Datasheet
9
FN3093.3
Power Supplies
The ICL7135 is designed to work from ±5V supplies.
However, 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.
See “differential input” for a discussion of the effects this will
have on the integrator swing without loss of linearity.
Typical Applications
The circuits which follow show some of the wide variety of
possibilities and serve to illustrate the exceptional versatility
of this A/D converter.
Figure 7 shows the complete circuit for a 4
1
/
2
digit (±2.000V)
full scale) A/D with LED readout using the ICL8069 as a
1.2V temperature compensated voltage reference. It uses
the band-gap principal to achieve excellent stability and low
noise at reverse currents down to 50µA. The circuit also
shows a typical R-C input filter. Depending on the
application, the time-constant of this filter can be made
faster, slower, or the filter deleted completely. The
1
/
2
digit
LED is driven from the 7 segment decoder, with a zero
reading blanked by connecting a D5 signal to RBl input of the
decoder. The 2-gate clock circuit should use CMOS gates to
maintain good power supply rejection.
A suitable circuit for driving a plasma-type display is shown
in Figure 8. The high voltage anode driver buffer is made by
Dionics. The 3 AND gates and caps driving “BI” are needed
for interdigit blanking of multiple-digit display elements, and
can be omitted if not needed. The 2.5kΩ and 3kΩ resistors
set the current levels in the display. A similar arrangement
can be used with Nixie
®
tubes.
The popular LCD displays can be interfaced to the outputs of
the ICL7135 with suitable display drivers, such as the
ICM7211A as shown in Figure 9. A standard CMOS 4030
QUAD XOR gate is used for displaying the
1
/
2
digit, the
polarity, and an “overrange” flag. A similar circuit can be
used with the ICL7212A LED driver and the ICM7235A
vacuum fluorescent driver with appropriate arrangements
made for the “extra” outputs. Of course, another full driver
circuit could be ganged to the one shown if required. This
would be useful if additional annunciators were needed. The
Figure shows the complete circuit for a 4
1
/
2
digit (±2.000V)
A/D.
Figure 10 shows a more complicated circuit for driving LCD
displays. Here the data is latched into the ICM7211 by the
STROBE signal and “Overrange” is indicated by blanking the
4 full digits.
FIGURE 7. 4
1
/
2
DIGIT A/D CONVERTER WITH A MULTIPLEXED COMMON ANODE LED DISPLAY
ANALOG
GND
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
+5V
6.8kΩ
V
REF
=
SIGNAL
0.1µF
-5V
ICL8069 1
2
5
B1
B
A
7447
34
B2
C
D
E
F
G
B4
B8
C
RC NETWORK
ƒ
OSC
= 0.45/RC
1.000V
10kΩ
100kΩ
27Ω
47K
150Ω
INPUT
1µF
R
RBI
100kΩ
+5V
100K
1µF
0.47µF
21
150Ω
150Ω
4.7K
+5V
ICL7135
V-
REF
ANALOG
INT OUT
AZ
IN
BUF OUT
RC1
RC2
INPUT LO
INPUT HI
V+
D5
B1
B2
UR
OR
STROBE
R/H
DIG. GND
POL
CLOCK
BUSY
D1
D2
D3
D4
B8
B4
COMMON
(NOTE 1)
NOTE:
1. For finer resolution on scale factor adjust, use a 10 turn pot or a small pot in series with
a fixed resistor.
ICL7135
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