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FUNCTIONAL BLOCK DIAGRAMS
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.
a
High Precision,
Wide-Band RMS-to-DC Converter
AD637
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.
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.
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
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
BUFFER
AD637
ABSOLUTE
VALUE
SQUARER/DIVIDER
BIAS
SECTION
FILTER
25kV
25kV
1
2
3
4
5
6
7
14
13
12
11
10
98
16
15
SOIC (R) Package
BUFFER
AD637
ABSOLUTE
VALUE
SQUARER/DIVIDER
BIAS
SECTION
FILTER
25kV
25kV
1
2
3
4
5
6
7
14
13
12
11
10
9
8
Ceramic DIP (D) and
Cerdip (Q) Packages
AD637–SPECIFICATIONS
(@ +25ⴗC, and ⴞ15 V dc unless otherwise noted)
REV. E
–2–
AD637J/A AD637K/B AD637S
Model Min Typ Max Min Typ Max Min Typ Max Units
TRANSFER FUNCTION
V
OUT
= avg .(V
IN
)
2
V
OUT
= avg .(V
IN
)
2
V
OUT
= avg .(V
IN
)
2
CONVERSION ACCURACY
Total Error, Internal Trim
1
(Fig. 2) ⴞ1 ⴞ 0.5 ⴞ0.5 ⴞ 0.2 ⴞ1 ⴞ 0.5 mV ± % of Reading
T
MIN
to T
MAX
ⴞ3.0 ⴞ 0.6 ⴞ2.0 ⴞ 0.3 ⴞ6 ⴞ 0.7 mV ± % of Reading
vs. Supply, + V
IN
= +300 mV 30 150 30 150 30 150 µV/V
vs. Supply, – V
IN
= –300 mV 100 300 100 300 100 300 µV/V
DC Reversal Error at 2 V 0.25 0.1 0.25 % of Reading
Nonlinearity 2 V Full Scale
2
0.04 0.02 0.04 % of FSR
Nonlinearity 7 V Full Scale 0.05 0.05 0.05 % of FSR
Total Error, External Trim ±0.5 ± 0.1 ±0.25 ± 0.05 ±0.5 ± 0.1 mV ± % of Reading
ERROR VS. CREST FACTOR
3
Crest Factor 1 to 2 Specified Accuracy Specified Accuracy Specified Accuracy
Crest Factor = 3 ±0.1 ±0.1 ±0.1 % of Reading
Crest Factor = 10 ±1.0 ±1.0 ±1.0 % of Reading
AVERAGING TIME CONSTANT 25 25 25 ms/µF C
AV
INPUT CHARACTERISTICS
Signal Range, ±15 V Supply
Continuous RMS Level 0 to 7 0 to 7 0 to 7 V rms
Peak Transient Input ±15 ±15 ±15 V p-p
Signal Range, ±5 V Supply
Continuous rms Level 0 to 4 0 to 4 0 to 4 V rms
Peak Transient Input ±6 ±6 ±6 V p-p
Maximum Continuous Nondestructive
Input Level (All Supply Voltages) ±15 ±15 ±15 V p-p
Input Resistance 6.4 8 9.6 6.4 8 9.6 6.4 8 9.6 kΩ
Input Offset Voltage ±0.5 ±0.2 ±0.5 mV
FREQUENCY RESPONSE
4
Bandwidth for 1% Additional Error (0.09 dB)
V
IN
= 20 mV 11 11 11 kHz
V
IN
= 200 mV 66 66 66 kHz
V
IN
= 2 V 200 200 200 kHz
±3 dB Bandwidth
V
IN
= 20 mV 150 150 150 kHz
V
IN
= 200 mV 1 1 1 MHz
V
IN
= 2 V 8 8 8 MHz
OUTPUT CHARACTERISTICS
Offset Voltage ⴞ1 ⴞ0.5 ⴞ1 mV
vs. Temperature ±0.05 ⴞ0.089 ±0.04 ⴞ0.056 ±0.04 ⴞ0.07 mV/°C
Voltage Swing, ±15 V Supply,
2 kΩ Load 0 to +12.0 +13.5 0 to +12.0 +13.5 0 to +12.0 +13.5 V
Voltage Swing, ±3 V Supply,
2 kΩ Load 0 to +2 +2.2 0 to +2 +2.2 0 to +2 +2.2 V
Output Current 66 6mA
Short Circuit Current 20 20 20 mA
Resistance, Chip Select “High” 0.5 0.5 0.5 Ω
Resistance, Chip Select “Low” 100 100 100 kΩ
dB OUTPUT
Error, V
IN
7 mV to 7 V rms, 0 dB = 1 V rms ±0.5 ±0.3 ±0.5 dB
Scale Factor –3 –3 –3 mV/dB
Scale Factor Temperature Coefficient +0.33 +0.33 +0.33 % of Reading/°C
–0.033 –0.033 –0.033 dB/°C
I
REF
for 0 dB = 1 V rms 5 20 80 52080 52080 µA
I
REF
Range 1 100 1 100 1 100 µA
BUFFER AMPLIFIER
Input Output Voltage Range –V
S
to (+V
S
–V
S
to (+V
S
–V
S
to (+V
S
– 2.5 V) – 2.5 V) – 2.5 V) V
Input Offset Voltage ±0.8 ⴞ2 ±0.5 ⴞ1 ±0.8 ⴞ2 mV
Input Current ±2 ⴞ10 ±2 ⴞ5 ±2 ⴞ10 nA
Input Resistance 10
8
10
8
10
8
Ω
Output Current (+5 mA, (+5 mA, (+5 mA,
–130 µA) –130 µA) –130 µA)
Short Circuit Current 20 20 20 mA
