AN023: Low Cost Digital Panel Meter Designs

Low Cost Digital Panel Meter Designs and
Complete Instructions for LCD and LED Kits
®
Application Note
AN023
Introduction
polarity features) it is only necessary to add display, 4
resistors, 4 capacitors, and an input filter if required (Figures
1 and 2).
The ICL7106 and ICL7107 are the first ICs to contain all the
active circuitry for a 31/2 digit panel meter on a single chip.
The ICL7106 is designed to interface with a liquid crystal
display (LCD) while the ICL7107 is intended for light-emitting
diode (LED) displays. In addition to a precision dual slope
converter, both circuits contain BCD to seven segment
decoders, display drivers, a clock and a reference. To build a
high performance panel meter (with auto zero and auto
A3 23
G3 22
BP 21
19 AB4
20 POL
C3 24
18 E3
G2 25
17 F3
V- 26
15 D3
16 B3
INT 27
14 E2
A-Z 29
BUFF 28
DISPLAY
13 F2
IN HI 31
IN LO 30
COM 32
CREF- 33
CREF+ 34
REF LO 35
TEST 37
REF HI 36
C3
TP4
R5
R
C2 2
C5
C1
9V
C4
OSC 3 38
OSC 2 39
OSC 1 40
R3
TP3
R1
R4
TP1TP2
+
TP5
IN
-
+
The ICL7136 is an ultra low power version of the ICL7106.
Except for the passive component values as shown in Figure
3 and Table 1, all references in this document to the
ICL7106 also apply to the ICL7136.
C1 = 0.1μF
C2 = 0.47μF
C3 = 0.22μF
C4 = 100pF
C5 = 0.01μF
R1 = 24kΩ
R2 = 47kΩ
R3 = 100kΩ
R4 = 1kΩ TRIMPOT
R5 = 1MΩ
12 A2
11 B2
10 C2
9 D2
8 E1
7 G1
6 F1
5 A1
4 B1
3 C1
2 D1
1 V+
ICL7106
DISPLAY
FIGURE 1. LCD DIGITAL PANEL METER USING ICL7106
C5
C1
-5V
R
C2 2
INT 27
V- 26
G2 25
C3 24
A3 23
G3 22
GND 21
14 E2
15 D3
16 B3
17 F3
18 E3
19 AB4
20 POL
DISPLAY
12 A2
11 B2
10 C2
9 D2
8 E1
7 G1
6 F1
5 A1
4 B1
3 C1
2 D1
ICL7107
1 V+
TO DECIMAL
POINT
C3
BUFF 28
A-Z 29
R6
13 F2
IN LO 30
IN HI 31
COM 32
CREF- 33
CREF+ 34
REF LO 35
REF HI 36
TEST 37
C4
OSC 3 38
OSC 2 39
OSC 1 40
R3
TP3
-
TP4
TP1TP2 R
1
R4
TP5
IN
R5
+
+5V
DISPLAY
C1 = 0.1μF
C2 = 0.47μF
C3 = 0.22μF
C4 = 100pF
C5 = 0.01μF
R1 = 24kΩ
R2 = 47kΩ
R3 = 100kΩ
R4 = 1kΩ TRIMPOT
R5 = 1MΩ
R6 = 150Ω
FIGURE 2. LED DIGITAL PANEL METER USING ICL7107
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 023
TP5
DISPLAY
1 V+
OSC 1 40
2 D1
OSC 2 39
3 C1
OSC 3 38
4 B1
TEST 37
5 A1
REF HI 36
6 F1
REF LO 35
7 G1
CREF 34
8 E1
CREF 33
9 D2
COMMON 32
10 C2
IN HI 31
11 B2
IN LO 30
12 A2
A-Z 29
13 F2
BUFF 28
14 E2
INT 27
15 D3
V - 26
16 B3
G2 25
17 F3
C3 24
18 E3
A3 23
19 AB4
G3 22
20 POL
BP 21
R3
C4
180kΩ
50pF
TP1
TP2
R4
10kΩ
R1
220kΩ
C1
0.1μF
TP3
C5
0.01μF
R5
+
IN
1MΩ
C2
-
0.01μF
C3
R2
+
9V
180kΩ
-
0.047μF
TP4
DISPLAY
FIGURE 3. LCD DIGITAL PANEL METER USING ICL7136
The Evaluation Kits
After purchasing a sample of the ICL7106 or the ICL7107,
the majority of users will want to build a simple voltmeter.
