TC7135 Datasheet, PDF (12/22 Pages) TelCom Semiconductor, Inc – 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER

TC7135
6.0 TYPICAL APPLICATIONS
6.1 Component Value Selection
6.1.1 INTEGRATING RESISTOR
The integrating resistor R_INT is determined by the full
scale input voltage and the output current of the buffer
used to charge the integrator capacitor, C_INT. Both the
buffer amplifier and the integrator have a class A output
stage, with 100µA of quiescent current. A 20µA drive
current gives negligible linearity errors. Values of 5µA
to 40µA give good results. The exact value of an
integrating resistor for a 20µA current is easily
calculated.
EQUATION 6-1:
R_INT =
Full scale voltage
20µA
6.1.2
INTEGRATING CAPACITOR (C_INT)
The product of integrating resistor and capacitor should
be selected to give the maximum voltage swing that
ensures the tolerance buildup will not saturate the inte-
grator swing (approximately 0.3V from either supply).
For ±5V supplies and ANALOG COMMON tied to sup-
ply ground, a ±3.5V to ±4V full scale integrator swing is
adequate. A 0.10µF to 0.47µF is recommended. In
general, the value of C_INT is given by:
EQUATION 6-2:
C_INT = ^{[10,000 x clock period] x IINT}
Integrator output voltage swing
(10,000) (clock period) (20µA)
=
Integrator output voltage swing
A very important characteristic of the integrating capac-
itor C_INT is that it has low dielectric absorption to pre-
vent rollover or ratiometric errors. A good test for
dielectric absorption is to use the capacitor with the
input tied to the reference. This ratiometric condition
should read half scale 0.9999, with any deviation
probably due to dielectric absorption. Polypropylene
capacitors give undetectable errors at reasonable cost.
Polystyrene and polycarbonate capacitors may also be
used in less critical applications.
6.1.3
AUTO ZERO AND REFERENCE
CAPACITORS
The size of the auto zero capacitor has some influence
on the noise of the system. A large capacitor reduces
the noise. The reference capacitor should be large
enough such that stray capacitance to ground from its
nodes is negligible.
The dielectric absorption of the reference and auto zero
capacitors are only important at power-on or when the
circuit is recovering from an overload. Smaller or
cheaper capacitors can be used if accurate readings
are not required for the first few seconds of recovery.
6.1.4 REFERENCE VOLTAGE
The analog input required to generate a full scale out-
put is V_IN = 2 V_REF.
The stability of the reference voltage is a major factor in
the overall absolute accuracy of the converter. For this
reason, it is recommended that a high-quality reference
be used where high-accuracy absolute measurements
are being made.
6.2 Conversion Timing
6.2.1 LINE FREQUENCY REJECTION
A signal integration period at a multiple of the 60Hz line
frequency will maximize 60Hz "line noise" rejection. A
100kHz clock frequency will reject 50Hz, 60Hz and
400Hz noise. This corresponds to five readings per
second (see Table 6-1 and Table 6-2).
TABLE 6-1: CONVERSION RATE VS.
CLOCK FREQUENCY
Oscillator Frequency
(kHz)
100
120
200
300
400
800
1200
Conversion Rate
(Conv./Sec.)
2.5
3
5
7.5
10
20
30
DS21460B-page 12
© 2002 Microchip Technology Inc.