ICL-7107 Intersil, ICL-7107 Datasheet - Page 10

• Manufacturer Part Number: ICL-7107
• Description: 31/2 Digit/ LCD/LED Display/ A/D Converters
• Manufacturer: Intersil

Component Value Selection
Integrating Resistor
Both the buffer amplifier and the integrator have a class A
output stage with 100 A of quiescent current. They can
supply 4 A 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, 470k
similarly a 47k for a 200mV scale.
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
(48kHz clock) nominal values for C
0.10 F, respectively. Of course, if different oscillator frequen-
cies 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.
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 0.47 F capacitor is recommended. On the
2V scale, a 0.047 F capacitor increases the speed of recov-
ery from overload and is adequate for noise on this scale.
Reference Capacitor
A 0.1 F 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 1 F will hold the roll-over error to 0.5 count
in this instance.
Oscillator Components
For all ranges of frequency a 100k resistor is recommended
and the capacitor is selected from the equation:
f
C
3.5V to +4V swing is nominal. For three readings/second
=
=
0.45
---------- - For 48kHz Clock (3 Readings/sec),
RC
100pF.
ICL7106, ICL7107, ICL7106S, ICL7107S
lNT
is near optimum and
are 0.22 F and
10
Reference Voltage
The analog input required to generate full scale output (2000
counts) is: V
V
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
resistor and capacitor would be 1 20k
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
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.
ICL7107 Power Supplies
The ICL7107 is designed to work from
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:
V+
1. The input signal can be referenced to the center of the
2. The signal is less than 1.5V.
3. An external reference is used.
REF
IN
FIGURE 10. GENERATING NEGATIVE SUPPLY FROM +5V
ICL7107
common mode range of the converter.
V+
should equal 100mV and 1V, respectively. However, in
0. Temperature and weighing systems with a variable
V-
OSC 1
OSC 2
OSC 3
GND
lN
REF
= 2V
= 0.341V. Suitable values for integrating
REF
. Thus, for the 200mV and 2V scale,
V- = 3.3V
CD4009
and 0.22 F. This
IN914
IN914
0.047
5V supplies.
F
+
-
10
F