AD AD637KR High precision, wide-band rms-to-dc converter Datasheet

a
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 integrated 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 permits measurements of signals with crest factors of up to 10 with
less than 1% additional error. The circuit’s wide bandwidth permits 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 facilitates 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 output 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.
High Precision,
Wide-Band RMS-to-DC Converter
AD637
FUNCTIONAL BLOCK DIAGRAMS
Ceramic DIP (D) and
Cerdip (Q) Packages
SOIC (R) Package
BUFFER
BUFFER
AD637
1
AD637
1
16
14
ABSOLUTE
VALUE
2
ABSOLUTE
VALUE
2
13
14
3
BIAS
SECTION
12
3
BIAS
SECTION
4
SQUARER/DIVIDER
4
11
SQUARER/DIVIDER
13
25kV
12
5
25kV
15
10
5
25kV
9
7
7
11
6
25kV
6
FILTER
FILTER
10
8
8
9
The AD637 is available in two accuracy grades (J, K) for commercial (0°C to +70°C) temperature range applications; two
accuracy grades (A, B) for industrial (–40°C to +85°C) applications; 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 performance without external trimming. The only external component required is a capacitor which sets the averaging time
period. The value of this capacitor also determines low frequency 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–SPECIFICATIONS (@ +25ⴗC, and ⴞ15 V dc unless otherwise noted)
Model
AD637J/A
Typ
Min
VOUT =
TRANSFER FUNCTION
CONVERSION ACCURACY
Total Error, Internal Trim1 (Fig. 2)
T MIN to TMAX
vs. Supply, + VIN = +300 mV
vs. Supply, – VIN = –300 mV
DC Reversal Error at 2 V
Nonlinearity 2 V Full Scale2
Nonlinearity 7 V Full Scale
Total Error, External Trim
Input Offset Voltage
Input Current
Input Resistance
Output Current
CHIP SELECT PROVISION (CS)
RMS “ON” Level
RMS “OFF” Level
IOUT of Chip Select
CS “LOW”
CS “HIGH”
On Time Constant
Off Time Constant
POWER SUPPLY
Operating Voltage Range
Quiescent Current
Standby Current
TRANSISTOR COUNT
VOUT =
avg . (VIN )
ⴞ0.5 ⴞ 0.2
ⴞ2.0 ⴞ 0.3
150
300
0.1
0.02
0.05
± 0.25 ± 0.05
ⴞ1 ⴞ 0.5
ⴞ6 ⴞ 0.7
150
300
0.25
0.04
0.05
± 0.5 ± 0.1
Specified Accuracy
± 0.1
± 1.0
25
25
25
0 to 7
0 to 4
6.4
8
0 to 7
±15
0 to 4
±6
±15
9.6
±0.5
6.4
8
0 to 7
± 15
0 to 4
±6
± 15
9.6
± 0.2
6.4
8
Units
2
Specified Accuracy
± 0.1
± 1.0
mV ± % of Reading
mV ± % of Reading
µV/V
µV/V
% of Reading
% of FSR
% of FSR
mV ± % of Reading
% of Reading
% of Reading
ms/µF CAV
± 15
V rms
V p-p
±6
V rms
V p-p
± 15
9.6
± 0.5
V p-p
kΩ
mV
11
66
200
11
66
200
11
66
200
kHz
kHz
kHz
150
1
8
150
1
8
150
1
8
kHz
MHz
MHz
±0.05
ⴞ1
ⴞ0.089
± 0.04
ⴞ0.5
ⴞ0.056
± 0.04
ⴞ1
ⴞ0.07
mV
mV/°C
0 to +12.0 +13.5
0 to +12.0 +13.5
0 to +12.0 +13.5
V
0 to +2
6
0 to +2
6
0 to +2
6
V
mA
mA
Ω
kΩ
5
1
–VS to (+V S
– 2.5 V)
+2.2
+2.2
20
0.5
100
20
0.5
100
±0.5
–3
+0.33
–0.033
20
± 0.3
–3
+0.33
–0.033
20
± 0.5
–3
+0.33
–0.033
20
±0.8
±2
108
80
100
5
1
–VS to (+V S
– 2.5 V)
ⴞ2
