4-1/2 DIGIT ANALOG-TO-DIGITAL
CONVERTER WITH ON-CHIP LCD DRIVERS
TC7129
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 remain-
ing 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 from the
equation:
C
INT
=
t
INT
x I
INT
,
V
SWING
1
Voltage Reference
(D
REF
, R
REF
, R
BIAS
, C
RF
)
A TC04 band-gap reference provides a high-stability
voltage reference of 1.25V. The reference potentiometer
(R
REF
) provides an adjustment for adjusting the reference
voltage; any value above 20 kΩ is adequate. The bias
resistor (R
BIAS
) limits the current through D
REF
to less than
150
µA.
The reference filter capacitor (C
RF
) forms an RC
filter with R
BIAS
to help eliminate noise.
2
3
4
5
6
7
Input filter
(R
IF
, C
IF
)
For added stability, an RC input noise filter is usually
included in the circuit. The input filter resistor value should
not exceed 100 kΩ. A typical RC time constant value is
16.7msec to help reject line-frequency noise. The input filter
capacitor should have low leakage for a high-impedance
input.
where t
INT
is the integration time.
Using the values derived above (assuming 60 Hz
operation), the equation becomes:
C
INT
= 16.7msec x 13.3
µA
= 0.1
µF.
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 ab-
sorption.
Battery
The typical circuit uses a 9V battery as a power source.
Any value between 6V and 12V can be used. For operation
from batteries with voltages lower than 6V and for operation
from power supplies, see "Powering the TC7129."
SPECIAL APPLICATIONS
The TC7129 as a Replacement Part
The TC7129 is a direct pin-for-pin replacement part for
the ICL7129. Note, however, that part requires a capacitor
and resistor between pins 26 and 28 for phase compensa-
tion. Since the TC7129 uses internal phase compensation,
these parts are not required and, in fact,
must be removed
from the circuit for stable operation.
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 component.
The value must be high enough to offset the effect of stray
capacitance at the capacitor terminals. A value of at least
1
µF
is recommended.
Powering the TC7129
While the most common power source for the TC7129
is a 9V battery, there are other possibilities. Some of the
more common ones are explained below.
8
TELCOM SEMICONDUCTOR, INC.
3-237
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