The parts can then be evaluated against the data sheet
specifications, and tried out in the intended application.
However, locating and purchasing even the small number of
additional components required, then wiring a breadboard,
can often cause delays of days or sometimes weeks. To
avoid this problem and facilitate evaluation of these unique
circuits, Intersil offers a kit which contains all the necessary
components to build a 31/2 digit panel meter. With the help
of this kit, an engineer or technician can have the system “up
and running” in about half an hour.
Two kits are offered, ICL7106EV/KIT and ICL7107EV/KIT.
Both contain the appropriate IC, a circuit board, a display
(LCD for ICL7106EV/KIT, LEDs for ICL7107EV/KIT),
passive components, and miscellaneous hardware.
Assembly Instructions
The circuit board layouts and assembly drawings for both
kits are given in Figures 10, 11. The boards are single-sided
to minimize cost and simplify assembly. Jumpers are used to
allow maximum flexibility. For example, provision has been
made for connecting an external clock (Test Point #5).
Provision has also been made for separating REF Lo from
COMMON when using an external reference zener. In a
production instrument, the board area could be reduced
2
dramatically. Aside from the display, all the components can
easily be placed in less than 4 square inches of board
space.
Molex™ pins are used to provide a low cost IC socket; one
circuit board can thus be used to evaluate several ICs.
(Strips of 20 pins should be soldered onto the PC boards;
the top of the strip holding the pins together can then be
broken off by bending it back and forth using needle-nose
pliers.) Solder terminals are provided for the five test points,
and for the ±5V input on the ICL7107 kit.
Full Scale Reading - 200mV or 2.000V?
The component values supplied with the kit are those
specified in the schematics of Figure 1 or Figure 2. They
have been optimized for 200mV full scale reading. The
complete absence of last digit jitter on this range illustrates
the exceptional noise performance of the ICL7106 and
ICL7107. In fact, the noise level (not exceeded 95% of time)
is about 15μV, a factor of 10 less than some competitive one
chip panel meters.
To modify the sensitivity for 2.000V full scale, the integrator
time constant and the reference should be changed by
substituting the component values given in Table 1. The
auto-zero capacitor (C2) should also be changed. These
additional components are not supplied in the kits. In
addition, the decimal point jumper should be changed so the
display reads 2.000.
Molex® is a registered trademark of Molex Incorporated.
Application Note 023
TABLE 1. COMPONENT VALUES FOR FULL SCALE OPTIONS
COMPONENT
200.0mV
FULL SCALE
2.000V
FULL SCALE
C2 (Mylar™)
0.47μF
0.047μF
R1
24kΩ
1.5kΩ (Note)
R2
47kΩ
470kΩ
C2
0.1μF
0.022μF
R1
220kΩ
150kΩ
R2
180kΩ
1.8MΩ
R4
10kΩ
100kΩ
NOTE: Changing R1 to 1.5kΩ will reduce the battery life of the ICL7106
kit. As an alternative, the potentiometer can be changed to 25kΩ.
Before soldering the display onto the circuit board, make
sure that it is inserted correctly. Many LCD packages do not
have pin #1 marked, but the segments of an unenergized
display can be seen by viewing with reflected light.
