TC7129(2002) View Datasheet(PDF) - Microchip Technology
Part Name
Description
Manufacturer
TC7129
(Rev.:2002)
(Rev.:2002)
Microchip Technology
TC7129 Datasheet PDF : 24 Pages
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TC7129
FIGURE 3-1:
STANDARD CIRCUIT
Low Battery
Continuity
V+
5pF
CO1
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
330kΩ
CINT
0.1µF
CREF+
1µF
+
9V
150kΩ
RINT
10kΩ
RBIAS
0.1
RREF DREF
µF
20
kΩ
CIF
RIF
100kΩ
–
+
VIN
RO
10pF
CO2
CRF
V+
0.1µF
3.3 Integrating Capacitor (CINT)
The charge stored in the integrating capacitor during
the integrate phase is directly proportional to the input
voltage. The primary selection criterion for CINT 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 3-3:
CINT =
tINT x IINT
VSWING
Where tINT is the integration time.
Using the values derived above (assuming 60Hz
operation), the equation becomes:
EQUATION 3-4:
CINT = 16.7msec x 13.3µA = 0.1µA
2V
The capacitor should have low dielectric absorption to
ensure good integration linearity. Polypropylene and
Teflon capacitors are usually suitable. A good mea-
surement of the dielectric absorption is to connect the
reference capacitor across the inputs by connecting:
Pin to Pin:
20 → 33 (CREF+ to IN HI)
30 → 32 (CREF- to IN LO)
A reading between 10,000 and 9998 is acceptable;
anything lower indicates unacceptably high dielectric
absorption.
3.4 Reference Capacitor (CREF)
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 1µF is recommended.
DS21459B-page 8
© 2002 Microchip Technology Inc.
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