ICL7106_14 Datasheet, PDF (10/17 Pages) Intersil Corporation – 31/2 Digit, LCD/LED Display, A/D Converters

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.
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.
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 rollover error. Generally
1µF will hold the rollover error to 0.5 count in this instance.
Oscillator Components
For all ranges of frequency a 100kΩ resistor is recommended and
the capacitor is selected as shown in Equation 2:
f = ⁰--_R--^.--⁴-_C--⁵-- For 48kHz Clock (3 Readings/sec),
C = 100pF.
(EQ. 2)
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.
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 the ICL7660 datasheet 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.
V+
CD4009
V+
OSC 1
OSC 2
OSC 3
ICL7107
GND
V-
1N914
0.047
µF
1N914
+
10µF
-
V- = 3.3V
FIGURE 10. GENERATING NEGATIVE SUPPLY FROM +5V
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FN3082.9
October 24, 2014