V+
V+
ICL7106
BP
TO LCD
DECIMAL
POINTS
SEGMENTS
DECIMAL
POINT
SELECT
CONTROL
(V+/GND)
TEST
CD4030
GND
Liquid Crystal Display (ICL7106)
Liquid crystal displays are generally driven by applying a
symmetrical square wave to the Back Plane (BP). To turn on
a segment, a waveform 180o out of phase with BP (but of
equal amplitude) is applied to that segment. Note that
excessive DC voltages (>50mV) will permanently damage
the display if applied for more than a few minutes. The
ICL7106 generates the segment drive waveform internally,
but the user should generate the decimal point front plane
drive by inverting the BP (pin 21) output (Note 1). In
applications where the decimal point remains fixed, a simple
MOS inverter can be used (Figure 4). For instruments where
the decimal point must be shifted, a quad exclusive OR gate
is recommended (Figure 5). Note that in both instances,
TEST (pin 37, TP1) is used as V- for the inverters. This pin is
capable of sinking about 1mA, and is approximately 5V
below V+. The BP output (pin 21) oscillates between V+ and
TEST.
NOTE:
1. In some displays, a satisfactory decimal point can be achieved by
tying the decimal front plan to COMMON (pin 32). This pin is
internally regulated at about 2.8V below V+. Prolonged use of
this technique, however, may permanently burn-in the decimal,
because COMMON is not exactly midway between BP high and
BP lo.
FIGURE 5. EXCLUSIVE ‘OR’ GATE FOR DECIMAL POINT DRIVE
Light Emitting Diode Display (ICL7107)
The ICL7107 pulldown FETs will sink about 8mA per
segment. Using standard common anode 0.3in or 0.43in red
LEDs, this drive level produces a bright display suitable for
almost any indoor application. However, additional brightness
can be achieved through the use of Hewlett Packard highefficiency LEDs. Note that the display contrast can be
increased substantially by using a red filter. Reference [4]
discusses filter techniques and lists manufacturers of suitable
materials.
A fixed decimal point can be turned on by tying the
appropriate cathode to ground through a 150Ω resistor. The
circuit boards supplied with the kit will accommodate either
HP 0.3in displays or the popular MAN 3700 types. The
difference between the two is that the HP has the decimal
point cathode on pin 6, whereas the MAN 3700 uses pin 9.
Due to the limited space on the circuit board, not all decimal
points are brought to jumper pads; it may be necessary to
wire directly from the 150Ω resistor to the display. For
multiple range instruments, a 7400 series CMOS quad gate
or buffer should be used. The majority of them are capable
of sinking about 8mA.
Capacitors
V+
1MΩ
IT1750
ICL7106
BP
TEST
TO LCD
DECIMAL
POINT
21
37
TO LCD
BACKPLANE
FIGURE 4. SIMPLE INVERTER FOR FIXED DECIMAL POINT
3
The integration capacitor should be a low dielectric-loss
type. Long term stability and temperature coefficient are
unimportant since the dual slope technique cancels the
effect of these variations. Polypropylene capacitors have
been found to work well; they have low dielectric loss
characteristics and are inexpensive. However, that is not to
say that they are the only suitable types. Mylar capacitors
are satisfactory for C1 (reference) and C2 (auto-zero).
For a more detailed discussion of recommended capacitor
types, see page three of Reference [2].
Mylar is a trademark of E. I. Du Pont De Nemours and Company.
Application Note 023
The Clock
The Reference
A simple RC oscillator is used in the kit. It runs at about
48kHz and is divided by 4 prior to being used as the system
clock (Figure 6). The internal clock period is thus 83.3μs,
and the signal integration period (1000 clock pulses) is
83.3ms. This gives a measurement frequency of 3 readings
per second since each conversion sequence requires 4000
clock pulses. Setting the clock oscillator at precisely 48kHz
will result in optimum line frequency (60Hz) noise rejection,
since the integration period is an integral number of line
frequency period. [2] Countries with 50Hz line frequencies
should set the clock at 50kHz.
