TC7129_13 Datasheet, PDF (8/28 Pages) Microchip Technology – 4-1/2 Digit Analog-to-Digital Converters with On-Chip LCD Drivers

TC7129
Low Battery
Continuity
V+
5 pF
C_O1
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
Display Drive Outputs
TC7129
120
kHz
Crystal
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
330 kΩ
C_INT
0.1 µF
C_REF+
1 µF
+
9V
150 kΩ
R_INT
10 kΩ
R_BIAS
0.1
R_REF D_REF
µF
20
kΩ
C_IF
R_IF
100 kΩ
–
+
V_IN
R_O
10 pF
C_O2
C_RF
V+
0.1 µF
Figure 3-1:
Standard Circuit.
3.3 Integrating Capacitor (C_INT)
The charge stored in the integrating capacitor during
the integrate phase is directly proportional to the input
voltage. The primary selection criterion for C_INT is to
choose a value that gives the highest voltage swing
while remaining within the high-linearity portion of the
integrator output range. An integrator swing of 2V is the
recommended value. The capacitor value can be
calculated using the following equation:
EQUATION 3-1:
C_INT =
t_INT x I_INT
V_SWING
Where t_INT is the integration time.
Using the values derived above (assuming 60 Hz
operation), the equation becomes:
EQUATION 3-2:
C_INT = ^{16.7 msec x 13.3 A} = 0.1 A
2V
The capacitor should have low dielectric absorption to
ensure good integration linearity. Polypropylene and
Teflon^® capacitors are usually suitable. A good
measurement of the dielectric absorption is to connect
the reference capacitor across the inputs by
connecting:
Pin-to-Pin:
20  33 (C_REF+ to IN HI)
30  32 (C_REF– to IN LO)
A reading between 10,000 and 9998 is acceptable;
anything lower indicates unacceptably high dielectric
absorption.
3.4 Reference Capacitor (C_REF)
The reference capacitor stores the reference voltage
during several phases of the measurement cycle. Low
leakage is the primary selection criterion for this com-
ponent. The value must be high enough to offset the
effect of stray capacitance at the capacitor terminals. A
value of at least 1 F is recommended.
DS21459E-page 8
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