ⴞ10
± 0.5
±2
108
80
100
0 to +10
25
±0.2
5
1
–VS to (+VS
– 2.5 V)
ⴞ1
ⴞ5
(+5 mA,
–130 µA)
20
1
5
20
+2.2
20
0.5
100
(+5 mA,
–130 µA)
Short Circuit Current
Small Signal Bandwidth
Slew Rate5
DENOMINATOR INPUT
Input Range
Input Resistance
Offset Voltage
2
Max
Specified Accuracy
±0.1
±1.0
±0.5 ± 0.1
dB OUTPUT
Error, VIN 7 mV to 7 V rms, 0 dB = 1 V rms
Scale Factor
Scale Factor Temperature Coefficient
BUFFER AMPLIFIER
Input Output Voltage Range
ⴞ1 ⴞ 0.5
ⴞ3.0 ⴞ 0.6
150
300
0.25
0.04
0.05
avg . (VIN )
AD637S
Typ
Min
30
100
FREQUENCY RESPONSE4
Bandwidth for 1% Additional Error (0.09 dB)
VIN = 20 mV
VIN = 200 mV
VIN = 2 V
±3 dB Bandwidth
VIN = 20 mV
VIN = 200 mV
VIN = 2 V
IREF for 0 dB = 1 V rms
IREF Range
VOUT =
Max
30
100
AVERAGING TIME CONSTANT
OUTPUT CHARACTERISTICS
Offset Voltage
vs. Temperature
Voltage Swing, ±15 V Supply,
2 kΩ Load
Voltage Swing, ±3 V Supply,
2 kΩ Load
Output Current
Short Circuit Current
Resistance, Chip Select “High”
Resistance, Chip Select “Low”
AD637K/B
Typ
Min
2
30
100
ERROR VS. CREST FACTOR3
Crest Factor 1 to 2
Crest Factor = 3
Crest Factor = 10
INPUT CHARACTERISTICS
Signal Range, ±15 V Supply
Continuous RMS Level
Peak Transient Input
Signal Range, ±5 V Supply
Continuous rms Level
Peak Transient Input
Maximum Continuous Nondestructive
Input Level (All Supply Voltages)
Input Resistance
Input Offset Voltage
avg . (VIN )
Max
0 to +10
25
± 0.2
20
ⴞ2
ⴞ10
V
mV
nA
Ω
(+5 mA,
–130 µA)
20
1
5
30
±0.5
± 0.8
±2
108
80
100
dB
mV/dB
% of Reading/°C
dB/°C
µA
µA
30
± 0.5
20
20
1
5
mA
MHz
V/µs
0 to +10
25
30
± 0.2
± 0.5
V
kΩ
mV
Open or +2.4 V < VC < +V S
VC < +0.2 V
Open or +2.4 V < VC < +VS
VC < +0.2 V
Open or +2.4 V < VC < +VS
VC < +0.2 V
10
Zero
10 µs + ((25 kΩ) × CAV )
10 µs + ((25 kΩ) × CAV )
10
Zero
10 µs + ((25 kΩ) × CAV )
10 µs + ((25 kΩ) × CAV )
10
Zero
10 µs + ((25 kΩ) × CAV )
10 µs + ((25 kΩ) × CAV )
µA
ⴞ3.0
V
mA
µA
ⴞ3.0
2.2
350
ⴞ18
3
450
ⴞ3.0
2.2
350
107
107
–2–
ⴞ18
3
450
2.2
350
ⴞ18
3
450
107
REV. E
AD637
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
ORDERING GUIDE
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
Model
Temperature
Range
Package
Description
Package
Option
AD637AR
AD637BR
AD637AQ
AD637BQ
AD637JD
AD637JD/+
AD637KD
AD637KD/+
AD637JQ
AD637KQ
AD637JR
AD637JR-REEL
AD637JR-REEL7
AD637KR
AD637SD
AD637SD/883B
AD637SQ/883B
AD637SCHIPS
5962-8963701CA*
–40°C to +85°C
–40°C to +85°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
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
–55°C to +125°C
–55°C to +125°C
–55°C to +125°C
0°C to +70°C
–55°C to +125°C
SOIC
SOIC
Cerdip
Cerdip
Side Brazed Ceramic DIP
Side Brazed Ceramic DIP
Side Brazed Ceramic DIP
Side Brazed Ceramic DIP
Cerdip
Cerdip
SOIC
SOIC
SOIC
SOIC
Side Brazed Ceramic DIP
Side Brazed Ceramic DIP
Cerdip
Die
Cerdip
R-16
R-16
Q-14
Q-14
D-14
D-14
D-14
D-14
Q-14
Q-14
R-16
R-16
R-16
R-16
D-14
D-14
Q-14
Q-14
*A standard microcircuit drawing is available.
