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PDF ( 数据手册 , 数据表 ) AD637

零件编号 AD637
描述 High Precision/ Wide-Band RMS-to-DC Converter
制造商 Analog Devices
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AD637 数据手册, 描述, 功能
a
High Precision,
Wide-Band RMS-to-DC Converter
AD637
FEATURES
High Accuracy
0.02% Max Nonlinearity, 0 V to 2 V RMS Input
0.10% Additional Error to Crest Factor of 3
Wide Bandwidth
8 MHz at 2 V RMS Input
600 kHz at 100 mV RMS
Computes:
True RMS
Square
Mean Square
Absolute Value
dB Output (60 dB Range)
Chip Select-Power Down Feature Allows:
Analog “3-State” Operation
Quiescent Current Reduction from 2.2 mA to 350 A
Side-Brazed DIP, Low Cost Cerdip and SOIC
PRODUCT DESCRIPTION
The AD637 is a complete high accuracy monolithic rms-to-dc
converter that computes the true rms value of any complex
waveform. It offers performance that is unprecedented in inte-
grated circuit rms-to-dc converters and comparable to discrete
and modular techniques in accuracy, bandwidth and dynamic
range. A crest factor compensation scheme in the AD637 per-
mits measurements of signals with crest factors of up to 10 with
less than 1% additional error. The circuit’s wide bandwidth per-
mits the measurement of signals up to 600 kHz with inputs of
200 mV rms and up to 8 MHz when the input levels are above
1 V rms.
As with previous monolithic rms converters from Analog Devices,
the AD637 has an auxiliary dB output available to the user. The
logarithm of the rms output signal is brought out to a separate
pin allowing direct dB measurement with a useful range of
60 dB. An externally programmed reference current allows the
user to select the 0 dB reference voltage to correspond to any
level between 0.1 V and 2.0 V rms.
A chip select connection on the AD637 permits the user to
decrease the supply current from 2.2 mA to 350 µA during
periods when the rms function is not in use. This feature facili-
tates the addition of precision rms measurement to remote or
hand-held applications where minimum power consumption is
critical. In addition when the AD637 is powered down the out-
put goes to a high impedance state. This allows several AD637s
to be tied together to form a wide-band true rms multiplexer.
The input circuitry of the AD637 is protected from overload
voltages that are in excess of the supply levels. The inputs will
not be damaged by input signals if the supply voltages are lost.
FUNCTIONAL BLOCK DIAGRAMS
Ceramic DIP (D) and
Cerdip (Q) Packages
BUFFER
1
AD637
14
2
3
BIAS
SECTION
4
5
25k
6
ABSOLUTE
VALUE
SQUARER/DIVIDER
25k
13
12
11
10
9
7
FILTER
8
SOIC (R) Package
BUFFER
1
AD637
16
2
3
BIAS
SECTION
4
5
25k
6
7
ABSOLUTE
VALUE
SQUARER/DIVIDER
25k
15
14
13
12
FILTER
11
10
89
The AD637 is available in two accuracy grades (J, K) for com-
mercial (0°C to +70°C) temperature range applications; two
accuracy grades (A, B) for industrial (–40°C to +85°C) applica-
tions; and one (S) rated over the –55°C to +125°C temperature
range. All versions are available in hermetically-sealed, 14-lead
side-brazed ceramic DIPs as well as low cost cerdip packages. A
16-lead SOIC package is also available.
PRODUCT HIGHLIGHTS
1. The AD637 computes the true root-mean-square, mean
square, or absolute value of any complex ac (or ac plus dc)
input waveform and gives an equivalent dc output voltage.
The true rms value of a waveform is more useful than an
average rectified signal since it relates directly to the power of
the signal. The rms value of a statistical signal is also related
to the standard deviation of the signal.
2. The AD637 is laser wafer trimmed to achieve rated perfor-
mance without external trimming. The only external compo-
nent required is a capacitor which sets the averaging time
period. The value of this capacitor also determines low fre-
quency accuracy, ripple level and settling time.
3. The chip select feature of the AD637 permits the user to
power down the device down during periods of nonuse,
thereby, decreasing battery drain in remote or hand-held
applications.
4. The on-chip buffer amplifier can be used as either an input
buffer or in an active filter configuration. The filter can be
used to reduce the amount of ac ripple, thereby, increasing
the accuracy of the measurement.
REV. E
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







