AD637BR [ADI]
High Precision, Wide-Band RMS-to-DC Converter; 高精度,宽波段RMS至DC转换器型号: | AD637BR |
厂家: | ADI |
描述: | High Precision, Wide-Band RMS-to-DC Converter |
文件: | 总10页 (文件大小:163K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
High Precision,
a
Wide-Band RMS-to-DC Converter
AD637
FUNCTIONAL BLOCK DIAGRAMS
FEATURES
High Accuracy
Ceramic DIP (D) and
SOIC (R) Package
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
Cerdip (Q) Packages
BUFFER
AD637
BUFFER
AD637
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
14
13
12
11
10
9
ABSOLUTE
VALUE
ABSOLUTE
VALUE
BIAS
SECTION
BIAS
SQUARER/DIVIDER
SECTION
SQUARER/DIVIDER
25k⍀
25k⍀
25k⍀
25k⍀
FILTER
FILTER
8
PRODUCT DESCRIPTION
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.
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.
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.
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.
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.
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.
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.
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.
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
Fax: 781/326-8703
World Wide Web Site: http://www.analog.com
© Analog Devices, Inc., 1999
(@ +25؇C, and ؎15 V dc unless otherwise noted)
AD637–SPECIFICATIONS
AD637J/A
AD637K/B
AD637S
Typ
Model
Min
Typ
Max
Min
Typ
Max
Min
Max
Units
2
2
2
TRANSFER FUNCTION
VOUT
=
avg . (VIN
)
VOUT
=
avg . (VIN
)
VOUT
=
avg . (VIN )
CONVERSION ACCURACY
Total Error, Internal Trim1 (Fig. 2)
؎1 ؎ 0.5
؎3.0 ؎ 0.6
150
؎0.5 ؎ 0.2
؎2.0 ؎ 0.3
150
؎1 ؎ 0.5 mV ± % of Reading
؎6 ؎ 0.7 mV ± % of Reading
T
MIN to TMAX
vs. Supply, + VIN = +300 mV
vs. Supply, – VIN = –300 mV
DC Reversal Error at 2 V
30
100
30
100
30
100
150
µV/V
300
300
300
µV/V
0.25
0.04
0.05
0.1
0.02
0.25
0.04
0.05
% of Reading
% of FSR
% of FSR
mV ± % of Reading
Nonlinearity 2 V Full Scale2
Nonlinearity 7 V Full Scale
Total Error, External Trim
0.05
± 0.25 ± 0.05
±0.5 ± 0.1
± 0.5 ± 0.1
ERROR VS. CREST FACTOR3
Crest Factor 1 to 2
Crest Factor = 3
Specified Accuracy
Specified Accuracy
Specified Accuracy
±0.1
±1.0
± 0.1
± 1.0
± 0.1
± 1.0
% of Reading
% of Reading
Crest Factor = 10
AVERAGING TIME CONSTANT
25
25
25
ms/µF CAV
INPUT CHARACTERISTICS
Signal Range, ±15 V Supply
Continuous RMS Level
Peak Transient Input
Signal Range, ±5 V Supply
Continuous rms Level
0 to 7
0 to 4
0 to 7
0 to 4
0 to 7
V rms
V p-p
±15
±6
± 15
± 6
± 15
0 to 4
V rms
V p-p
Peak Transient Input
± 6
Maximum Continuous Nondestructive
Input Level (All Supply Voltages)
Input Resistance
±15
9.6
±0.5
± 15
9.6
± 0.2
± 15
9.6
± 0.5
V p-p
kΩ
mV
6.4
8
6.4
8
6.4
8
Input Offset Voltage
FREQUENCY RESPONSE4
Bandwidth for 1% Additional Error (0.09 dB)
V
V
V
IN = 20 mV
IN = 200 mV
IN = 2 V
11
66
200
11
66
200
11
66
200
kHz
kHz
kHz
±3 dB Bandwidth
V
V
IN = 20 mV
IN = 200 mV
150
1
8
150
1
8
150
1
8
kHz
MHz
MHz
VIN = 2 V
OUTPUT CHARACTERISTICS
Offset Voltage
؎1
؎0.089
؎0.5
؎0.056
؎1
؎0.07
mV
mV/°C
vs. Temperature
±0.05
± 0.04
± 0.04
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
Output Current
Short Circuit Current