For 200.0mV full scale, the voltage applied between REF Hi
and REF Lo should be set at 100.0mV. For 2.000V full scale,
set the reference voltage at 1.000V. The reference inputs
are floating, and the only restriction on the applied voltage is
that it should lie in the range V- to V+.
ICL7106/ICL7107
40
100kΩ
39
100pF
38
÷2
÷2
SYSTEM CLOCK
FIGURE 6. ICL7106/ICL7107 INTERNAL OSCILLATOR CLOCK
An external clock can also be used. In the ICL7106, the
internal logic is referenced to TEST. External clock
waveforms should therefore swing between TEST and V+
(Figure 7A). In the ICL7107, the internal logic is referenced
to GND so any generator whose output swings from ground
to +5V will work well (Figure 7B).
OSC1
TEST
40
5V
37
FIGURE 7A. ICL7106
5V
40
5V
0V
ICL7107
FIGURE 7B. ICL7107
FIGURE 7. EXTERNAL CLOCK OPTIONS
4
The limitations of the on-chip reference should also be
recognized, however. With the ICL7107, the internal heating
which results from the LED drivers can cause some
degradation in performance. Due to its high thermal
resistance, plastic parts are poorer in this respect than
ceramic. The user is cautioned against extrapolating from
the performance of the kit, which is supplied with a ceramic
ICL7107, to a system using the plastic part. The combination
of reference TC, internal chip dissipation, and package
thermal resistance can increase noise near fullscale from
25μV to 80μVP-P .
The linearity in going from a high dissipation count such as
1000 (19 segments on) to a low dissipation count such as
1111 (8 segments on) can also suffer by a count or more.
Devices with a positive TC reference may require several
counts to pull out of an overload condition. This is because
overload is a low dissipation mode, with the three least
significant digits blanked. Similarly, units with a negative TC
may cycle between overload and a nonoverload count as the
die alternately heats and cools. These problems are of
course eliminated if an external reference is used.
The ICL7106, with its negligible dissipation, suffers from
none of these problems. In either case, an external
reference can easily be added as shown in Figures 8A or
8B.
ICL7106
OSC1
The voltage between V+ and COMMON is internally
regulated at about 2.8V. This reference is adequate for
many applications and is used in the evaluation kits. It has a
typical temperature coefficient of 100ppm/oC.
Application Note 023
V+
V+
CD4009
V+
REF HI
6.8V
ZENER
REF LO
IZ
ICL7106
ICL7107
V+
OSC 1
IN914
OSC 2
OSC 3
0.047
μF
ICL7107
IN914
GND
V-
+
10
μF
-
V-
FIGURE 8A.
V- = 3.3V
V+
FIGURE 9. GENERATING NEGATIVE SUPPLY FROM +5V
Input Filters
V+
ICL7106
ICL7107
ICL8069
1.2V
REFERENCE
REF HI
REF LO
COMMON
FIGURE 8B.
FIGURE 8. USING AN EXTERNAL REFERENCE
Power Supplies
The ICL7106 kit is intended to be operated from a 9V dry
cell. INPUT Lo is shorted to COMMON, causing V+ to sit
2.8V positive with respect to INPUT Lo, and V- 6.2V
negative with respect to INPUT Lo.
The ICL7107 kit should be operated from ±5V. Noisy
supplies should be bypassed with 6.8μF capacitors to
ground at the point where the supplies enter the board.
INPUT Lo has an effective common mode range with
respect to GND of a couple of volts.
The precise value is determined by the point at which the
integrator output ramps within ~0.3V of one or other of the
supply rails. This is governed by the integrator time constant,
the magnitude and polarity of the input, the common mode
voltage, and the clock frequency: for further details, consult
the data sheet. Where the voltage being measured is
floating with respect to the supplies, INPUT Lo should be
tied to some voltage within the common mode range such as
GROUND or COMMON. If a -5V supply is unavailable,
suitable negative rail can be generated locally using the
circuit shown in Figure 9.
