TC7135
DS21460D-page 12 © 2007 Microchip Technology Inc.
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 100 µA of quiescent current. A 20 µA drive
current gives negligible linearity errors. Values of 5 µA
to 40 µA give good results. The exact value of an
integrating resistor for a 20 µA current is easily
calculated.
EQUATION 6-1:
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 build-up will not saturate the
integrator swing (approximately 0.3V from either
supply). For ±5V supplies and ANALOG COMMON tied
to supply ground, a ±3.5V to ±4V full scale integrator
swing is adequate. A 0.10 µF to 0.47 µF is
recommended. In general, the value of C
INT
is given
by:
EQUATION 6-2:
A very important characteristic of the integrating
capacitor C
INT
is that it has low dielectric absorption to
prevent 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, with a larger capacitor
reducing 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
output 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 60 Hz
line frequency will maximize 60 Hz “line noise”
rejection. A 100 kHz clock frequency will reject 50 Hz,
60 Hz and 400 Hz noise. This corresponds to five
readings per second (see Table 6-1 and Table 6-2).
TABLE 6-1: CONVERSION RATE VS.
CLOCK FREQUENCY
R
INT
Full Scale Voltage
20
μ
A
--------------------------------------------=
C
INT
10 000
,
clock period
×
[]I
INT
×
integrator output voltage swing
---------------------------------------------------------------------------=
10 000
,
()clock period()20
μ
A
×
integrator output voltage swing
------------------------------------------------------------------------------=
Oscillator Frequency
(kHz)
Conversion Rate
(Conv./Sec.)
100 2.5
120 3
200 5
300 7.5
400 10
800 20
1200 30
