a
FEATURES
COMPUTES
True RMS Value
Average Rectified Value
Absolute Value
PROVIDES
200 mV Full-Scale Input Range
(Larger Inputs with Input Attenuator)
Direct Interfacing with 3 1/2 Digit
CMOS A/D Converters
High Input Impedance of 10
12
Low Input Bias Current: 25 pA max
High Accuracy: 0.2 mV 0.3% of Reading
RMS Conversion with Signal Crest Factors Up to 5
Wide Power Supply Range: +2.8 V, –3.2 V to 16.5 V
Low Power: 160 A max Supply Current
No External Trims Needed for Specified Accuracy
AD736—A General Purpose, Buffered Voltage
Output Version Also Available
PRODUCT DESCRIPTION
Low Cost, Low Power,
True RMS-to-DC Converter
AD737*
FUNCTIONAL BLOCK DIAGRAM
8k
C
C
1
FULL
WAVE
RECTIFIER
INPUT
AMPLIFIER
BIAS
SECTION
RMS CORE
AD737
8
COM
V
IN
2
8k
7 +V
S
POWER
DOWN
3
6 OUTPUT
–V
S
4
5 C
AV
The AD737 is a low power, precision, monolithic true rms-to-dc
converter. It is laser trimmed to provide a maximum error of
±
0.2 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 per-
formance of non-rms “precision rectifiers” in many applications.
Compared to these circuits, the AD737 offers higher accuracy at
equal or lower cost.
The AD737 can compute the rms value of both ac and dc input
voltages. It can also be operated ac coupled by adding one ex-
ternal capacitor. In this mode, the AD737 can resolve input sig-
nal 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 AD737 has no output buffer amplifier, thereby significantly
reducing dc offset errors occuring at the output. This allows the
device to be highly compatible with high input impedance A/D
converters.
Requiring only 160
µA
of power supply current, the AD737 is
optimized for use in portable multimeters and other battery
powered applications. This converter also provides a “power
down” feature which reduces the power supply standby current
to less than 30
µA.
*Protected
under U.S. Patent Number 5,495,245.
The AD737 allows the choice of two signal input terminals: a
high impedance (10
12
Ω)
FET input which will directly interface
with high Z input attenuators and a low impedance (8 kΩ) input
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 AD737 achieves a 1% of reading error bandwidth exceed-
ing 10 kHz for input amplitudes from 20 mV rms to 200 mV
rms while consuming only 0.72 mW.
The AD737 is available in four performance grades. The
AD737J and AD737K grades are rated over the commercial
temperature range of 0°C to +70°C. The AD737A and AD737B
grades are rated over the industrial temperature range of –40°C
to +85°C.
The AD737 is available in three low-cost, 8-lead packages: plas-
tic DIP, plastic SO and hermetic cerdip.
PRODUCT HIGHLIGHTS
1. The AD737 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 AD737 to perform true rms measurement.
3. The low power consumption of 0.72 mW makes the AD737
suitable for many battery powered applications.
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: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1999
AD737–SPECIFICATIONS
Model
Conditions
(@ +25 C, 5 V supplies, ac coupled with 1 kHz sine-wave input applied unless
otherwise noted.)