Resistance, Chip Select “High”
Resistance, Chip Select “Low”
0 to +2
6
+2.2
0 to +2
6
+2.2
0 to +2
6
+2.2
V
mA
mA
Ω
20
0.5
100
20
0.5
100
20
0.5
100
kΩ
dB OUTPUT
Error, VIN 7 mV to 7 V rms, 0 dB = 1 V rms
Scale Factor
±0.5
–3
± 0.3
–3
± 0.5
–3
dB
mV/dB
Scale Factor Temperature Coefficient
+0.33
–0.033
20
+0.33
–0.033
20
+0.33
–0.033
20
% of Reading/°C
dB/°C
µA
I
REF for 0 dB = 1 V rms
5
1
80
100
5
1
80
100
5
1
80
100
IREF Range
µA
BUFFER AMPLIFIER
Input Output Voltage Range
–VS to (+VS
– 2.5 V)
–VS to (+VS
– 2.5 V)
–VS to (+VS
– 2.5 V)
V
Input Offset Voltage
Input Current
Input Resistance
Output Current
±0.8
±2
؎2
؎10
± 0.5
± 2
؎1
؎5
± 0.8
± 2
؎2
؎10
mV
nA
Ω
108
108
108
(+5 mA,
(+5 mA,
(+5 mA,
–130 µA)
–130 µA)
–130 µA)
Short Circuit Current
Small Signal Bandwidth
Slew Rate5
20
1
5
20
1
5
20
1
5
mA
MHz
V/µs
DENOMINATOR INPUT
Input Range
Input Resistance
Offset Voltage
0 to +10
25
±0.2
0 to +10
25
± 0.2
0 to +10
25
± 0.2
V
kΩ
mV
20
30
±0.5
20
30
± 0.5
20
30
± 0.5
CHIP SELECT PROVISION (CS)
RMS “ON” Level
RMS “OFF” Level
IOUT of Chip Select
CS “LOW”
Open or +2.4 V < VC < +VS
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
Zero
10
Zero
µA
CS “HIGH”
On Time Constant
Off Time Constant
10 µs + ((25 kΩ) × CAV
10 µs + ((25 kΩ) × CAV
)
)
10 µs + ((25 kΩ) × CAV
10 µs + ((25 kΩ) × CAV
)
)
10 µs + ((25 kΩ) × CAV
10 µs + ((25 kΩ) × CAV
)
)
POWER SUPPLY
Operating Voltage Range
Quiescent Current
Standby Current
؎3.0
2.2
؎18
3
450
؎3.0
2.2
؎18
3
450
؎3.0
؎18
3
450
V
mA
µA
2.2
350
350
350
TRANSISTOR COUNT
107
107
107
–2–
REV. E
AD637
NOTES
1Accuracy specified 0-7 V rms dc with AD637 connected as shown in Figure 2.
2Nonlinearity is defined as the maximum deviation from the straight line connecting the readings at 10 mV and 2 V.
3Error vs. crest factor is specified as additional error for 1 V rms.
4Input voltages are expressed in volts rms. % are in % of reading.
5With external 2 kΩ pull down resistor tied to –VS.
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
Temperature
Range
Package
Description
Package
Option
Model
AD637AR
AD637BR
AD637AQ
AD637BQ
AD637JD
AD637JD/+
AD637KD
AD637KD/+
AD637JQ
–40°C to +85°C SOIC
–40°C to +85°C SOIC
–40°C to +85°C Cerdip
–40°C to +85°C Cerdip
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
R-16
R-16
Q-14
Q-14
Side Brazed Ceramic DIP D-14
Side Brazed Ceramic DIP D-14
Side Brazed Ceramic DIP D-14
Side Brazed Ceramic DIP D-14
Cerdip
Cerdip
SOIC
SOIC
SOIC
SOIC
Q-14
Q-14
R-16
R-16
R-16
R-16
AD637KQ
AD637JR
AD637JR-REEL
AD637JR-REEL7 0°C to +70°C
AD637KR
0°C to +70°C
AD637SD
–55°C to +125°C Side Brazed Ceramic DIP D-14
–55°C to +125°C Side Brazed Ceramic DIP D-14
AD637SD/883B
AD637SQ/883B
AD637SCHIPS
–55°C to +125°C Cerdip
0°C to +70°C Die
Q-14
5962-8963701CA* –55°C to +125°C Cerdip
Q-14
*A standard microcircuit drawing is available.
FILTER/AMPLIFIER
CAV
+V
BUFF OUT
ONE QUADRANT
SQUARER/DIVIDER
24k⍀
S
BUFF IN
BUFFER
AMPLIFIER
A5
RMS
OUT
A4
I
4
dB
OUT
I
1
24k⍀
COM
Q4
Q1
ABSOLUTE VALUE VOLTAGE –
CURRENT CONVERTER
CS
Q5
BIAS
DEN
INPUT
I
24k⍀
Q2
Q3
A3
3
6k⍀
6k⍀
OUTPUT
OFFSET
A2
12k⍀
125⍀
AD637
V
IN
A1
–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.