One of the attractive features of the ICL7106 and ICL7107 is
the extremely low input leakage current, typically 1pA at
25oC. This minimizes the errors caused by high impedance
passive filters on the input. For example, the simple RC
(1MΩ/0.01μF) combination used in the evaluation kits
introduces a negligible 1μV error.
Preliminary Tests
Auto Zero
With power on and the inputs shorted, the display should
read zero. The negative sign should be displayed about 50%
of the time, an indication of the effectiveness of the autozero system used in the ICL7106 and ICL7107. Note that
some competitive circuits flash negative on every alternate
conversion for inputs near zero. While this may look good to
the uninitiated, it is not a true auto zero system!
Over-Range
Inputs greater than full scale will cause suppression of the
three least significant digits; i.e., only 1 or -1 will be displayed.
Polarity
The absence of a polarity signal indicates a positive reading.
A negative reading is indicated by a negative sign.
Further evaluation should be performed with the help of a
precision DC voltage calibrator such as Fluke Model 343A.
Alternatively a high quality 41/2 digit DVM can be used,
provided its performance has been measured against that of
a reliable standard.
DPM Components: Sources of Supply
It has already been shown that the ICL7106 and ICL7107
require an absolute minimum of additional components. The
only critical ones are the display and the integration capacitor.
The following list of possible suppliers is intended to be of
assistance in putting a converter design into production. It
should not be interpreted as a comprehensive list of
suppliers, nor does it constitute an endorsement by Intersil.
5
Application Note 023
References
Liquid Crystal Displays
1. LXD Inc., Cleveland, Ohio
2. Hamlin Inc., Lake Mills, Wisconsin
3. IEE Inc., Van Nuys, California
4. Shelley Associates, Irvine, California
5. Crystaloid Electronics, Stow, Ohio
LED Displays (Common Anode)
1. Hewlett Packard Components, Palo Alto, California
2. Itac Inc., Santa Clara, California
3. Litronix Inc., Cupertino, California
4. Monsanto Inc., Palo Alto, California
Polypropylene Capacitors
1. Plessey Capacitors, West Lake Village, California
[1] AN016 Application Note, Intersil Corporation, “Selecting
A/D Converters”, Dave Fullagar.
[2] AN017 Application Note, Intersil Corporation, “The
Integrating A/D Converter”, Lee Evans.
[3] AN018 Application Note, Intersil Corporation, “Do’s and
Don’ts of Applying A/D Converters”, Peter Bradshaw
and Skip Osgood.
[4] Hewlett Packard (Opto Electronics Div.) Application
Note 964, “Contrast Enhancement Techniques”.
[5] AN032 Application Note, Intersil Corporation,
“Understanding the Auto-Zero and Common Mode
Performance of the ICL7106/7107/7109 Family”, Peter
Bradshaw.
2. IMB Electronic Products, Santa Fe Springs, California
3. Elcap Components, Santa Ana, CaliforniaTRW
Capacitors, Ogallala, Nebraska
CAUTION: Potential trouble areas when constructing the evaluation kits:
1. Certain LCD displays have a protective plastic sheet
covering the plastic top. This sheet may be removed after
installing the display to maximize display viewing.
2. Solder flux or other impurities on PC board may cause
leakage paths between IC pins and board traces reducing
performance and should be removed with rubbing alcohol
or some other suitable cleaning agent. Displays should be
removed when cleaning as damage could result to them.
3. Blue PC board material (PC75) has been treated with a
chemical which may cause surface leakage between the
input traces. It is suggested that the board be scribed
between the input traces and adjacent traces to eliminate
this surface leakage.
In order to ensure that unused segments on the LCD displays
do not turn on, tie them to the backplane pin (pin 21).
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com
6
Application Note 023
FIGURE 10. ICL7107 PRINTED CIRCUIT BOARD AND COMPONENT PLACEMENT
7
Application Note 023
FIGURE 11. ICL7107 PRINTED CIRCUIT BOARD AND COMPONENT PLACEMENT
8
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