FILTER/AMPLIFIER
BUFF OUT
ONE QUADRANT
SQUARER/DIVIDER
BUFF IN
CAV
24kV
+VS
BUFFER
AMPLIFIER
A5
RMS
OUT
A4
I4
dB
OUT
I1
24kV
COM
Q4
Q1
ABSOLUTE VALUE VOLTAGE –
CURRENT CONVERTER
6kV
Q5
Q2
6kV
Q3
BIAS
I3
A3
24kV
A2
12kV
125V
VIN
CS
DEN
INPUT
OUTPUT
OFFSET
AD637
A1
–VS
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.
REV. E
–3–
WARNING!
ESD SENSITIVE DEVICE
AD637
the AD637 can be ac coupled through the addition of a nonpolar capacitor in series with the input as shown in Figure 2.
FUNCTIONAL DESCRIPTION
The AD637 embodies an implicit solution of the rms equation
that overcomes the inherent limitations of straightforward rms
computation. The actual computation performed by the AD637
follows the equation
BUFFER
AD637
1
V 2 
V rms = Avg  IN 
V rms 
Figure 1 is a simplified schematic of the AD637, it is subdivided
into four major sections; absolute value circuit (active rectifier),
square/divider, filter circuit and buffer amplifier. The input voltage VIN which can be ac or dc is converted to a unipolar current
I1 by the active rectifier A1, A2. I1 drives one input of the
squarer divider which has the transfer function
2
I
I4 = 1
I3
The output current of the squarer/divider, I4 drives A4 which
forms a low-pass filter with the external averaging capacitor. If
the RC time constant of the filter is much greater than the longest period of the input signal than A4s output will be proportional to the average of I4. The output of this filter amplifier is
used by A3 to provide the denominator current I3 which equals
Avg. I4 and is returned to the squarer/divider to complete the
implicit rms computation.
14 NC
ABSOLUTE
VALUE
2
3
12
BIAS
SECTION
4
SQUARER/DIVIDER
OPTIONAL
AC COUPLING
VIN CAPACITOR
13
NC
11
+VS
10
–VS
25kV
5
25kV
6
VO =
9
VIN3
CAV
FILTER
7
8
Figure 2. Standard RMS Connection
The performance of the AD637 is tolerant of minor variations in
the power supply voltages, however, if the supplies being used
exhibit a considerable amount of high frequency ripple it is
advisable to bypass both supplies to ground through a 0.1 µF
ceramic disc capacitor placed as close to the device as possible.
The output signal range of the AD637 is a function of the supply voltages, as shown in Figure 3. The output signal can be
used buffered or nonbuffered depending on the characteristics
of the load. If no buffer is needed, tie buffer input (Pin 1) to
common. The output of the AD637 is capable of driving 5 mA
into a 2 kΩ load without degrading the accuracy of the device.
