AD AD736JR Low cost, low power, true rms-to-dc converter Datasheet

a
FEATURES
COMPUTES
True RMS Value
Average Rectified Value
Absolute Value
PROVIDES
200 mV Full-Scale Input Range
(Larger Inputs with Input Attenuator)
High Input Impedance of 1012 V
Low Input Bias Current: 25 pA max
High Accuracy: 60.3 mV 60.3% of Reading
RMS Conversion with Signal Crest Factors Up to 5
Wide Power Supply Range: +2.8 V, –3.2 V to 616.5 V
Low Power: 200 mA max Supply Current
Buffered Voltage Output
No External Trims Needed for Specified Accuracy
AD737—An Unbuffered Voltage Output Version with
Chip Power Down Is Also Available
PRODUCT DESCRIPTION
The AD736 is a low power, precision, monolithic true
rms-to-dc converter. It is laser trimmed to provide a maximum
error of ± 0.3 mV ± 0.3% of reading with sine-wave inputs. Furthermore, it maintains high accuracy while measuring a wide
range of input waveforms, including variable duty cycle pulses
and triac (phase) controlled sine waves. The low cost and small
physical size of this converter make it suitable for upgrading the
performance of non-rms “precision rectifiers” in many applications. Compared to these circuits, the AD736 offers higher accuracy at equal or lower cost.
The AD736 can compute the rms value of both ac and dc input
voltages. It can also be operated ac coupled by adding one external capacitor. In this mode, the AD736 can resolve input signal levels of 100 µV rms or less, despite variations in
temperature or supply voltage. High accuracy is also maintained
for input waveforms with crest factors of 1 to 3. In addition,
crest factors as high as 5 can be measured (while introducing
only 2.5% additional error) at the 200 mV full-scale input level.
The AD736 has its own output buffer amplifier, thereby providing a great deal of design flexibility. Requiring only 200 µA of
power supply current, the AD736 is optimized for use in portable multimeters and other battery powered applications.
The AD736 allows the choice of two signal input terminals: a
high impedance (1012 Ω) FET input which will directly interface
with high Z input attenuators and a low impedance (8 kΩ) input
Low Cost, Low Power,
True RMS-to-DC Converter
AD736
FUNCTIONAL BLOCK DIAGRAM
which allows the measurement of 300 mV input levels, while
operating from the minimum power supply voltage of +2.8 V,
–3.2 V. The two inputs may be used either singly or differentially.
The AD736 achieves a 1% of reading error bandwidth exceeding
10 kHz for input amplitudes from 20 mV rms to 200 mV rms
while consuming only 1 mW.
The AD736 is available in four performance grades. The
AD736J and AD736K grades are rated over the commercial temperature range of 0°C to +70°C. The AD736A and AD736B
grades are rated over the industrial temperature range of –40°C
to +85°C.
The AD736 is available in three low-cost, 8-pin packages: plastic
mini-DIP, plastic SO and hermetic cerdip.
PRODUCT HIGHLIGHTS
1. The AD736 is capable of computing the average rectified
value, absolute value or true rms value of various input
signals.
2. Only one external component, an averaging capacitor, is
required for the AD736 to perform true rms measurement.
3. The low power consumption of 1 mW makes the AD736
suitable for many battery powered applications.
4. A high input impedance of 1012 Ω eliminates the need for an
external buffer when interfacing with input attenuators.
5. A low impedance input is available for those applications
requiring up to 300 mV rms input signal operating from low
power supply voltages.
REV. C
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices 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 Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
(@ +258C 65 V supplies, ac coupled with 1 kHz sine-wave input applied unless
AD736–SPECIFICATIONS otherwise noted.)