Min
AD737J/A
Typ
Max
2
Min
AD737K/B
Typ
V
OUT
=
2
Max
Units
TRANSFER FUNCTION
CONVERSION ACCURACY
Total Error, Internal Trim
1
All Grades
1 kHz Sine Wave
ac Coupled Using C
C
0–200 mV rms
200 mV–1 V rms
V
OUT
=
Avg.(V
IN
)
Avg.(V
IN
)
0.2/0.3
–1.2
0.007
0
0
0
+0.06
–0.18
1.3
+0.25
0.1/0.2
0.7
2.5
0.4/0.5
2.0
0.5/0.7
0.2/0.2
–1.2
0.007
0.2/0.3
2.0
0.3/0.5
±
mV/± % of Reading
% of Reading
±
mV/± % of Reading
±
% of Reading/°C
%/V
%/V
% of Reading
% of Reading
±
mV/± % of Reading
% Additional Error
% Additional Error
T
MIN
-T
MAX
A&B Grades
@ 200 mV rms
J&K Grades
@ 200 mV rms
vs. Supply Voltage
@ 200 mV rms Input
V
S
=
±5
V to
±
16.5 V
@ 200 mV rms Input
V
S
=
±5
V to
±
3 V
dc Reversal Error, dc Coupled @ 600 mV dc
Nonlinearity
2
, 0–200 mV
@ 100 mV rms
Total Error, External Trim
0–200 mV rms
3
ERROR vs. CREST FACTOR
Crest Factor 1 to 3
C
AV
, C
F
= 100
µF
Crest Factor = 5
C
AV
, C
F
= 100
µF
INPUT CHARACTERISTICS
High Impedance Input (Pin 2)
Signal Range
Continuous rms Level
V
S
= +2.8 V, –3.2 V
Continuous rms Level
V
S
=
±5
V to
±
16.5 V
Peak Transient Input
V
S
= +2.8 V, –3.2 V
Peak Transient Input
V
S
=
±5
V
Peak Transient Input
V
S
=
±16.5
V
Input Resistance
Input Bias Current
V
S
=
±5
V
Low Impedance Input (Pin 1)
Signal Range
Continuous rms Level
V
S
= +2.8 V, –3.2 V
Continuous rms Level
V
S
=
±5
V to
±
16.5 V
Peak Transient Input
V
S
= +2.8 V, –3.2 V
Peak Transient Input
V
S
=
±5
V
Peak Transient Input
V
S
=
±16.5
V
Input Resistance
Maximum Continuous
Nondestructive Input
All Supply Voltages
Input Offset Voltage
4
ac Coupled
J&K Grades
A&B Grades
vs. Temperature
vs. Supply
V
S
=
±5
V to
±
16.5 V
vs. Supply
V
S
=
±5
V to
±
3 V
OUTPUT CHARACTERISTICS
Output Voltage Swing
No Load
V
S
= +2.8 V, –3.2 V
No Load
V
S
=
±5
V
No Load
V
S
=
±16.5
V
Output Resistance
@ dc
FREQUENCY RESPONSE
High Impedance Input (Pin 2)
For 1% Additional Error
Sine-Wave Input
V
IN
= 1 mV rms
V
IN
= 10 mV rms
V
IN
= 100 mV rms
V
IN
= 200 mV rms
±
3 dB Bandwidth
Sine-Wave Input
V
IN
= 1 mV rms
V
IN
= 10 mV rms
V
IN
= 100 mV rms
V
IN
= 200 mV rms
+0.1
–0.3
2.5
+0.35
0
0
0
+0.06
–0.18
1.3
+0.25
0.1/0.2
0.7
2.5
+0.1
–0.3
2.5
+0.35
200
1
0.9
4.0
10
12
1
25
±
2.7
0.9
4.0
10
12
1
±
2.7
200
1
25
mV rms
V rms
V
V
V
Ω
pA
6.4
±
1.7
±
3.8
±
11
8
300
l
9.6
±
12
3
3
30
150
6.4
±
1.7
±
3.8
±
11
8
300
l
9.6
±
12
3
3
30
150
mV rms
V rms
V
V
V
kΩ
V p-p
mV
mV
µV/°C
µV/V
µV/V
8
50
80
8
50
80
0 to –1.6
0 to –3.3
0 to –4
6.4
–1.7
–3.4
–5
8
9.6
0 to –1.6
0 to –3.3
0 to –4
6.4
–1.7
–3.4
–5
8
9.6
V
V
V
kΩ
1
6
37
33
5
55
170
190
1
6
37
33
5
55
170
190
kHz
kHz
kHz
kHz
kHz
kHz
kHz
kHz
–2–
REV. C
AD737
Model
Conditions
Min
AD737J/A
Typ
Max
Min
AD737K/B
Typ
Max
Units
FREQUENCY RESPONSE
Low Impedance Input (Pin 1)
For 1% Additional Error
V
IN
= 1 mV rms
V
IN
= 10 mV rms
V
IN
= 100 mV rms
V
IN
= 200 mV rms
±
3 dB Bandwidth
V
IN
= 1 mV rms
V
IN
= 10 mV rms
V
IN
= 100 mV rms
V
IN
= 200 mV rms
POWER SUPPLY
Operating Voltage Range
Quiescent Current
V
IN
= 200 mV rms, No Load
Power Down Mode Current
TEMPERATURE RANGE
Operating, Rated Performance
Commercial (0°C to +70°C)
Industrial (–40°C to +85°C)
Sine-Wave Input
1
6
90
90
Sine-Wave Input
5
55
350
460
+2.8, –3.2
±
5
120
170
25
±
16.5
160
210
40
+2.8, –3.2
5
55
350
460
±
5
120
170
25
±
16.5
160
210
40
kHz
kHz
kHz
kHz
V
µA
µA
µA
1
6
90
90
kHz
kHz
kHz
kHz
Zero Signal
Sine-Wave Input
Pin 3 Tied to +V
S
AD737J
AD737A
AD737K
AD737B
NOTES
l
Accuracy is specified with the AD737 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.