I 2 
I 4 = Avg  1  = I1 rms
 I 4 
and
VOUT = VIN rms
If the averaging capacitor is omitted, the AD637 will compute the
absolute value of the input signal. A nominal 5 pF capacitor should
be used to insure stability. The circuit operates identically to that of
the rms configuration except that I3 is now equal to I4 giving
20
2
I1
I4
MAX VOUT – Volts 2kV Load
I4 =
I 4 = I1
The denominator current can also be supplied externally by providing a reference voltage, VREF, to Pin 6. The circuit operates
identically to the rms case except that I3 is now proportional to
VREF. Thus:
2
I
I 4 = Avg 1
I3
and
2
V
V O = IN
V DEN
This is the mean square of the input signal.
15
10
5
0
0
63
65
610
615
SUPPLY VOLTAGE – DUAL SUPPLY – Volts
618
Figure 3. AD637 Max VOUT vs. Supply Voltage
CHIP SELECT
STANDARD CONNECTION
The AD637 includes a chip select feature which allows the user
to decrease the quiescent current of the device from 2.2 mA to
350 µA. This is done by driving the CS, Pin 5, to below 0.2 V
dc. Under these conditions, the output will go into a high impedance state. In addition to lowering power consumption, this
feature permits bussing the outputs of a number of AD637s to
form a wide bandwidth rms multiplexer. If the chip select is not
being used, Pin 5 should be tied high.
The AD637 is simple to connect for a majority of rms measurements. In the standard rms connection shown in Figure 2, only
a single external capacitor is required to set the averaging time
constant. In this configuration, the AD637 will compute the
true rms of any input signal. An averaging error, the magnitude
of which will be dependent on the value of the averaging capacitor, will be present at low frequencies. For example, if the filter
capacitor CAV, is 4 µF this error will be 0.1% at 10 Hz and increases to 1% at 3 Hz. If it is desired to measure only ac signals,
–4–
REV. E
AD637
functions of input signal frequency f, and the averaging time
constant τ (τ: 25 ms/µF of averaging capacitance). As shown in
Figure 6, the averaging error is defined as the peak value of the
ac component, ripple, plus the value of the dc error.
OPTIONAL TRIMS FOR HIGH ACCURACY
The AD637 includes provisions to allow the user to trim out
both output offset and scale factor errors. These trims will result
in significant reduction in the maximum total error as shown in
Figure 4. This remaining error is due to a nontrimmable input
offset in the absolute value circuit and the irreducible nonlinearity of the device.
The trimming procedure on the AD637 is as follows:
The peak value of the ac ripple component of the averaging error is defined approximately by the relationship:
50
in % of reading where (t > 1/f)
6.3 τf
l. Ground the input signal, VIN and adjust R1 to give 0 V output from Pin 9. Alternatively R1 can be adjusted to give the
correct output with the lowest expected value of VIN.
EO
IDEAL
EO
DC ERROR = AVERAGE OF OUTPUT–IDEAL
2. Connect the desired full scale input to VIN, using either a dc
or a calibrated ac signal, trim R3 to give the correct output at
Pin 9, i.e., 1 V dc should give l.000 V dc output. Of course, a
2 V peak-to-peak sine wave should give 0.707 V dc output.
Remaining errors are due to the nonlinearity.
AVERAGE ERROR
DOUBLE-FREQUENCY
RIPPLE
TIME
5.0
Figure 6. Typical Output Waveform for a Sinusoidal Input
AD637K MAX
ERROR – mV
2.5
This ripple can add a significant amount of uncertainty to the
accuracy of the measurement being made. The uncertainty can
be significantly reduced through the use of a post filtering network or by increasing the value of the averaging capacitor.
INTERNAL TRIM
AD637K
EXTERNAL TRIM
0
The dc error appears as a frequency dependent offset at the
output of the AD637 and follows the equation:
1
in % of reading
0.16 + 6.4τ 2 f 2
Since the averaging time constant, set by CAV , directly sets the
time that the rms converter “holds” the input signal during
computation, the magnitude of the dc error is determined only
by CAV and will not be affected by post filtering.