Model
Conditions
Min
V OUT =
TRANSFER FUNCTION
CONVERSION ACCURACY
Total Error, Internal Trim1
All Grades
AD736J/A
Typ
1 kHz Sine Wave
ac Coupled Using CC
0–200 mV rms
200 mV–1 V rms
TMIN–TMAX
A&B Grades
@ 200 mV rms
J&K Grades
@ 200 mV rms
vs. Supply Voltage
@ 200 mV rms Input
VS = ± 5 V to ± 16.5 V
@ 200 mV rms Input
VS = ± 5 V to ± 3 V
dc Reversal Error, dc Coupled @ 600 mV dc
Nonlinearity2, 0 mV–200 mV @ 100 mV rms
Total Error, External Trim
0–200 mV rms
ERROR vs. CREST FACTOR3
Crest Factor 1 to 3
CAV, CF = 100 µF
Crest Factor = 5
CAV, CF = 100 µF
INPUT CHARACTERISTICS
High Impedance Input (Pin 2)
Signal Range
Continuous rms Level
VS = +2.8 V, –3.2 V
Continuous rms Level
VS = ± 5 V to ± 16.5 V
Peak Transient Input
VS = +2.8 V, –3.2 V
Peak Transient Input
VS = ± 5 V
Peak Transient Input
VS = ± 16.5 V
Input Resistance
Input Bias Current
VS = ± 3 V to ± 16.5 V
Low Impedance Input (Pin 1)
Signal Range
Continuous rms Level
VS = +2.8 V, –3.2 V
Continuous rms Level
VS = ± 5 V to ± 16.5 V
Peak Transient Input
VS = +2.8 V, –3.2 V
Peak Transient Input
VS = ± 5 V
Peak Transient Input
VS = ± 16.5 V
Input Resistance
Maximum Continuous
Nondestructive Input
All Supply Voltages
Input Offset Voltage4
ac Coupled
J&K Grades
A&B Grades
vs. Temperature
vs. Supply
VS = ± 5 V to ± 16.5 V
vs. Supply
VS = ± 5 V to ± 3 V
OUTPUT CHARACTERISTICS
Output Offset Voltage
J&K Grades
A&B Grades
vs.Temperature
vs. Supply
VS = ± 5 V to ± 16.5 V
VS = ± 5 V to ± 3 V
Output Voltage Swing
2 kΩ Load
VS = +2.8 V, –3.2 V
2 kΩ Load
VS = ± 5 V
2 kΩ Load
VS = ± 16.5 V
No Load
VS = ± 16.5 V
Output Current
Short-Circuit Current
Output Resistance
@ dc
FREQUENCY RESPONSE
High Impedance Input (Pin 2)
For 1% Additional Error
Sine-Wave Input
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
Max
2
V OUT =
Avg.(V IN )
0.3/0.3
–1.2
AD736K/B
Typ
Min
0.5/0.5
62.0
2
0.7/0.7
0
0
0
+0.06
–0.18
1.3
+0.25
0.1/0.5
0
0
0
0.7
2.5
60.9
64.0
6.4
± 2.7
± 1.7
± 3.8
± 11
8
8
50
80
± 0.1
1
50
50
0 to +1.6
0 to +3.6
0 to +4
0 to +4
2
+1.7
+3.8
+5
+12
± mV/± % of Reading
% of Reading
0.5/0.5
± mV/± % of Reading
± % of Reading/°C
+0.06
–0.18
1.3
+0.25
0.1/0.3
+0.1
–0.3
2.5
+0.35
%/V
%/V
% of Reading
% of Reading
± mV/± % of Reading
0.7
2.5
200
1
1012
1
0.3/0.3
62.0
0.007
+0.1
–0.3
2.5
+0.35
60.9
64.0
25
300
l
Units
Avg.(V IN )
0.2/0.2
–1.2
0.007
Max
% Additional Error
% Additional Error
200
1
± 2.7
1012
1
25
9.6
mV rms
V rms
V
V
V
kΩ
± 12
± 12
V p-p
63
63
30
150
63
63
30
150
mV
mV
µV/°C
µV/V
µV/V
60.3
60.3
20
130
mV
mV
µV/°C
µV/V
µV/V
9.6
6.4
± 1.7
± 3.8
± 11
8
8
50
80
± 0.1
60.5
60.5
20
130
1
50
50
3
0.2
3
0.2
V
V
V
V
mA
mA
Ω
1
6
37
33
1
6
37
33
kHz
kHz
kHz
kHz
–2–
0 to +1.6
0 to +3.6
0 to +4
0 to +4
2
+1.7
+3.8
+5
+12
300
l
mV rms
V rms
V
V
V
Ω
pA
REV. C
AD736
Model
Conditions
± 3 dB Bandwidth
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
Min
AD736J/A
Typ
Min
AD736K/B
Typ
Max
Units
Sine-Wave Input
FREQUENCY RESPONSE
Low Impedance Input (Pin 1)