WARNING!
ESD SENSITIVE DEVICE
REV. E
–3–
AD637
FUNCTIONAL DESCRIPTION
the AD637 can be ac coupled through the addition of a non-
polar capacitor in series with the input as shown in Figure 2.
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
NC
14
13
VIN2
V rms = Avg
V rms
OPTIONAL
AC COUPLING
CAPACITOR
ABSOLUTE
VALUE
2
3
V
IN
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 volt-
age 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
12 NC
11
BIAS
SECTION
SQUARER/DIVIDER
+V
4
5
6
7
S
25k⍀
–V
10
S
25k⍀
I12
I3
V
3
V
=
9
8
IN
O
C
I4
=
AV
FILTER
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 long-
est period of the input signal than A4s output will be propor-
tional 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.
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.
2
I1
I4
The output signal range of the AD637 is a function of the sup-
ply 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.
I4 = Avg
= I1 rms
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
15
10
5
I12
I4
I4
=
I4 = I1
The denominator current can also be supplied externally by pro-
viding a reference voltage, VREF, to Pin 6. The circuit operates
identically to the rms case except that I3 is now proportional to
VREF. Thus:
I12
I4 = Avg
I3
0
and
0
؎3
؎5
؎10
؎15
؎18
2
SUPPLY VOLTAGE – DUAL SUPPLY – Volts
VIN
VO
=
VDEN
Figure 3. AD637 Max VOUT vs. Supply Voltage
CHIP SELECT
This is the mean square of the input signal.
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 im-
pedance 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 measure-
ments. 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 capaci-
tor, 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 in-
creases to 1% at 3 Hz. If it is desired to measure only ac signals,
–4–
REV. E
AD637
OPTIONAL TRIMS FOR HIGH ACCURACY
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.
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 non-
linearity of the device.
The peak value of the ac ripple component of the averaging er-
ror is defined approximately by the relationship:
50
6.3 τf
The trimming procedure on the AD637 is as follows:
in % of reading where (t > 1/f)
l. Ground the input signal, VIN and adjust R1 to give 0 V out-
put from Pin 9. Alternatively R1 can be adjusted to give the
E
O
IDEAL
O
E
correct output with the lowest expected value of VIN
.
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
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 net-
work or by increasing the value of the averaging capacitor.
2.5
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 + 6.4τ2 f 2
2.5
AD637K: 0.5mV ؎0.2%
0.25mV ؎0.05%
EXTERNAL
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.
5.0
0
0.5
1.0
1.5
2.0
INPUT LEVEL – Volts
100
Figure 4. Max Total Error vs. Input Level AD637K
Internal and External Trims
BUFFER
AD637
10
1
14
13
12
11
10
9
R4
147⍀
ABSOLUTE
VALUE
2
3
PEAK RIPPLE
V
IN
+V
S
1.0
BIAS
SECTION
OUTPUT
OFFSET
ADJUST
SQUARER/DIVIDER
R1
50k⍀
+V
–V
4
5
S
DC ERROR
R2
1M⍀
25k⍀
S
–V
S
0.1
10
25k⍀
100
1k
10k
6
7
+
V rms
OUT
SINEWAVE INPUT FREQUENCY – Hz
C
AV
FILTER
Figure 7. Comparison of Percent DC Error to the Percent
Peak Ripple over Frequency Using the AD637 in the Stan-
dard RMS Connection with a 1 × µF CAV
8
R3
1k⍀
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 configura-
tion. For most applications the single pole filter will give the
best overall compromise between ripple and settling time.
SCALE FACTOR ADJUST,
؎2%
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 ap-
proach 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
–5–
AD637
100
10
100
10
BUFFER
AD637
RMS
BUFFER
OUTPUT
OUTPUT
BUFFER INPUT
1
14
SIGNAL
INPUT
NC
ABSOLUTE
VALUE
13
2
3
ANALOG COM
+
12 NC
11
C3
BIAS
SECTION
1.0
1.0
OUTPUT
OFFSET
SQUARER/DIVIDER
+V
S
4
5
25k⍀
VALUES FOR C AND
AV
CHIP
SELECT
–V
S
10
1% SETTLING TIME
0.1
0.1
FOR STATED % OF READING
AVERAGING ERROR*
ACCURACY ؎2% DUE TO
COMPONENT TOLERANCE
25k⍀
DENOMINATOR
INPUT
9
+
6
7
* %dc ERROR + %RIPPLE (Peak)
C
AV
FILTER
0.01
8
0.01
100k
dB
1
10
100
1k
10k
INPUT FREQUENCY – Hz
Figure 9a.
