AN9336: Multi-Meter Display Converter Eases DMM Design

Multi-Meter Display Converter Eases DMM
Design
Application Note
Introduction
Dual-Slope Analog to Digital converters have enjoyed enormous popularity in the past due to such features as: autozero, input polarity detection, differential inputs, and excellent
linearity. Although running at slower conversion speeds than
SAR, and FLASH type converters, Dual-Slope converters are
cost effective for digital multi-meters (DMMs), temperature
systems, weigh scales, and panel meters. Current usage of
dual-slope products has reached 10 million units per year.
Even with this large number of parts being shipped each year,
there is significant room for improvement.
At present, it requires approximately 60 external components
(many precision in nature) to bring the typical 26-range, 31/2
digit DVM to market. Many of these external components are
for LCD annunciator-driving, range, and function selection, AC
to DC conversion, and fault protection. The ICL7139 with the
addition of only 20 external components, builds a complete
autoranging 33/4 digit DVM. The major features of the
ICL7139 are: fast autoranging, continuity detection, low power
CMOS (20mW), and LCD display drivers, including annunciators and decimal points. Add to these only one external
adjustment, and the already low cost of 31/2 digit DVMs can
be further decreased.
It may be beneficial to first review the concept of dual-slope
conversion. It is an Analog to Digital conversion technique that
is a subclass of integrating/charge-balancing conversion techniques. Some typical features are: high input impedance, excellent linearity, and excellent rejection to line-frequency noise
(normal mode rejection). Two disadvantages are the slow conversion rate and the need for several external components.
CINT
-
-
IIN
+
+
INT
INTEGRATOR COMPARATOR
IREF
DEINT
CLOCK
CONTROL LOGIC
AND
COUNTERS
DATA
FIGURE 1. DUAL SLOPE A/D CONVERTER
The basic block diagram of a dual-slope converter is shown in
Figure 1. The unknown input current (IIN) is integrated across
an integration capacitor (CINT) for a fixed amount of time
(TINT), and then deintegrated with a reference current (IREF)
for a variable amount of time (TDEINT). The ratio of IIN/IREF is
equal to TDEINT/TINT. Therefore, by making Tint equal to one
thousand clock cycles, TDEINT is equal to (IIN/IREF)*1000. A
voltage-to-current converter is generally added at the input to
isolate the user from the low input impedance of the system.
1
January 1994
AN9336
Figure 2 is a simplified block diagram of the ICL7139. The
digital section contains the control logic, the counters, and
the display decoders and drivers. The digital section is powered by VCC and by Digital Common, which is about 3.1V
below VCC. The oscillator, normally 120kHz for measurement of 60Hz AC, is divided by two, to generate the internal
master clock. The analog section Figure 3 includes the integrator, the comparator, the reference, the analog buffers, and
several analog switches. The analog section is either powered from VCC to VEE, or from VCC to Analog Common,
depending on function.
Analog Common is about 3V below VCC and is derived from
a temperature-compensated zener diode. Its typical temperature coefficient is ±50ppm/oC. It can source a large amount
of current (20mA), but can only sink about 20µA.
Description of Modes and Functions
The key to lowering the overall cost of a typical DVM is to
reduce the number of range-select switches and precision
range resistors. To accomplish this, a system that changes
range by changing the integration time instead of using
range select resistors was developed (see the Autoranging
Concept insert). This allowed a large reduction in cost for the
average DVM by removing most of the range select resistors
and the range select switches
The various modes of conversion, DC Volts, DC current,
ohms, and AC to DC conversion, are now described. In the
DC voltage mode, the input voltage is first converted to a
current (VIN/RINTV) by the 100mΩ integrate resistor. It is
then integrated for ten, one hundred, one thousand, or ten
thousand clock-cycles by the integrator. The integrated voltage is then deintegrated by a known current VREF/RDEINT).
The display counters are started at the beginning of the
deintegration and continue to count the master clock until the
integrator output has crossed zero. The digital count is then
checked for underrange and if necessary, the next range is
entered (see Figure 4 for timing diagram). If underrange is
not detected, the count is sent to the display latches.
DC current is handled by forcing the input current through
one of two selected current shunts (RS1 or RS2). The voltage that is developed across the current shunt is integrated
as in the voltage mode. The integration resistor (RINTI) is
smaller than RINTV to allow the least possible voltage across
the current shunts. The ICL7139 allows a maximum of
400mV across the current shunts. There are eight current
ranges (four DC and four AC). The ranges are split into two
sets (high and low) of two ranges each. The ICL7139 autoranges between the two ranges in each set.