2.5
AD637K: 0.5mV 60.2%
0.25mV 60.05%
EXTERNAL
5.0
0
0.5
1.0
INPUT LEVEL – Volts
2.0
1.5
Figure 4. Max Total Error vs. Input Level AD637K
Internal and External Trims
DC ERROR OR RIPPLE % OF READING
BUFFER
100
AD637
14
1
R4
147V
ABSOLUTE
VALUE
2
3
+VS
OUTPUT
R1
OFFSET 50kV
ADJUST
–VS
VIN
12
BIAS
SECTION
R2
1MV
13
4
SQUARER/DIVIDER
11
+VS
10
–VS
25kV
5
9
FILTER
7
+
8
CAV
V rms
OUT
1.0
DC ERROR
100
1k
SINEWAVE INPUT FREQUENCY – Hz
10k
Figure 7. Comparison of Percent DC Error to the Percent
Peak Ripple over Frequency Using the AD637 in the Standard RMS Connection with a 1 × µ F CAV
R3
1kV
The ac ripple component of averaging error can be greatly
reduced by increasing the value of the averaging capacitor.
There are two major disadvantages to this: first, the value of the
averaging capacitor will become extremely large and second, the
settling time of the AD637 increases in direct proportion to the
value of the averaging capacitor (Ts = 115 ms/µF of averaging
capacitance). A preferable method of reducing the ripple is
through the use of the post filter network, shown in Figure 8.
This network can be used in either a one or two pole configuration. For most applications the single pole filter will give the
best overall compromise between ripple and settling time.
SCALE FACTOR ADJUST,
62%
Figure 5. Optional External Gain and Offset Trims
CHOOSING THE AVERAGING TIME CONSTANT
The AD637 will compute the true rms value of both dc and ac
input signals. At dc the output will track the absolute value of
the input exactly; with ac signals the AD637’s output will approach the true rms value of the input. The deviation from the
ideal rms value is due to an averaging error. The averaging error
is comprised of an ac and dc component. Both components are
REV. E
PEAK RIPPLE
0.1
10
25kV
6
10
–5–
AD637
100
100
+VS
10
–VS
25kV
9
FILTER
8
+
CAV
1.0
1.0
VALUES FOR CAV AND
1% SETTLING TIME
0.1 FOR STATED % OF READING
AVERAGING ERROR*
ACCURACY 62% DUE TO
COMPONENT TOLERANCE
0.1
* %dc ERROR + %RIPPLE (Peak)
0.01
1
10
100
1k
INPUT FREQUENCY – Hz
10k
0.01
100k
Figure 9a.
24kV
Figure 8. Two Pole Sallen-Key Filter
10
* %dc ERROR + % PEAK RIPPLE
ACCURACY 620% DUE TO
COMPONENT TOLERANCE
R
O
R
R
ER
O
R
%
01
R
ER
0.
O
R
R
1%
0.
ER RO
ER
1.0
1.0
5%
Figure 9b shows the relationship between averaging error, signal
frequency settling time and averaging capacitor value. This
graph is drawn for filter capacitor values of 3.3 times the averaging capacitor value. This ratio sets the magnitude of the ac and
dc errors equal at 50 Hz. As an example, by using a 1 µF averaging capacitor and a 3.3 µF filter capacitor, the ripple for a 60 Hz
input signal will be reduced from 5.3% of reading using the
averaging capacitor alone to 0.15% using the single pole filter.
This gives a factor of thirty reduction in ripple and yet the settling time would only increase by a factor of three. The values of
CAV and C2, the filter capacitor, can be calculated for the desired
value of averaging error and settling time by using Figure 9b.
10
1%
Figure 9a shows values of CAV and the corresponding averaging
error as a function of sine-wave frequency for the standard rms
connection. The 1% settling time is shown on the right side of
the graph.
100
VALUES OF CAV, C2 AND
1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
FOR 1 POLE POST FILTER
FOR 1 POLE
FILTER, SHORT
RX AND
REMOVE C3
0.1
0.1
0.01
1
10
100
1k
INPUT FREQUENCY – Hz
10k
0.01
100k
Figure 9b.
R
O
R
R
ER
O
%
R
01
R
ER
0.
O
R
R
ER RO
ER
1.0
0.1
0.01
Figure 9c can be used to determine the required value of CAV,
C2 and C3 for the desired level of ripple and settling time.