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.
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
±
16.5 V
Internal Power Dissipation
2
. . . . . . . . . . . . . . . . . . . . . 200 mW
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Short-Circuit Duration . . . . . . . . . . . . . . . . . Indefinite
Differential Input Voltage . . . . . . . . . . . . . . . . . . +V
S
and –V
S
Storage Temperature Range (Q) . . . . . . –65°C to +150°C
Storage Temperature Range (N, R) . . . . . –65°C to +125°C
Operating Temperature Range
AD737J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD737A/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 perma-
nent damage to the device. This is a stress rating only; 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-Lead Plastic DIP Package:
θ
JA
= 165°C/W
8-Lead Cerdip Package:
θ
JA
= 110°C/W
8-Lead Small Outline Package:
θ
JA
= 155°C/W
ABSOLUTE MAXIMUM RATINGS
1
ORDERING GUIDE
Temperature
Range
–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
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
Package
Description
Cerdip
Cerdip
Plastic DIP
SOIC
13" Tape and Reel
7" Tape and Reel
Plastic DIP
SOIC
13" Tape and Reel
7" Tape and Reel
Package
Option
Q-8
Q-8
N-8
SO-8
SO-8
SO-8
N-8
SO-8
SO-8
SO-8
Model
AD737AQ
AD737BQ
AD737JN
AD737JR
AD737JR-REEL
AD737JR-REEL7
AD737KN
AD737KR
AD737KR-REEL
AD737KR-REEL7
PIN CONFIGURATIONS
Plastic DIP (N-8), Cerdip (Q-8), SOIC (SO-8)
AD737
8
FULL
WAVE
RECTIFIER
INPUT
AMPLIFIER
BIAS
SECTION
RMS CORE
COM
8k
C
C
1
V
IN
2
8k
7 +V
S
POWER
DOWN
3
6 OUTPUT
–V
S
4
5 C
AV
REV. C
–3–
AD737 –Typical Characteristics
Figure 1. Additional Error vs.
Supply Voltage
Figure 2. Maximum Input Level
vs. Supply Voltage
Figure 3. Power Down Current 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. C
AV
Figure 7. Additional Error vs.
Temperature
Figure 8. DC Supply Current vs.
RMS lnput Level
Figure 9. 23 dB Frequency vs.
RMS Input Level (Pin 2)
–4–
REV. C
Applying the AD737
Figure 10. Error vs. RMS Input
Voltage (Pin 2) Using Circuit
of Figure 21
Figure 11. C
AV
vs. Frequency for
Specified Averaging Error
Figure 12. RMS Input Level 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 C
AV
Figure 15. Pin 2 Input Bias Current
vs. Temperature
CALCULATING SETTLING TIME USING FIGURE 14
TYPES OF AC MEASUREMENT
The graph of Figure 14 may be used to closely approximate the
time required for the AD737 to settle when its input level is re-
duced 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 (re-
duced) 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. 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 AD737
will settle very quickly with increasing input levels.
The AD737 is capable of measuring ac signals by operating as
either an average responding or a true rms-to-de 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 aver-
age absolute value of a sine-wave voltage is 0.636 that of V
PEAK
;
the corresponding rms value is 0.707 times V
PEAK
. 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 measure-
ment 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.
REV. C
–5–
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