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Application Note 9336
SWITCHES
CRYSTAL
CONTROL LOGIC
INCLUDING
AUTORANGING LOGIC
OSC
BEEPER
DRIVER
PIEZO
ELECTRIC
BEEPER
COUNTERS
DISPLAY
DRIVER
AND
LATCHES
DISPLAY
DIGITAL COMMON
POWER
SUPPLY
SECTION
VCC
ANALOG SECTION
ANALOG SWITCHES, INTEGRATION
AND COMPARATOR
VEE COM
EXTERNAL
RESISTORS
AND CAPACITORS
FIGURE 2. ICL7139 BLOCK DIAGRAM
The ohms mode is a slight modification of the standard ratiometric technique. The standard ratiometric measurement is
accomplished by first forcing the same current through a
known and an unknown resistor. Then, by integrating the
voltage across the unknown resistor (effectively the input
voltage), and deintegrating with the voltage across the
known resistor (effectively the reference voltage), the converter can measure the ratio of unknown/known. This is the
technique that most 31/2 digit meters use. The ICL7139 cannot integrate voltages without drawing some input current.
Fortunately, this input current (the integrate and deintegrate
currents) has no effect on the accuracy of the reading, as it
exactly cancels between the integrate and deintegrate
phases. The ICL7139 has four ohm ranges that are split into
two sets of two ranges. Again, the ICL7139 autoranges
between the two ranges in each set.
AC to DC conversion is limited to line-frequency/sine-wave
measurements only. The ICL7139 starts the AC measurement by disconnecting the integrator capacitor and using the
integrator as an auto-zeroed comparator. Once synchronized to the AC input, the ICL7139 closes two autozero
loops and begins a normal integrate/deintegrate cycle. Since
diode D4 is in series with the integrator capacitor (CINT).
The timing is similar to the DC voltage mode, except that the
integration period is doubled and the ICL7139. resynchronizes to the AC input before each range’s reading. Since the
voltage across CINT is proportional to the average AC input
voltage, the ICL7139 modifies the deintegration clock to
scale the reading to RMS.
2
Additional Features
Although the line-frequency/sine-wave limitation for AC measurements seems severe, most measurements are line-frequency oriented. An alternative is to use an external RMS
converter such as an AD536 or an AD636. This technique
could extend measurements of arbitrary inputs from 40Hz to
20kHz.
Continuity detection is enabled on the lowest ohms range.
When the voltage across the unknown resistor (RX) is less
than approximately 100mV, the beeper output will drive a
2kHz square wave. Since continuity detection is independent
of the state of the conversion, it appears instantaneous to
the user. The beeper output is designed to drive a piezoelectric transducer.
Application Note 9336
CAZ
RDEINT
CINT
CINT
CAZ
TRIPLE POINT
DEINT
5
ACINT
D1
DEINT
D4
DEINT-
ACS
D2
VREF
~
ACINT D3
AZ
T
INT V/Ω
AZ
ACS
RINTV
T
AZ
+
+
INTEGRATOR
COMPARATOR
AC IN
V+
6.7V
COMMON
~
S = AZ • ACS • ACINT
T = (INT + ACS) AZ AR
ACS = AC SYNC
AR = AUTORANGE CHOPPER
AZ = AUTOZERO
INT = INTEGRATE
+
80µA
V-
FIGURE 3. ANALOG BLOCK DIAGRAM
FIRST AUTO ZERO
FIRST INTEGRATE
FIRST DEINTEGRATE
UNDERRANGE
AUTO ZERO
SECOND AUTO ZERO
SECOND INTEGRATE
SECOND DEINTEGRATE
UNDERRANGE
AUTO ZERO
THIRD AUTO ZERO
THIRD INTEGRATE
THIRD DEINTEGRATE
UNDERRANGE
AUTO ZERO
FOURTH AUTO ZERO
FOURTH INTEGRATE
FOURTH DEINTEGRATE
AUTO ZERO
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
FIGURE 4. LINE FREQUENCY CYCLES (1 CYCLE = 1000 INTERNAL CLOCK PULSES = 2000 OSCILLATION CYCLES)
3
Application Note 9336
tion current (VREF/RDEINT) directly from the reference for
negative voltage inputs, but not for positive voltage inputs. To
eliminate rollover error (the difference between a positive
voltage reading and the same negative voltage reading), the
output impedance of the reference should be smaller than
RDEINT/4000.