AD637 pdf, 数据表
AD637
2.0
1.8
1.6
1.4
1.2
CF = 10
1.0
0.8
CF = 7
0.6
0.4
0.2
CF = 3
0.0
0.5 1.0 1.5 2.0
VIN – V rms
Figure 13. Error vs. RMS Input Level for Three Common
Crest Factors
the energy is contained in the peaks). The duty cycle and peak
amplitude were varied to produce crest factors from l to 10
while maintaining a constant 1 volt rms input amplitude.
CONNECTION FOR dB OUTPUT
Another feature of the AD637 is the logarithmic or decibel out-
put. The internal circuit which computes dB works well over a
60 dB range. The connection for dB measurement is shown in
Figure 14. The user selects the 0 dB level by setting R1 for the
proper 0 dB reference current (which is set to exactly cancel the
log output current from the squarer/divider circuit at the desired
0 dB point). The external op amp is used to provide a more
convenient scale and to allow compensation of the +0.33%/°C
temperature drift of the dB circuit. The special T.C. resistor R3
is available from Tel Labs in Londenderry, New Hampshire
(model Q-81) and from Precision Resistor Inc., Hillside, N.J.
(model PT146).
DB CALIBRATION
1. Set VIN = 1.00 V dc or 1.00 V rms
2. Adjust R1 for 0 dB out = 0.00 V
3. Set VIN = 0.1 V dc or 0.10 V rms
4. Adjust R2 for dB out = – 2.00 V
Any other dB reference can be used by setting VIN and R1
accordingly.
LOW FREQUENCY MEASUREMENTS
If the frequencies of the signals to be measured are below
10 Hz, the value of the averaging capacitor required to deliver
even 1% averaging error in the standard rms connection be-
comes extremely large. The circuit shown in Figure 15 shows an
alternative method of obtaining low frequency rms measure-
ments. The averaging time constant is determined by the prod-
uct of R and CAV1, in this circuit 0.5 s/µF of CAV. This circuit
permits a 20:1 reduction in the value of the averaging capacitor,
permitting the use of high quality tantalum capacitors. It is
suggested that the two pole Sallen-Key filter shown in the dia-
gram be used to obtain a low ripple level and minimize the value
of the averaging capacitor.
If the frequency of interest is below 1 Hz, or if the value of the
averaging capacitor is still too large, the 20:1 ratio can be
increased. This is accomplished by increasing the value of R. If
this is done it is suggested that a low input current, low offset
voltage amplifier like the AD548 be used instead of the internal
buffer amplifier. This is necessary to minimize the offset error
introduced by the combination of amplifier input currents and
the larger resistance.
SIGNAL
INPUT
BUFFER INPUT
1
BUFFER
AD637
NC 2
ANALOG COM
3
OUTPUT
OFFSET 4
BIAS
SECTION
CHIP
SELECT
5
DENOMINATOR
25k
INPUT
6
dB
7
ABSOLUTE
VALUE
SQUARER/DIVIDER
25k
FILTER
10k
R1
500k
+VS
+2.5 VOLTS
AD508J
BUFFER
14 OUTPUT
SIGNAL
INPUT
13
12 NC
R3
60.4
1k*
R2
33.2k
5k
+VS
dB SCALE
FACTOR
ADJUST
AD707JN
COMPENSATED
dB OUTPUT
+ 100mV/dB
–VS
11 +VS
10 –VS
RMS OUTPUT
9
+
1F
8
CAV
*1k+ 3500ppm
TC RESISTOR TEL LAB Q81
PRECISION RESISTOR PT146
OR EQUIVALENT
0dB ADJUST
Figure 14. dB Connection
–8– REV. E














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