* %dc ERROR + % PEAK RIPPLE
ACCURACY 620% DUE TO
COMPONENT TOLERANCE
10
1.0
1%
0.
For applications that are extremely sensitive to ripple, the two pole
configuration is suggested. This configuration will minimize
capacitor values and settling time while maximizing performance.
10
5%
The symmetry of the input signal also has an effect on the magnitude of the averaging error. Table I gives practical component
values for various types of 60 Hz input signals. These capacitor
values can be directly scaled for frequencies other than 60 Hz,
i.e., for 30 Hz double these values, for 120 Hz they are halved.
100
VALUES OF CAV, C2 AND C3
AND 1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
2 POLL SALLEN-KEY FILTER
1%
REQUIRED CAV (AND C2 + C3)
C2 = C3 = 2.2 3 CAV
100
1
10
100
1k
INPUT FREQUENCY – Hz
0.1
10k
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.365
+
C2
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.400
100
REQUIRED CAV (AND C2)
C2 = 3.3 3 CAV
RX
24kV
R
O
R
ER
11
R
O
R
ER
7
C3
25kV
CHIP
SELECT 5
dB
SQUARER/DIVIDER
10
R
O
R
ER
4
DENOMINATOR
6
INPUT
+
12 NC
BIAS
SECTION
R
O
R
ER
3
%
10
OUTPUT
OFFSET
1%
0.
ANALOG COM
10
SIGNAL
INPUT
13
1%
ABSOLUTE
VALUE
NC 2
RMS
OUTPUT
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.115
14
1
BUFFER
OUTPUT
%
01
0.
BUFFER INPUT
AD637
REQUIRED CAV – mF
BUFFER
0.01
100k
Figure 9c.
–6–
REV. E
AD637
Table I. Practical Values of CAV and C2 for Various Input
Waveforms
Input Waveform
and Period
Absolute Value
Circuit Waveform
and Period
Recommended CAV and C2
Values for 1% Averaging
Minimum
Error@60Hz
with T = 16.6ms
R 3 CAV
1%
Recommended Recommended Settling
Time
Standard
Standard
Time
Constant Value C
Value C2
AV
1/2T
T
A
0V
1/2T
0.47mF
1.5mF
181ms
T
0.82mF
2.7mF
325ms
Symmetrical Sine Wave
T
T
B
AC MEASUREMENT ACCURACY AND CREST FACTOR
Crest factor is often overlooked in determining the accuracy of
an ac measurement. Crest factor is defined as the ratio of the
peak signal amplitude to the rms value of the signal (C.F. = Vp/
V rms). Most common waveforms, such as sine and triangle
waves, have relatively low crest factors (≤2). Waveforms which
resemble low duty cycle pulse trains, such as those occurring in
switching power supplies and SCR circuits, have high crest
factors. For example, a rectangular pulse train with a 1% duty
cycle has a crest factor of 10 (C.F. = 1 η ).
T
0V
Sine Wave with dc Offset
T
C
0
T
T2
Vp
T2
0V
10(T – T2)
6.8mF
22mF
h = DUTY CYCLE =
CF = 1/
e0
eIN (rms) = 1 Volt rms
100mF
2.67sec
100ms
T
h
10
Pulse Train Waveform
T
T
T2
T2
CAV = 22mF
10(T – 2T2)
5.6mF
18mF
2.17sec
0V
INCREASE IN ERROR – %
D
FREQUENCY RESPONSE
The frequency response of the AD637 at various signal levels is
shown in Figure 10. The dashed lines show the upper frequency
limits for 1%, 10% and ± 3 dB of additional error. For example,
note that for 1% additional error with a 2 V rms input the highest frequency allowable is 200 kHz. A 200 mV signal can be
measured with 1% error at signal frequencies up to 100 kHz.