Range jumping is a common problem in autoranging DVMs.
This phenomenon occurs when the DVM is unable to lock
into on range due to noise near the range entry/exit point. A
small amount of digital hystersis was added to keep the
ICL7139 from range jumping.
The duplexed display-driver section has 2 backplane drivers
and 21 segments drivers. The typical display has 3 decimal
points, 11 annunciators, and 33/4 digits (up to 3999) of seven
segment digits. The display-driver section is powered from
VCC to Digital Common. The ICL7139 is designed to use a
standard 9V battery. When the supply voltage drops below
7V, the low battery annunicator is turned on. This is approximately 200mV before the internal reference (Analog Common) drops out of regulation.
The ratiometric ohms can be modified to read Siemens (the
inverse of ohms). By reversing the roles of the reference
resistor and the unknown resistor, the display will read Siemens. This extends the upper limit of resistance from 4mΩ
to well beyond 10MΩ.
Putting the ICL7139 to Work
The real advantage of the ICL7139 is the simplicity and low
cost of building a DVM. Three applications are discussed.
Design Trade-offs and System
Considerations
The ICL7139 retains many of the features of the popular
ICL7106, but some new design constraints must now be
dealt with. The reference capacitor has been removed from
the ICL7139, thus lowering the system cost and improving
the response speed in the ohms mode. However, this also
requires the internal amplifiers to have high common-mode
rejection, causing parasitic capacitances to play a more
important role in the determination of offset errors.
The first application is a 20-range DVM (see Figure 5). There
are 4 AC/DC voltage ranges, 4 AC/DC current ranges, and 4
resistance ranges. The maximum voltage/current/resistance
that can be measured is 400V, 4A, and 4MΩ, respectively. The
minimum resolution of each range (voltage/current/resistance)
is 100µV, 1µA, and 1Ω, respectively. The only external components are; 2 switches, 2 capacitors, 7 precision resistors, 1
trimpot, 1 beeper, 1 fuse, 1 crystal, and a display. This system
should only be used in an environment where minimal fault
protection is needed.
The output impedance of the reference must also be looked
at more carefully than in previous designs. During the voltage and current modes, the ICL7139 draws the deintegra-
The second application is shown in Figure 6. This configuration has the same 20 ranges as the first application, but
includes a few extra features.
10MΩ
3.3nF
0.1mF
13
9
14
120kHz
CRYSTAL
15
21
10MΩ 12
S3A
A
S4A
V
Ω
10kΩ
A
1MΩ
µA
1MΩ 11
a
1-3
23-40
c
c
V-
A
c
d
BEEPER
4
V+
+
ICL7139
ON/OFF
10
9V
BATTERY
V+
S1
5
COMMON
V-
20kΩ
6
PIN 10
VREF
18
V/Ω/A
19
S2
V+
HIΩ-DC/LOΩ-AC
S3B
17
mA/µA
S2 CLOSED: HIΩ-DC
mA
FIGURE 5. MINIMUM SYSTEM CONFIGURATION
4
c
d
16
INT (I)
b
f
kΩMΩ
-
V+ µA V+
f
c
d
a
e
b
BEEPER
0.1Ω
2W
V
Ω
e
AC
HIΩ
9.9Ω
COMMON
f
e
LOΩ
mA
30K50K
a
b
d
DISPLAY
DRIVE
OUTPUTS
INT (V/Ω)
a
e
b
7
8
mAVµA
22
TRIPLE CAZ CINT OSC OSC
OUT IN
POINT
DEINT
INPUTS
V/Ω
LO BAT
Application Note 9336
10MΩ
3.3nF
0.1µF
13
S4A
A
S5A
15
21
Ω
A
µA
mA
COMMON
V+ µA V+
V-
a
c
1-3
23-40
COMMON
a
b
d
f
c
a
e
b
d
18
V/Ω/A
c
c
d
d
AC
kΩMΩ
16
BEEPER
4
PIN 4
+
9V
BATTERY
+
1µF
S1
6
4.7µF +
10kΩ
TANT
10kΩ
ICL8069
19
S2
V+
20
S3
V+
PIN 10
HIΩ-DC/LOΩ-AC
S4B
A
b
f
VREF
V
Ω
e
f
c
e
5
V-
e
b
ON/OFF
10
mAVµA
a
0.1Ω
2W
30K50K
LO BAT
22
TRIPLE CAZ CINT OSC OSC
IN
OUT
POINT
9
DEINT
DISPLAY
10MΩ 12
DRIVE
INT (V/Ω)
OUTPUTS
10kΩ
7
LOΩ
1MΩ 8
HIΩ
BEEPER
1MΩ 11
INT (I)
V+
ICL7139
9.9Ω
ICL7149
INPUTS
V/Ω
V
14
120kHz
CRYSTAL
17
mA/µA
HOLD
mA
S2 CLOSED: HIΩ-DC
S3 CLOSED: HOLD READING
FIGURE 6. HIGH PERFORMANCE CONFIGURATION
Fault protection is significantly improved. This protection
could easily provide ±250V protection for both the external
components and the ICL7139. An external reference was
added for improved performance over temperature. Note
that the voltage reference or the deintegration resistor can
be trimmed for initial accuracy. AC coupling is used on the
AC ranges to isolate any DC voltage from affecting the accuracy of the AC reading. The HOLD Feature is switch accessible. The duplexed LCD display has two extra segments.