For 1% Additional Error
Sine-Wave Input
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
± 3 dB Bandwidth
Sine-Wave Input
VIN = l mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
POWER SUPPLY
OperatingVoltageRange
Quiescent Current
200 mV rms, No Load
Max
Zero Signal
Sine-Wave Input
5
55
170
190
5
55
170
190
kHz
kHz
kHz
kHz
1
6
90
90
1
6
90
90
kHz
kHz
kHz
kHz
5
55
350
460
5
55
350
460
kHz
kHz
kHz
kHz
+2.8, –3.2 ± 5
160
230
TEMPERATURE RANGE
Operating, Rated Performance
Commercial (0°C to +70°C)
Industrial (–40°C to +85°C)
± 16.5
200
270
AD736J
AD736A
+2.8, –3.2 ± 5
160
230
± 16.5
200
270
Volts
µA
µA
AD736K
AD736B
NOTES
l
Accuracy is specified with the AD736 connected as shown in Figure 16 with capacitor C C.
2
Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 and 200 mV rms. Output offset voltage is adjusted to zero.
3
Error vs. Crest Factor is specified as additional error for a 200 mV rms signal. C.F. = V PEAK/V rms.
4
DC offset does not limit ac resolution.
Specifications are subject to change without notice.
Specifications shown in boldface are tested on all production units at final electrical test.
Results from those tests are used to calculate outgoing quality levels.
ABSOLUTE MAXIMUM RATINGS 1
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 16.5 V
Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . . 200 mW
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . ± VS
Output Short-Circuit Duration . . . . . . . . . . . . . . . . . Indefinite
Differential Input Voltage . . . . . . . . . . . . . . . . . . +VS and –VS
Storage Temperature Range (Q) . . . . . . –65°C to +150°C
Storage Temperature Range (N, R) . . . . . –65°C to +125°C
Operating Temperature Range
AD736J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD736A/B . . . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300°C
ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V
NOTES
1
Stresses above those listed under “Absolute Maximum Ratings” may cause
permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any other conditions above those indicated in the
operational section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability .
2
8-Pin Plastic Package: θJA = 165°C/W
8-Pin Cerdip Package: θJA = 110°C/W
8-Pin Small Outline Package: θJA = 155°C/W
ORDERING GUIDE
Model
Temperature
Range
Package
Description
Package
Option
AD736JN
AD736KN
AD736JR
AD736KR
AD736AQ
AD736BQ
AD736JR-REEL
AD736JR-REEL-7
AD736KR-REEL
AD736KR-REEL-7
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
–40°C to +85°C
–40°C to +85°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
Plastic Mini-DIP
Plastic Mini-DIP
Plastic SOIC
Plastic SOIC
Cerdip
Cerdip
Plastic SOIC
Plastic SOIC
Plastic SOIC
Plastic SOIC
N-8
N-8
SO-8
SO-8
Q-8
Q-8
SO-8
SO-8
SO-8
SO-8
REV. C
PIN CONFIGURATION
8-Pin Mini-DIP (N-8), 8-Pin SOIC (R-8),
8-Pin Cerdip (Q-8)
–3–
AD736 –Typical Characteristics
Figure 1. Additional Error vs.