1.0
CF = 10
0.1
CF = 3
0.01
10
VOUT – Volts
1000
Figure 12 is a curve of additional reading error for the AD637
for a 1 volt rms input signal with crest factors from 1 to 11. A
rectangular pulse train (pulsewidth 100 µs) was used for this test
since it is the worst-case waveform for rms measurement (all
1V RMS INPUT
1%
0.1
10
100
PULSEWIDTH – ms
Figure 11. AD637 Error vs. Pulsewidth Rectangular Pulse
7V RMS INPUT
2V RMS INPUT
1
1
10%
100mV RMS INPUT
63dB
+1.5
1k
10mV RMS INPUT
10k
100k
1M
INPUT FREQUENCY – Hz
+1.0
INCREASE IN ERROR – %
0.01
10M
Figure 10. Frequency Response
To take full advantage of the wide bandwidth of the AD637 care
must be taken in the selection of the input buffer amplifier. To
insure that the input signal is accurately presented to the converter, the input buffer must have a –3 dB bandwidth that is
wider than that of the AD637. A point that should not be overlooked is the importance of slew rate in this application. For
example, the minimum slew rate required for a 1 V rms 5 MHz
sine-wave input signal is 44 V/µs. The user is cautioned that this
is the minimum rising or falling slew rate and that care must be
exercised in the selection of the buffer amplifier as some amplifiers exhibit a two-to-one difference between rising and falling slew
rates. The AD845 is recommended as a precision input buffer.
REV. E
+0.5
0
+0.5
POSITIVE INPUT PULSE
CAV = 22mF
–1.0
–1.5
1
2
3
4
5
6
7
CREST FACTOR
8
9
10
Figure 12. Additional Error vs. Crest Factor
–7–
11
MAGNITUDE OF ERROR – % OF rms LEVEL
AD637
2.0
DB CALIBRATION
1.8
1.
2.
3.
4.
1.6
1.4
1.2
CF = 10
1.0
Set VIN = 1.00 V dc or 1.00 V rms
Adjust R1 for 0 dB out = 0.00 V
Set VIN = 0.1 V dc or 0.10 V rms
Adjust R2 for dB out = – 2.00 V
Any other dB reference can be used by setting VIN and R1
accordingly.
0.8
CF = 7
0.6
LOW FREQUENCY MEASUREMENTS
0.4
0.2
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 becomes extremely large. The circuit shown in Figure 15 shows an
alternative method of obtaining low frequency rms measurements. The averaging time constant is determined by the product 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 diagram be used to obtain a low ripple level and minimize the value
of the averaging capacitor.
CF = 3
0.0
0.5
1.0
VIN – V rms
1.5
2.0
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 output. 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).
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.
R2
33.2kV
SIGNAL
INPUT
dB SCALE
FACTOR
ADJUST
5kV
+VS
BUFFER
BUFFER INPUT
AD637
1
14
ABSOLUTE
VALUE
NC 2
ANALOG COM
OUTPUT
OFFSET
3
4
SQUARER/DIVIDER
*
1kV
AD707JN
COMPENSATED
dB OUTPUT
+ 100mV/dB
–VS
11 +VS
25kV
CHIP
SELECT 5
dB
SIGNAL
INPUT
12 NC
BIAS
SECTION
DENOMINATOR
6
INPUT
13
R3
60.4V
BUFFER
OUTPUT
10 –VS
RMS OUTPUT
25kV
9
FILTER
7
8
+
1mF
CAV
10kV
+VS
R1
500kV
+2.5 VOLTS
*1kV + 3500ppm
TC RESISTOR TEL LAB Q81
PRECISION RESISTOR PT146
OR EQUIVALENT
AD508J
0dB ADJUST
Figure 14. dB Connection
–8–
REV. E
AD637
1mF
NOTE: VALUES CHOSEN TO GIVE 0.1%
AVERAGING ERROR @ 1Hz
BUFFER
3.3MV
1
OUTPUT
OFFSET 50kV
ADJUST
AD548JN
FILTERED
V rms OUTPUT
14
ABSOLUTE
VALUE
NC 2
+VS
3.3MV
1mF
AD637
+VS
3
–VS
SIGNAL
INPUT
13
6.8MV
12 NC
BIAS
SECTION
1MV
4
SQUARER/DIVIDER
1000pF
11
+VS
10
–VS
25kV
5
–VS
25kV
9
6
FILTER
7
499kV
CAV1
3.3mF
8
+
VIN2
V rms
100mF
CAV
1%
R
Figure 15. AD637 as a Low Frequency RMS Converter
VECTOR SUMMATION
EXPANDABLE
Vector summation can be accomplished through the use of two
AD637s as shown in Figure 16. Here the averaging capacitors
are omitted (nominal 100 pF capacitors are used to insure
stability of the filter amplifier), and the outputs are summed as
shown. The output of the circuit is
2
V O = V X +V Y
BUFFER
14
ABSOLUTE
VALUE
2
3
2
13
VX IN
12
BIAS
SECTION
This concept can be expanded to include additional terms by
feeding the signal from Pin 9 of each additional AD637 through
a 10 kΩ resistor to the summing junction of the AD711, and tying all of the denominator inputs (Pin 6) together.