R
24k⍀
X
24k⍀
100
10
100
VALUES OF C , C2 AND
AV
+
C2
FOR 1 POLE
FILTER, SHORT
AND
1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
R
X
REMOVE C3
FOR 1 POLE POST FILTER
10
* %dc ERROR + % PEAK RIPPLE
ACCURACY ؎20% DUE TO
COMPONENT TOLERANCE
Figure 8. Two Pole Sallen-Key Filter
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.
1.0
1.0
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 averag-
ing 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 averag-
ing 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 set-
tling 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.
0.1
0.1
0.01
100k
0.01
1
10
100
1k
10k
INPUT FREQUENCY – Hz
Figure 9b.
100
10
100
10
VALUES OF C , C2 AND C3
AV
AND 1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
2 POLL SALLEN-KEY FILTER
* %dc ERROR + % PEAK RIPPLE
ACCURACY ؎20% DUE TO
COMPONENT TOLERANCE
The symmetry of the input signal also has an effect on the mag-
nitude 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.
1.0
1.0
0.1
0.1
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.
0.01
100k
0.01
1
10
100
1k
10k
Figure 9c can be used to determine the required value of CAV
C2 and C3 for the desired level of ripple and settling time.
,
INPUT FREQUENCY – Hz
Figure 9c.
–6–
REV. E
AD637
Table I. Practical Values of CAV and C2 for Various Input
Waveforms
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
Recommended C and C2
AV
Values for 1% Averaging
Error@60Hz with T = 16.6ms
Minimum
Absolute Value
Circuit Waveform
and Period
Input Waveform
and Period
R
؋
C 1%
Settling
Time
AV
Recommended Recommended
Time
Constant
Standard
Value CAV
Standard
Value C2
1/2T
T
1/2T
0.47F
0.82F
1.5F
2.7F
181ms
325ms
A
0V
Symmetrical Sine Wave
T
η
cycle has a crest factor of 10 (C.F. = 1
).
T
T
B
C
D
100s
T
T
= DUTY CYCLE =
0V
Sine Wave with dc Offset
Vp
e0
CF = 1/
0
T
T
e
IN
(rms) = 1 Volt rms
100F
10(T – T )
2
T
2
T
6.8F
5.6F
22F
18F
2.67sec
2.17sec
2
0V
10
Pulse Train Waveform
T
C
AV
= 22F
T
T
2
10(T – 2T )
2
T
2
0V
1.0
0.1
CF = 10
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 high-
est frequency allowable is 200 kHz. A 200 mV signal can be
measured with 1% error at signal frequencies up to 100 kHz.
CF = 3
0.01
1
10
100
1000
PULSEWIDTH – s
10
Figure 11. AD637 Error vs. Pulsewidth Rectangular Pulse
7V RMS INPUT
2V RMS INPUT
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
1%
10%
؎3dB
100mV RMS INPUT
0.1
+1.5
+1.0
+0.5
0
0.01
10mV RMS INPUT
1k
10k
100k
1M
10M
INPUT FREQUENCY – Hz
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 con-
verter, the input buffer must have a –3 dB bandwidth that is
wider than that of the AD637. A point that should not be over-
looked 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 amplifi-
ers exhibit a two-to-one difference between rising and falling slew
rates. The AD845 is recommended as a precision input buffer.
+0.5
POSITIVE INPUT PULSE
C
= 22F
AV
–1.0
–1.5
1
2
3
4
5
6
7
8
9
10
11
CREST FACTOR
Figure 12. Additional Error vs. Crest Factor
REV. E
–7–
AD637
2.0
1.8
1.6
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
1.4
1.2
CF = 10
CF = 7
1.0
0.8
Any other dB reference can be used by setting VIN and R1
accordingly.
0.6
0.4
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.
0.2
0.0
CF = 3
0.5
1.0
– V rms
1.5
2.0
V
IN
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).