Supply Voltage
Figure 2. Maximum Input Level
vs. SupplyVoltage
Figure 3. Peak Buffer Output vs.
Supply Voltage
Figure 4. Frequency Response
Driving Pin 1
Figure 5. Frequency Response
Driving Pin 2
Figure 6. Additional Error vs.
Crest Factor vs. CAV
Figure 7. Additional Error vs.
Temperature
Figure 8. DC Supply Current vs.
RMS lnput Level
Figure 9. –3 dB Frequency vs.
RMS Input Level (Pin2)
–4–
REV. C
Typical Characteristics– AD736
Figure 10. Error vs. RMS Input
Voltage (Pin 2), Output Buffer Offset Is Adjusted To Zero
Figure 11. CAV vs. Frequency for
Specified Averaging Error
Figure 13. Pin 2 Input Bias Current
vs. Supply Voltage
Figure 14. Settling Time vs. RMS
Input Level for Various
Values of CAV
Figure 12. RMS Input Level vs.
Frequency for Specified Averaging Error
Figure 15. Pin 2 Input Bias Current vs. Temperature
CALCULATING SETTLING TIME USING FIGURE 14
The graph of Figure 14 may be used to closely approximate the
time required for the AD736 to settle when its input level is reduced in amplitude. The net time required for the rms converter
to settle will be the difference between two times extracted from
the graph – the initial time minus the final settling time. As an
example, consider the following conditions: a 33 µF averaging
capacitor, an initial rms input level of 100 mV and a final (reduced) input level of 1 mV. From Figure 14, the initial settling
time (where the 100 mV line intersects the 33 µF line) is around
80 ms.
REV. C
The settling time corresponding to the new or final input level
of 1 mV is approximately 8 seconds. Therefore, the net time for
the circuit to settle to its new value will be 8 seconds minus
80 ms which is 7.92 seconds. Note that, because of the smooth
decay characteristic inherent with a capacitor/diode combination, this is the total settling time to the final value (i.e., not the
settling time to 1%, 0.1%, etc., of final value). Also, this graph
provides the worst case settling time, since the AD736 will settle
very quickly with increasing input levels.
–5–
AD736
tions: input amplifier, full-wave rectifier, rms core, output amplifier and bias sections. The FET input amplifier allows
both a high impedance, buffered input (Pin 2) or a low impedance, wide-dynamic-range input (Pin 1). The high impedance
input, with its low input bias current, is well suited for use with
high impedance input attenuators.
The output of the input amplifier drives a full wave precision
rectifier, which in turn, drives the rms core. It is in the core that
the essential rms operations of squaring, averaging and square
rooting are performed, using an external averaging capacitor,
CAV. Without CAV, the rectified input signal travels through the
core unprocessed, as is done with the average responding connection (Figure 17).
A final subsection, an output amplifier, buffers the output from
the core and also allows optional low-pass filtering to be performed via external capacitor, CF, connected across the feedback path of the amplifier. In the average responding
connection, this is where all of the averaging is carried out. In
the rms circuit, this additional filtering stage helps reduce any
output ripple which was not removed by the averaging capacitor, CAV.
TYPES OF AC MEASUREMENT
The AD736 is capable of measuring ac signals by operating as
either an average responding or a true rms-to-dc converter. As
its name implies, an average responding converter computes the
average absolute value of an ac (or ac and dc) voltage or current
by full wave rectifying and low-pass filtering the input signal;
this will approximate the average. The resulting output, a dc
“average” level, is then scaled by adding (or reducing) gain; this
scale factor converts the dc average reading to an rms equivalent
value for the waveform being measured. For example, the average absolute value of a sine-wave voltage is 0.636 that of VPEAK;
the corresponding rms value is 0.707 times VPEAK. Therefore,
for sine-wave voltages, the required scale factor is 1.11 (0.707
divided by 0.636).