These are used for the DC segment and to separate mA/V
segments.
The third application Figure 7 shows a bridge circuit for measurement of pressure or temperature. Also note that the integrator capacitor from 3.3nF to 3.9nF. The voltage range has
been changed to include a maximum of 1000V. A simple current-divider was all that was necessary to simultaneously
maintain an input impedance of 10MΩ and a maximum of
±40µA of input integration current. The ICL8069’s biasing
resistor was decreased to 3.3kΩ to force more current into
Analog Common (Analog Common has only 20µA of pullup
current).
Customization
The internal design of the ICL7139 is set up to facilitate custom versions. A number of features (the annunciators, the
ranges, the overrange trip-points, and the beeper frequency)
of the ICL7139 are mask programmable.
Conclusion
An inexpensive system can now be integrated or customized for
a large number of applications that were too expensive before.
5
Autoranging Concept Insert
In the ICL7106, the 200mV input is normally used, while the voltage applied to it is converted from higher voltages by resistive
dividers, to obtain a variety of ranges. The ICL7139, on the other
hand, changes the integration time to scale the input voltage.
The first range to be integrated has only ten clock-cycles of integration time. For all but large input voltages, this produces a small
voltage across the integration capacitor (CINT). Next, a deintegration current of VREF/RDEINT allows the integrator to ramp
back toward ground. If the digital count is larger than 360 counts,
the display latches are updated, and an autozero cycle is entered
for the remainder of the conversion. If the digital count is smaller
than 360 counts, a short auto-zero cycle is entered, after which
range-two integration begins.
Figure 4 is a timing diagram showing the various phases.
Range-two integration lasts for ten times longer than rangeone integration or one hundred clock cycles. Again, if the
digital count is larger than 360 counts, the display latches
are updated and an autozero cycle is entered. If the digital
count is less than 360 counts, the short autozero cycle is
entered and a range-three integration of one thousand clock
cycles is started. The digital count from range three deintegration is again checked for underrange and if necessary,
range four integration is begun. The fourth range deintegration count is always sent to the display drivers. All of this
happens each reading (normally every 400ms), making the
ICL7139 one of the fastest autoranging meters available.
Application Note 9336
10MΩ
3.3nF
0.1µF
13
INPUTS
V/Ω
10MΩ
10kΩ
14
120kHz
CRYSTAL
15
21
LO BAT
22
a
TRIPLE CAZ CINT OSC OSC
IN
OUT
POINT
9
DEINT
DISPLAY
12
DRIVE
INT (V/Ω)
OUTPUTS
PIN 10
1MΩ
a
e
b
c
d
1-3
23-40
1MΩ
PRESSURE
OR
TEMPERATURE
mAVµA
a
b
e
f
c
11
c
d
d
PIN 4
+
9V
BATTERY
ON/OFF
10
18
N/C
c
d
kΩMΩ
4
INT (I)
ICL7139
VOLTS
b
f
AC
V+
COMMON
e
f
c
e
a
b
S1
5
COMMON
V-
10kΩ
6
ICL8069
PIN 10
VREF
V/Ω/A
10kΩ
19
S2
V+
20
S3
V+
HIΩ-DC/LOΩ-AC
S4
VTEMP/PRES
HOLD
S2 CLOSED: HIΩ-DC
S3 CLOSED: HOLD READING
FIGURE 7. TEMPERATURE AND PRESSURE CONFIGURATION
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