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
dB SCALE
FACTOR
ADJUST
33.2k⍀
SIGNAL
INPUT
5k⍀
+V
S
BUFFER
AD637
R3
60.4⍀
BUFFER
OUTPUT
BUFFER INPUT
1
14
13
12
11
10
9
*
1k⍀
AD707JN
SIGNAL
INPUT
ABSOLUTE
VALUE
NC
2
3
4
5
6
7
COMPENSATED
dB OUTPUT
+ 100mV/dB
ANALOG COM
NC
–V
S
BIAS
OUTPUT
OFFSET
SECTION
SQUARER/DIVIDER
+V
S
25k⍀
CHIP
–V
S
SELECT
RMS OUTPUT
25k⍀
DENOMINATOR
INPUT
+
1F
dB
FILTER
8
C
AV
10k⍀
+V
S
*1k⍀ + 3500ppm
R1
500k⍀
TC RESISTOR TEL LAB Q81
PRECISION RESISTOR PT146
OR EQUIVALENT
+2.5 VOLTS
AD508J
0dB ADJUST
Figure 14. dB Connection
–8–
REV. E
AD637
+V
S
1F
NOTE: VALUES CHOSEN TO GIVE 0.1%
AVERAGING ERROR @ 1Hz
3.3M⍀ 3.3M⍀
1F
AD548JN
BUFFER
AD637
FILTERED
V rms OUTPUT
1
2
3
4
5
6
7
14
13
12
11
10
9
–V
S
SIGNAL
INPUT
ABSOLUTE
VALUE
NC
6.8M⍀
+V
NC
S
BIAS
SECTION
1000pF
1M⍀
OUTPUT
OFFSET
ADJUST
SQUARER/DIVIDER
+V
S
50k⍀
25k⍀
–V
S
–V
S
2
25k⍀
V
IN
V rms
+
100F
FILTER
8
C
AV
1%
499k⍀
R
C
3.3F
AV1
Figure 15. AD637 as a Low Frequency RMS Converter
EXPANDABLE
VECTOR SUMMATION
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
BUFFER
AD637
1
2
3
14
13
12
11
10
9
ABSOLUTE
VALUE
V
IN
X
2
VO = VX 2 +VY
BIAS
SECTION
SQUARER/DIVIDER
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 ty-
ing all of the denominator inputs (Pin 6) together.
+V
S
4
5
25k⍀
–V
S
25k⍀
6
7
If CAV is added to IC1 in this configuration, the output is
100pF
5pF
FILTER
AD637
8
2 . If the averaging capacitor is included on both
VX 2 +VY
10k⍀
10k⍀
BUFFER
2
VX 2 +VY
IC1 and IC2, the output will be
.
1
2
3
14
13
12
11
10
9
AD711K
This circuit has a dynamic range of 10 V to 10 mV and is lim-
ited only by the 0.5 mV offset voltage of the AD637. The useful
bandwidth is 100 kHz.
ABSOLUTE
VALUE
V
IN
X
10k⍀
BIAS
SECTION
SQUARER/DIVIDER
+V
4
5
S
20k⍀
25k⍀
–V
S
25k⍀
6
7
100pF
FILTER
8
2
2
V
=
V
+ V
V
OUT
X
Figure 16. AD637 Vector Sum Configuration
REV. E
–9–
AD637
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
TO-116 Package
(D-14)
Cerdip Package
(Q-14)
0.005 (0.13) MIN
0.098 (2.49) MAX
0.005 (0.13) MIN
14
0.098 (2.49) MAX
8
14
8
0.310 (7.87)
0.310 (7.87)
0.220 (5.59)
7
0.220 (5.59)
7
1
1
0.320 (8.13)
0.290 (7.37)
0.320 (8.13)
0.290 (7.37)
PIN 1
PIN 1
0.060 (1.52)
0.785 (19.94) MAX
0.060 (1.52)
0.015 (0.38)
0.785 (19.94) MAX
0.015 (0.38)
0.200 (5.08)
MAX
0.200 (5.08)
0.125 (3.18)
0.200 (5.08)
MAX
0.150
(3.81)
MAX
0.150
(3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.015 (0.38)
0.015 (0.38)
0.008 (0.20)
SEATING
PLANE
0.100
(2.54)
BSC
SEATING
PLANE
0.023 (0.58)
0.014 (0.36)
0.070 (1.78)
0.023 (0.58)
0.100
(2.54)
BSC
0.070 (1.78)
0.008 (0.20)
15°
0°
0.030 (0.76)
0.014 (0.36)
0.030 (0.76)
SOIC Package
(R-16)
0.4133 (10.50)
0.3977 (10.00)
16
9
0.2992 (7.60)
0.2914 (7.40)
0.4193 (10.65)
0.3937 (10.00)
1
8
PIN 1
0.1043 (2.65)
0.0926 (2.35)
0.050 (1.27)
BSC
0.0291 (0.74)
0.0098 (0.25)
؋
45؇ 8؇
0؇
0.0192 (0.49)
0.0138 (0.35)
0.0118 (0.30)
0.0040 (0.10)
SEATING
PLANE
0.0500 (1.27)
0.0157 (0.40)
0.0125 (0.32)
0.0091 (0.23)
–10–
REV. E
相关型号:
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