AD737JN ,Low Cost, Low Power, True RMS-to-DC ConverterFEATURES FUNCTIONAL BLOCK DIAGRAMCOMPUTESTrue RMS ValueAD737Average Rectified Value8kVC 1 8COMAbsol ..
AD737JR ,Low Cost, Low Power, True RMS-to-DC ConverterCHARACTERISTICSHigh Impedance Input (Pin 2)Signal RangeContinuous rms Level V = +2.8 V, –3.2 V 200 ..
AD737JR-REEL7 ,Low Cost, Low Power, True RMS-to-DC ConverterSPECIFICATIONSAD737J/A AD737K/BModel Conditions Min Typ Max Min Typ Max Units2 2TRANSFER FUNCTION V ..
AD737JRZ , Low Cost, Low Power, True RMS-to-DC Converter
AD737JRZ , Low Cost, Low Power, True RMS-to-DC Converter
AD7390AN ,+3 Volt Serial-Input Micropower 10-Bit & 12-Bit DACsapplications.(40°C to 85°C) temperature range. The AD7391AR isThe full-scale voltage output is d ..
ADF06 , DIP Switches
ADF4001 ,200MHz Clock Generator PLLSpecifications subject to change without notice.t t3 4CLOCKt t1 2DB20 DB1 DB0 (LSB)DATA DB2DB19(MSB ..
ADF4001BRU ,200 MHz Clock Generator PLLSpecifications subject to change without notice.t t3 4CLOCKt t1 2DB20 DB1 DB0 (LSB)DATA DB2DB19(MSB ..
ADF4001BRUZ-R7 , 200 MHz Clock Generator PLL
ADF4002BRUZ , Phase Detector/Frequency Synthesizer
ADF4002BRUZ-RL7 , Phase Detector/Frequency Synthesizer
AD737JN-AD737JR-AD737JR-REEL7
Low Cost, Low Power, True RMS-to-DC Converter
REV.C
Low Cost, Low Power,
True RMS-to-DC Converter
FUNCTIONAL BLOCK DIAGRAM
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 1012 V
Low Input Bias Current: 25 pA max
High Accuracy: 60.2 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: 160 mA max Supply Current
No External Trims Needed for Specified Accuracy
AD736—A General Purpose, Buffered Voltage
Output Version Also Available
PRODUCT DESCRIPTIONThe AD737 is a low power, precision, monolithic true rms-to-dc
converter. It is laser trimmed to provide a maximum error of0.2 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 mV 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 mA 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 mA.
The AD737 allows the choice of two signal input terminals: a
high impedance (1012 W) FET input which will directly interface
with high Z input attenuators and a low impedance (8 kW) 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 HIGHLIGHTSThe AD737 is capable of computing the average rectified
value, absolute value or true rms value of various input
signals.Only one external component, an averaging capacitor, is
required for the AD737 to perform true rms measurement.The low power consumption of 0.72 mW makes the AD737
suitable for many battery powered applications.
*Protected under U.S. Patent Number 5,495,245.
AD737–SPECIFICATIONS
(@ +258C, 65 V supplies, ac coupled with 1 kHz sine-wave input applied unless
otherwise noted.)ERROR vs. CREST FACTOR
INPUT CHARACTERISTICS
OUTPUT CHARACTERISTICS
FREQUENCY RESPONSE
TEMPERATURE RANGE
NOTES
lAccuracy is specified with the AD737 connected as shown in Figure 16 with capacitor CC.Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 and 200 mV rms.
3Error vs. Crest Factor is specified as additional error for a 200 mV rms signal. C.F. = VPEAK/V rms.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.
ORDERING GUIDE
PIN CONFIGURATIONS
Plastic DIP (N-8), Cerdip (Q-8), SOIC (SO-8)
ABSOLUTE MAXIMUM RATINGS1Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .–16.5 V
Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . .200 mW
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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
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
NOTESStresses 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.8-Lead Plastic DIP Package: qJA = 165°C/W
8-Lead Cerdip Package: qJA = 110°C/W
8-Lead Small Outline Package: qJA = 155°C/W
AD737
AD737
–Typical CharacteristicsFigure 1.Additional Error vs.
Supply Voltage
Figure 4.Frequency Response
Driving Pin 1
Figure 7.Additional Error vs.
Temperature
Figure 3.Power Down Current vs.
Supply Voltage
Figure 6.Additional Error vs.
Crest Factor vs. CAV
Figure 9.23 dB Frequency vs.
RMS Input Level (Pin 2)
Figure 2.Maximum Input Level
vs. Supply Voltage
Figure 5.Frequency Response
Driving Pin 2
Figure 8.DC Supply Current vs.
RMS lnput Level
Figure 12.RMS Input Level vs.
Frequency for Specified Averaging
Error
Figure 15.Pin 2 Input Bias Current
vs. Temperature
Figure 11.CAV vs. Frequency for
Specified Averaging Error
Figure 14.Settling Time vs. RMS
Input Level for Various Values of CAV
Figure 10.Error vs. RMS Input
Voltage (Pin 2) Using Circuit
of Figure 21
Figure 13. Pin 2 Input Bias
Current vs. Supply Voltage
TYPES OF AC MEASUREMENTThe 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 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 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
CALCULATING SETTLING TIME USING FIGURE 14The 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 mF 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 mF 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.
AD737
Amplitudeinput (Pin 1). The high impedance input, with its low input
bias current, is well suited for use with high impedance input
attenuators. The input signal may be either dc or ac coupled
to the input amplifier. Unlike other rms converters, the AD737
permits both direct and indirect ac coupling of the inputs. AC
coupling is provided by placing a series capacitor between the
input signal and Pin 2 (or Pin 1) for direct coupling and
between Pin 1 and ground (while driving Pin 2) for indirect
coupling.
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 con-
nection (Figure 17).
A final subsection, the bias section, permits a “power down”
function. This reduces the idle current of the AD737 from 160A down to a mere 30 mA. This feature is selected by tying Pin
3 to the +VS terminal. In the average responding connection, all
of the averaging is carried out by an RC post filter consisting of
an 8 kW internal scale-factor resistor connected between Pins 6
and 8 and an external averaging capacitor, CF. In the rms cir-
cuit, this additional filtering stage helps reduce any output
ripple which was not removed by the averaging capacitor, CAV.
RMS MEASUREMENT – CHOOSING THE OPTIMUM
VALUE FOR CAVSince the external averaging capacitor, CAV, “holds” the recti-
fied input signal during rms computation, its value directly af-
fects the accuracy of the rms measurement, especially at low
frequencies. Furthermore, because the averaging capacitor ap-
pears across a diode in the rms core, the averaging time con-
stant 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 cir-
cuit to settle to the new rms level will increase. Therefore, lower
input levels allow the circuit to perform better (due to increased
Mathematically, the rms value of a voltage is defined (using a
simplified equation) as:
This involves squaring the signal, taking the average, and then
obtaining the square root. True rms converters are “smart recti-
fiers”: 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 de voltages, the
converter will have a computational error 11% (of reading)
higher than the true rms value (see Table I).
AD737 THEORY OF OPERATIONAs shown by Figure 16, the AD737 has four functional subsec-
tions: input amplifier, full-wave rectifier, rms core and bias sec-
tions. The FET input amplifier allows both a high impedance,
buffered input (Pin 2) or a low impedance, wide-dynamic-range
+VS
CAV–VS
POWER
DOWN
VIN
+VSPOSITIVE SUPPLY
COMMON
CAV
33mF
10mF
(OPTIONAL)
VOUT
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
