TC7129_06 Datasheet, PDF (15/28 Pages) Microchip Technology – 4-1/2 Digit Analog-to-Digital Converters with On-Chip LCD Drivers

4.11 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
during the separate measurement phases described
earlier. (See Figure 4-12).
TABLE 4-2: SWITCH LEGENDS
Label
Description
Label
DE
DE–
DE+
INT₁
INT₂
INT
REST
ZI
X10
X10
Meaning.
Open during all de-integrate phases.
Closed during all de-integrate phases when
input voltage is negative.
Closed during all de-integrate phases when
input voltage is positive.
Closed during the first integrate phase
(measurement of the input voltage).
Closed during the second integrate phase
(measurement of the amplifier offset).
Open during both integrate phases.
Closed during the rest phase.
Closed during the zero integrate phase.
Closed during the X10 phase.
Open during the X10 phase.
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.
However, 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.
4.12 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
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 (see Figure 4-13).
A schematic of the input/output nature of this pin is also
shown in Figure 4-14.
TC7129
IN HI
–
+
Buffer
COM
IN LO
CONT
–
200 mV
V
+
TC7129
500 kΩ
To Display Driver
(Not Latched)
Figure 4-13:
Continuity Indicator Circuit.
TC7129
DP₄/OR, Pin 20
DP₃/UR, Pin 21
LATCH/HOLD Pin 22
CONTINUITY, Pin 27
500 kΩ
Figure 4-14:
Input/Output Pin Schematic.
4.13 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 12 μA; DGND has essentially no
source capability (see Figure 4-15).
© 2006 Microchip Technology Inc.
DS21459D-page 15