In contrast to measuring the “average” value, true rms measurement is a “universal language” among waveforms, allowing the
magnitudes of all types of voltage (or current) waveforms to be
compared to one another and to dc. RMS is a direct measure of
the power or heating value of an ac voltage compared to that of
dc: an ac signal of 1 volt rms will produce the same amount of
heat in a resistor as a 1 volt dc signal.
Mathematically, the rms value of a voltage is defined (using a
simplified equation) as:
V rms =
Avg.(V 2 )
This involves squaring the signal, taking the average, and then
obtaining the square root. True rms converters are “smart rectifiers”: they provide an accurate rms reading regardless of the
type of waveform being measured. However, average responding
converters can exhibit very high errors when their input signals
deviate from their precalibrated waveform; the magnitude of the
error will depend upon the type of waveform being measured.
As an example, if an average responding converter is calibrated
to measure the rms value of sine-wave voltages, and then is used
to measure either symmetrical square waves or dc voltages, the
converter will have a computational error 11% (of reading)
higher than the true rms value (see Table I).
AD736 THEORY OF OPERATION
As shown by Figure 16, the AD736 has five functional subsec-
Figure 16. AD736 True RMS Circuit
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
Waveform Type
1 Volt Peak
Amplitude
Crest Factor
(VPEAK/V rms)
True rms Value
Average Responding
Circuit Calibrated to
Read rms Value of
Sine Waves Will Read
% of Reading Error*
Using Average
Responding Circuit
Undistorted
Sine Wave
1.414
0.707 V
0.707 V
0%
Symmetrical
Square Wave
1.00
1.00 V
1.11 V
+11.0%
Undistorted
Triangle Wave
1.73
0.577 V
0.555 V
–3.8%
Gaussian
Noise (98% of
Peaks <1 V)
3
0.333 V
0.295 V
–11.4%
Rectangular
Pulse Train
2
10
0.5 V
0.1 V
0.278 V
0.011 V
–44%
–89%
SCR Waveforms
50% Duty Cycle
25% Duty Cycle
2
4.7
0.495 V
0.212 V
0.354 V
0.150 V
–28%
–30%
*%of Reading Error =
Average Responding Value – True rmsValue
× 100%
True rmsValue
–6–
REV. C
AD736
RMS MEASUREMENT – CHOOSING THE OPTIMUM
VALUE FOR CAV
Since the external averaging capacitor, CAV, “holds” the rectified input signal during rms computation, its value directly affects the accuracy of the rms measurement, especially at low
frequencies. Furthermore, because the averaging capacitor appears across a diode in the rms core, the averaging time constant
will increase exponentially as the input signal is reduced. This
means that as the input level decreases, errors due to nonideal
averaging will reduce while the time it takes for the circuit to
settle to the new rms level will increase. Therefore, lower input
levels allow the circuit to perform better (due to increased averaging) but increase the waiting time between measurements.
Obviously, when selecting CAV, a trade-off between computational accuracy and settling time is required.
As shown, the dc error is the difference between the average of
the output signal (when all the ripple in the output has been
removed by external filtering) and the ideal dc output. The dc
error component is therefore set solely by the value of averaging
capacitor used-no amount of post filtering (i.e., using a very
large CF) will allow the output voltage to equal its ideal value.
The ac error component, an output ripple, may be easily removed by using a large enough post filtering capacitor, CF.
In most cases, the combined magnitudes of both the dc and ac
error components need to be considered when selecting appropriate values for capacitors CAV and CF. This combined error,
representing the maximum uncertainty of the measurement is
termed the “averaging error” and is equal to the peak value of
the output ripple plus the dc error.
As the input frequency increases, both error components decrease rapidly: if the input frequency doubles, the dc error and
ripple reduce to 1/4 and 1/2 their original values, respectively,
and rapidly become insignificant.
