ICL7126_14 Datasheet, PDF (10/15 Pages) Intersil Corporation – 3 1/2 Digit, Low Power, Single Chip A/D Converter

ICL7126
Component Value Selection
Integrating Resistor
Both the buffer amplifier and the integrator have a class A
output stage with 6µA of quiescent current. They can
supply ~1µA of drive current with negligible nonlinearity.
The integrating resistor should be large enough to remain
in this very linear region over the input voltage range, but
small enough that undue leakage requirements are not
placed on the PC board. For 2V full-scale, 1.8MΩ is near
optimum and similarly a 180kΩ for a 200mV scale.
Integrating Capacitor
The integrating capacitor should be selected to give the
maximum voltage swing that ensures tolerance build-up will
not saturate the integrator swing (approximately. 0.3V from
either supply). When the analog COMMON is used as a
reference, a nominal ±2V full-scale integrator swing is fine. For
three readings/second (48kHz clock) nominal values for C_lNT
are 0.047µF, for 1/s (16kHz) 0.15µF. Of course, if different
oscillator frequencies are used, these values should be
changed in inverse proportion to maintain the same
output swing.
The integrating capacitor should have a low dielectric
absorption to prevent roll-over errors. While other types may
be adequate for this application, polypropylene capacitors
give undetectable errors at reasonable cost.
At three readings/sec, a 750Ω resistor should be placed in
series with the integrating capacitor, to compensate for
comparator delay.
Auto-Zero Capacitor
The size of the auto-zero capacitor has some influence on
the noise of the system. For 200mV full-scale where noise is
very important, a 0.32µF capacitor is recommended. On the
2V scale, a 0.33µF capacitor increases the speed of
recovery from overload and is adequate for noise on this
scale.
Reference Capacitor
A 0.1µF capacitor gives good results in most applications.
However, where a large common mode voltage exists (i.e.,
the REF LO pin is not at analog COMMON) and a 200mV
scale is used, a larger value is required to prevent roll-over
error. Generally 1µF will hold the roll-over error to 0.5 count
in this instance.
Oscillator Components
For all ranges of frequency a 50pF capacitor is recommended
and the resistor is selected from the approximation equation
f
∼
0_----._--4_---5_--
RC
•
For
48kHz
clock
(3
readings/sec),
R
=
180k
Ω
Reference Voltage
The analog input required to generate full-scale output (2000
counts) is: V_lN = 2V_REF. Thus, for the 200mV and 2V scale,
10
V_REF should equal 100mV and 1V, respectively. However, in
many applications where the A/D is connected to a transducer,
there will exist a scale factor other than unity between the input
voltage and the digital reading. For instance, in a weighing
system, the designer might like to have a full-scale reading
when the voltage from the transducer is 0.682V. Instead of
dividing the input down to 200mV, the designer should use the
input voltage directly and select V_REF = 0.341V. Suitable values
for integrating resistor 330kΩ. This makes the system slightly
quieter and also avoids a divider network on the input. Another
advantage of this system occurs when a digital reading of zero
is desired for V_IN ≠ 0. Temperature and weighing systems with
a variable fare are examples. This offset reading can be
conveniently generated by connecting the voltage transducer
between IN HI and COMMON and the variable (or fixed) offset
voltage between COMMON and IN LO.
Typical Applications
The ICL7126 may be used in a wide variety of
configurations. The circuits which follow show some of the
possibilities, and serve to illustrate the exceptional versatility
of these A/D converters.
The following application notes contain very useful
information on understanding and applying this part and are
available from Intersil Corporation.
Application Notes
NOTE #
DESCRIPTION
AN016 “Selecting A/D Converters”
AN017 “The Integrating A/D Converter”
AN018
AN023
AN032
AN046
AN052
“Do’s and Don’ts of Applying A/D Converters”
“Low Cost Digital Panel Meter Designs”
“Understanding the Auto-Zero and Common Mode
Performance of the ICL7136/7/9 Family”
“Building a Battery-Operated Auto Ranging DVM with
the ICL7106”
“Tips for Using Single-Chip 3¹/₂ Digit A/D Converters”
FN3084.5