ICL7106, ICL7107, ICL7107S
Component Value Selection
Integrating Resistor
Both the buffer amplifier and the integrator have a class A
output stage with 100µA of quiescent current. They can
supply 4µA 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, 470kΩ is near optimum and
similarly a 47kΩ for a 200mV scale.
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,
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 0.662V. 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 and capacitor would be 120kΩ and 0.22µF. This
makes the system slightly quieter and also avoids a divider
network on the input. The ICL7107 with
±5V
supplies can
accept input signals up to
±4V.
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.
Integrating Capacitor
The integrating capacitor should be selected to give the
maximum voltage swing that ensures tolerance buildup will
not saturate the integrator swing (approximately. 0.3V from
either supply). In the ICL7106 or the ICL7107, when the
analog COMMON is used as a reference, a nominal +2V full-
scale integrator swing is fine. For the ICL7107 with +5V
supplies and analog COMMON tied to supply ground, a
±3.5V
to +4V swing is nominal. For three readings/second
(48kHz clock) nominal values for C
lNT
are 0.22µF and
0.10µF, respectively. Of course, if different oscillator
frequencies are used, these values should be changed in
inverse proportion to maintain the same output swing.
An additional requirement of the integrating capacitor is that
it must have a low dielectric absorption to prevent roll-over
errors. While other types of capacitors are adequate for this
application, polypropylene capacitors give undetectable
errors at reasonable cost.
ICL7107 Power Supplies
The ICL7107 is designed to work from
±5V
supplies.
However, if a negative supply is not available, it can be
generated from the clock output with 2 diodes, 2 capacitors,
and an inexpensive lC. Figure 10 shows this application. See
ICL7660 data sheet for an alternative.
In fact, in selected applications no negative supply is
required. The conditions to use a single +5V supply are:
1. The input signal can be referenced to the center of the
common mode range of the converter.
2. The signal is less than
±1.5V.
3. An external reference is used.
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 0.47µF capacitor is recommended. On the
2V scale, a 0.047µF 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.
V+
CD4009
V+
OSC 1
OSC 2
OSC 3
ICL7107
GND
V-
1N914
1N914
+
10
µF
Oscillator Components
For all ranges of frequency a 100kΩ resistor is recommended
and the capacitor is selected from the equation:
0.45
-
f
= ----------
For 48kHz Clock (3 Readings/sec),
RC
C
=
100pF.
0.047
µF
-
V- = 3.3V
FIGURE 10. GENERATING NEGATIVE SUPPLY FROM +5V
10
FN3082.8
INTERSIL [ INTERSIL CORPORATION ]