AC MEASUREMENT ACCURACY AND CREST FACTOR
The crest factor of the input waveform is often overlooked when
determining the accuracy of an ac measurement. Crest factor is
defined as the ratio of the peak signal amplitude to the rms amplitude (C.F. = VPEAK/V rms). Many common waveforms, such
as sine and triangle waves, have relatively low crest factors (≤2).
Other waveforms, such as low duty cycle pulse trains and SCR
waveforms, have high crest factors. These types of waveforms
require a long averaging time constant (to average out the long
time periods between pulses). Figure 6 shows the additional
error vs. crest factor of the AD736 for various values of CAV.
SELECTING PRACTICAL VALUES FOR INPUT
COUPLING (CC), AVERAGING (CAV) AND FILTERING
(CF) CAPACITORS
Table II provides practical values of CAV and CF for several
common applications.
Figure 17. AD736 Average Responding Circuit
Table II. AD737 Capacitor Selection Chart
Application
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION (FIGURE 17)
rms
Input
Level
Because the average responding connection does not use the
CAV averaging capacitor, its settling time does not vary with input signal level; it is determined solely by the RC time constant
of CF and the internal 8 kΩ resistor in the output amplifier’s
feedback path.
General Purpose 0–1 V
rms Computation
DC ERROR, OUTPUT RIPPLE, AND AVERAGING
ERROR
General Purpose
Average
Responding
Figure 18 shows the typical output waveform of the AD736 with
a sine-wave input applied. As with all real-world devices, the
ideal output of VOUT = VIN is never exactly achieved; instead,
the output contains both a dc and an ac error component.
Low
Max
CAV
Frequency Crest
Cutoff
Factor
(–3dB)
CF
Settling
Time*
to 1%
20 Hz
200 Hz
5
5
150 µF 10 µF 360 ms
15 µF 1 µF 36 ms
0–200 mV 20 Hz
200 Hz
5
5
33 µF 10 µF 360 ms
3.3 µF 1 µF 36 ms
20 Hz
200 Hz
None
None
33 µF 1.2 sec
3.3 µF 120 ms
0–200 mV 20 Hz
200 Hz
None
None
33 µF 1.2 sec
3.3 µF 120 ms
0–1 V
0–200 mV 50 Hz
60 Hz
5
5
100 µF 33 µF 1.2 sec
82 µF 27 µF 1.0 sec
0–100 mV 50 Hz
60 Hz
5
5
50 µF
47 µF
Speech
0–200 mV 300 Hz
3
1.5 µF 0.5 µF 18 ms
Music
0–100 mV 20 Hz
10
100 µF 68 µF 2.4 sec
SCR Waveform
Measurement
33 µF 1.2 sec
27 µF 1.0 sec
Audio
Applications
*Settling time is specified over the stated rms input level with the input signal increasing
from zero. Settling times will be greater for decreasing amplitude input signals.
Figure 18. Output Waveform for Sine-Wave Input Voltage
REV. C
–7–
AD736
Note that at FL, the amplitude error will be approximately
–30% (–3 dB) of reading. To reduce this error to 0.5% of reading, choose a value of CC that sets FL at one tenth the lowest
frequency to be measured.
In addition, if the input voltage has more than 100 mV of dc
offset, than the ac coupling network shown in Figure 21 should
be used in addition to capacitor CC.
Applications Circuits
C1174a–10–9/88
The input coupling capacitor, CC, in conjunction with the 8 kΩ
internal input scaling resistor, determine the –3 dB low frequency rolloff. This frequency, FL, is equal to:
1
FL =
2π(8,000)(TheValue of CC in Farads )
Figure 22. Battery Powered Option
Figure 19. AD736 with a High Impedance Input Attenuator
Figure 23. Low Z, AC Coupled Input Connection
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
PRINTED IN U.S.A.
Figure 20. Differential Input Connection
Figure 21. External Output VOS Adjustment
–8–
REV. C
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