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參數資料
| 型號： | ICL7107S |
| 廠商： | Intersil Corporation |
| 元件分類： | 串行ADC |
| 英文描述： | LED Display, A/D Converters(3 1/2 位LED 顯示器驅動 A/D轉換器) |
| 中文描述： | LED顯示，A / D轉換（3 1 / 2位發(fā)光顯示器驅動的A / D轉換器） |
| 文件頁數： | 10/16頁 |
| 文件大小： | 534K |
| 代理商： | ICL7107S |
10
FN3082.8
Component Value Selection
Integrating Resistor
Both the buffer amplifier and the integrator have a class A
output stage with 100
μ
A of quiescent current. They can
supply 4
μ
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
is near optimum and
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
±
3.5V to +4V swing is nominal. For three readings/second
(48kHz clock) nominal values for C
lNT
are 0.22
μ
F and
0.10
μ
F, 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 0.47
μ
F capacitor is recommended. On the
2V scale, a 0.047
μ
F capacitor increases the speed of
recovery from overload and is adequate for noise on this
scale.
Reference Capacitor
A 0.1
μ
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
μ
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:
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 120k
and 0.22
μ
F. 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
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.
f
-----------
For 48kHz Clock (3 Readings/sec),
=
C
100pF.
=
ICL7107
V+
OSC 1
V-
OSC 2
OSC 3
GND
V+
V- = 3.3V
0.047
μ
F
10
μ
F
-
+
1N914
1N914
CD4009
FIGURE 10. GENERATING NEGATIVE SUPPLY FROM +5V
ICL7106, ICL7107, ICL7107S
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