Small Signal Bandwidth 1 1 1 MHz
Slew Rate
5
55 5V/µs
DENOMINATOR INPUT
Input Range 0 to +10 0 to +10 0 to +10 V
Input Resistance 20 25 30 20 25 30 20 25 30 kΩ
Offset Voltage ±0.2 ±0.5 ±0.2 ±0.5 ±0.2 ±0.5 mV
CHIP SELECT PROVISION (CS)
RMS “ON” Level Open or +2.4 V < V
C
< +V
S
Open or +2.4 V < V
C
< +V
S
Open or +2.4 V < V
C
< +V
S
RMS “OFF” Level V
C
< +0.2 V V
C
< +0.2 V V
C
< +0.2 V
I
OUT
of Chip Select
CS “LOW” 10 10 10 µA
CS “HIGH” Zero Zero Zero
On Time Constant 10 µs + ((25 kΩ) × C
AV
) 10 µs + ((25 kΩ) × C
AV
) 10 µs + ((25 kΩ) × C
AV
)
Off Time Constant 10 µs + ((25 kΩ) × C
AV
) 10 µs + ((25 kΩ) × C
AV
) 10 µs + ((25 kΩ) × C
AV
)
POWER SUPPLY
Operating Voltage Range ⴞ3.0 ⴞ18 ⴞ3.0 ⴞ18 ⴞ3.0 ⴞ18 V
Quiescent Current 2.2 3 2.2 3 2.2 3 mA
Standby Current 350 450 350 450 350 450 µA
TRANSISTOR COUNT 107 107 107
NOTES
1
Accuracy specified 0-7 V rms dc with AD637 connected as shown in Figure 2.
2
Nonlinearity is defined as the maximum deviation from the straight line connecting the readings at 10 mV and 2 V.
3
Error vs. crest factor is specified as additional error for 1 V rms.
4
Input voltages are expressed in volts rms. % are in % of reading.
5
With external 2 kΩ pull down resistor tied to –V
S
.
Specifications 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. All min
and max specifications are guaranteed, although only those shown in boldface are tested on all production units.
ABSOLUTE MAXIMUM RATINGS
ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±18 V dc
Internal Quiescent Power Dissipation . . . . . . . . . . . . 108 mW
Output Short-Circuit Duration . . . . . . . . . . . . . . . . . Indefinite
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature Range (Soldering 10 secs) . . . . . . . +300°C
Rated Operating Temperature Range
AD637J, K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD637A, B . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C
AD637S, 5962-8963701CA . . . . . . . . . . . –55°C to +125°C
ORDERING GUIDE
Temperature Package Package
Model Range Description Option
AD637AR –40°C to +85°C SOIC R-16
AD637BR –40°C to +85°C SOIC R-16
AD637AQ – 40°C to +85°C Cerdip Q-14
AD637BQ –40°C to +85°C Cerdip Q-14
AD637JD 0°C to +70°C Side Brazed Ceramic DIP D-14
AD637JD/+ 0°C to +70°C Side Brazed Ceramic DIP D-14
AD637KD 0°C to +70°C Side Brazed Ceramic DIP D-14
AD637KD/+ 0°C to +70°C Side Brazed Ceramic DIP D-14
AD637JQ 0°C to +70°C Cerdip Q-14
AD637KQ 0°C to +70°C Cerdip Q-14
AD637JR 0°C to +70°C SOIC R-16
AD637JR-REEL 0°C to +70°C SOIC R-16
AD637JR-REEL7 0°C to +70°C SOIC R-16
AD637KR 0°C to +70°C SOIC R-16
AD637SD –55°C to +125°C Side Brazed Ceramic DIP D-14
AD637SD/883B –55°C to +125°C Side Brazed Ceramic DIP D-14
AD637SQ/883B –55°C to +125°C Cerdip Q-14
AD637SCHIPS 0°C to +70°CDie
5962-8963701CA* –55°C to +125°C Cerdip Q-14
*A standard microcircuit drawing is available.
FILTER/AMPLIFIER
24kV
24kV
ONE QUADRANT
SQUARER/DIVIDER
BUFFER
AMPLIFIER
Q1
Q2
Q3
Q4
125V
6kV
6kV
12kV
24kV
A5
A1
A2
ABSOLUTE VALUE VOLTAGE –
CURRENT CONVERTER
I
1
I
3
I
4
A4
A3
BIAS
Q5
CAV
+V
S
RMS
OUT
COM
CS
DEN
INPUT
OUTPUT
OFFSET
dB
OUT
AD637
V
IN
BUFF OUT
BUFF IN
–V
S
Figure 1. Simplified Schematic
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD637 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
AD637
REV. E –3–
WARNING!
ESD SENSITIVE DEVICE
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