4
SQUARER/DIVIDER
11
+VS
10
–VS
25kV
5
25kV
9
6
If CAV is added to IC1 in this configuration, the output is
100pF
FILTER
7
2
AD637
1
8
5pF
2
V X +V Y . If the averaging capacitor is included on both
10kV
BUFFER
IC1 and IC2, the output will be
2
2
V X +V Y .
AD637
1
This circuit has a dynamic range of 10 V to 10 mV and is limited only by the 0.5 mV offset voltage of the AD637. The useful
bandwidth is 100 kHz.
10kV
14
ABSOLUTE
VALUE
2
3
13
VX IN
AD711K
12
BIAS
SECTION
4
SQUARER/DIVIDER
11
10kV
+VS
20kV
25kV
5
10
–VS
25kV
9
6
100pF
7
FILTER
8
VOUT =
Figure 16. AD637 Vector Sum Configuration
REV. E
–9–
VX2 + VV2
AD637
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
TO-116 Package
(D-14)
0.005 (0.13) MIN
0.098 (2.49) MAX
14
8
0.098 (2.49) MAX
14
8
0.310 (7.87)
0.220 (5.59)
1
0.320 (8.13)
0.290 (7.37)
0.060 (1.52)
0.015 (0.38)
7
PIN 1
0.785 (19.94) MAX
0.200 (5.08)
MAX
0.150
(3.81)
MAX
0.200 (5.08)
0.125 (3.18)
0.023 (0.58)
0.014 (0.36)
0.015 (0.38)
0.008 (0.20)
0.100 0.070 (1.78) SEATING
(2.54) 0.030 (0.76) PLANE
BSC
0.060 (1.52)
0.015 (0.38)
0.150
(3.81)
MIN
0.100 0.070 (1.78) SEATING
(2.54) 0.030 (0.76) PLANE
BSC
0.320 (8.13)
0.290 (7.37)
15°
0°
0.015 (0.38)
0.008 (0.20)
SOIC Package
(R-16)
0.4133 (10.50)
0.3977 (10.00)
16
9
0.2992 (7.60)
0.2914 (7.40)
1
PIN 1
0.4193 (10.65)
0.3937 (10.00)
8
0.050 (1.27)
BSC
0.0118 (0.30)
0.0040 (0.10)
0.1043 (2.65)
0.0926 (2.35)
88
0.0192 (0.49) SEATING
08
0.0125 (0.32)
0.0138 (0.35) PLANE
0.0091 (0.23)
0.0291 (0.74)
3 458
0.0098 (0.25)
0.0500 (1.27)
0.0157 (0.40)
PRINTED IN U.S.A.
0.200 (5.08)
MAX
0.200 (5.08)
0.125 (3.18)
0.023 (0.58)
0.014 (0.36)
0.310 (7.87)
0.220 (5.59)
1
7
PIN 1
0.785 (19.94) MAX
C804f–0–12/99 (rev. E)
0.005 (0.13) MIN
Cerdip Package
(Q-14)
–10–
REV. E