator, compar-
ator, input buffer amplifier, and analog switches (see
Table 4-2), used to change the circuit configuration dur-
ing the separate measurement phases described ear-
lier. See Integrator Block Diagram (Figure 4-12).
–
+
IN HI
COM
Buffer
TABLE 4-2:
SWITCH LEGENDS
Label
Description
Label
DE
Meaning.
TC7129
Open during all de-integrate phases.
DE–
Closed during all de-integrate phases when
input voltage is negative.
IN LO
–
500kΩ
DE+
INT1
INT2
Closed during all de-integrate phases when
input voltage is positive.
200mV
V
To Display Driver
(Not Latched)
+
Closed during the first integrate phase (mea-
surement of the input voltage).
CONT
Closed during the second integrate phase
(measurement of the amplifier offset).
INT
REST
ZI
Open during both integrate phases.
Closed during the rest phase.
FIGURE 4-14:
INPUT/OUTPUT PIN
SCHEMATIC
Closed during the zero integrate phase.
Closed during the X10 phase.
Open during the X10 phase.
X10
X10
TC7129
The buffer amplifier has a Common mode input voltage
range from 1.5V above V- to 1V below V+. The integra-
tor 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 80µA of out-
put current, but should be limited to 20µA for good
linearity.
500kΩ
DP /OR, Pin 20
4
DP /UR, Pin 21
3
LATCH/HOLD Pin 22
CONTINUITY, Pin 27
4.15 Continuity Indicator
A comparator with a 200mV threshold is connected
between IN HI (Pin 33) and IN LO (Pin 32). Whenever
the voltage between inputs is less than 200mV, the
CONTINUITY output (Pin 27) will be pulled HIGH, acti-
vating 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
(see Figure 4-13).
4.16 Common and Digital Ground
The common and digital ground (DGND) outputs are
generated from internal zener diodes. The voltage
between V+ and DGND is the internal supply voltage
for the digital section of the TC7129. Common can
source approximately 12µA; DGND has essentially no
source capability (see Figure 4-15).
A schematic of the input/output nature of this pin is also
shown in Figure 4-14.
2002 Microchip Technology Inc.
DS21459B-page 15
MICROCHIP [ MICROCHIP ]
