AD636_13 [ADI]
Low Level, True RMS-to-DC Converter; 低的水平，真RMS至DC转换器
| 型号： | AD636_13 |
| 厂家： | 
ADI |
| 描述： | Low Level, True RMS-to-DC Converter |
| 文件： | 总16页 (文件大小：444K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Low Level, True RMS-to-DC Converter
AD636
Data Sheet
FEATURES
FUNCTIONAL BLOCK DIAGRAM
True rms-to-dc conversion
200 mV full scale
ABSOLUTE
VALUE
V
IN
Laser-trimmed to high accuracy
0.5% maximum error (AD636K)
1.0% maximum error (AD636J)
Wide response capability
dB
SQUARER
DIVIDER
COM
C
AV
+V
S
+V
S
Computes rms of ac and dc signals
1 MHz, −3 dB bandwidth: V rms > 100 mV
Signal crest factor of 6 for 0.5% error
dB output with 50 dB range
Low power: 800 μA quiescent current
Single or dual supply operation
Monolithic integrated circuit
Low cost
CURRENT
MIRROR
10kΩ
R
L
+V
S
I
OUT
BUFFER IN
BUF
10kΩ
BUFFER OUT
40kΩ
AD636
GENERAL DESCRIPTION
–V
S
The AD636 is a low power monolithic IC that performs true
rms-to-dc conversion on low level signals. It offers performance
that is comparable or superior to that of hybrid and modular
converters costing much more. The AD636 is specified for a
signal range of 0 mV to 200 mV rms. Crest factors up to 6 can
be accommodated with less than 0.5% additional error, allowing
accurate measurement of complex input waveforms.
–V
S
Figure 1.
The AD636 is available in two accuracy grades. The total error of the
J-version is typically less than ±0.5 mV ± ±.0% of reading, while
the total error of the AD636K is less than ±0.2 mV to ±0.5% of
reading. Both versions are temperature rated for operation
between 0°C and 70°C and available in ±4-lead SBDIP and ±0-lead
TO-±00 metal can.
The low power supply current requirement of the AD636,
typically 800 μA, is ideal for battery-powered portable
The AD636 computes the true root-mean-square of a complex ac
(or ac plus dc) input signal and gives an equivalent dc output level.
The true rms value of a waveform is a more useful quantity than
the average rectified value because it is a measure of the power
in the signal. The rms value of an ac-coupled signal is also its
standard deviation.
instruments. It operates from a wide range of dual and single
power supplies, from ±2.5 V to ±±6.5 V or from +5 V to +24 V.
The input and output terminals are fully protected; the input
signal can exceed the power supply with no damage to the device
(allowing the presence of input signals in the absence of supply
voltage), and the output buffer amplifier is short-circuit protected.
The AD636 includes an auxiliary dB output derived from an
internal circuit point that represents the logarithm of the rms
output. The 0 dB reference level is set by an externally supplied
current and can be selected to correspond to any input level from
0 dBm (774.6 mV) to −20 dBm (77.46 mV). Frequency response
ranges from ±.2 MHz at 0 dBm to greater than ±0 kHz at −50 dBm.
The 200 mV full-scale range of the AD636 is compatible with
many popular display-oriented ADCs. The low power supply
current requirement permits use in battery-powered hand-held
instruments. An averaging capacitor is the only external
component required to perform measurements to the fully
specified accuracy is. Its value optimizes the trade-off between
low frequency accuracy, ripple, and settling time.
The AD636 is easy to use. The device is factory-trimmed at the
wafer level for input and output offset, positive and negative
waveform symmetry (dc reversal error), and full-scale accuracy
at 200 mV rms. Therefore, no external trims are required to
achieve full-rated accuracy.
An optional on-chip amplifier acts as a buffer for the input or the
output signals. Used in the input, it provides accurate
performance from standard ±0 MΩ input attenuators. As an
output buffer, it sources up to 5 mA.
Rev. E
Document Feedback
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responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
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Tel: 781.329.4700
Technical Support
©2013 Analog Devices, Inc. All rights reserved.
www.analog.com
AD636
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Applications..................................................................................... 10
Standard Connection................................................................. 10
Optional Trims for High Accuracy.......................................... 10
Single-Supply Connection ........................................................ 10
Choosing the Averaging Time Constant................................. 11
A Complete AC Digital Voltmeter........................................... 12
A Low Power, High Input, Impedance dB Meter....................... 12
Circuit Description ................................................................ 12
Performance Data .................................................................. 12
Frequency Response 3 dBm ............................................... 13
Calibration .............................................................................. 13
Outline Dimensions....................................................................... 14
Ordering Guide .......................................................................... 14
Functional Block Diagram .............................................................. 1
General Description ......................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Absolute Maximum Ratings............................................................ 5
ESD Caution.................................................................................. 5
Pin Configurations and Function Descriptions ........................... 6
Typical Performance Characteristics ............................................. 7
Theory of Operation ........................................................................ 8
RMS Measurements ..................................................................... 8
The AD636 Buffer Amplifier ...................................................... 8
Frequency Response..................................................................... 9
AC Measurement Accuracy and Crest Factor (CF)................. 9
REVISION HISTORY
5/13—Rev. D to Rev. E
11/06—Rev. C to Rev. D
Reorganized Layout............................................................Universal
Changes to Figure 1...........................................................................1
Change to Table 1 ..............................................................................4
Added Typical Performance Characteristics Section ...................7
Added Theory of Operation Section; Changes to Figure 7 and
Figure 8 ...............................................................................................8
Changed Applying the AD636 Section to Applications Section;
Changes to Figure 9, Figure 10, and Single-Supply Connection
Section...............................................................................................10
Changes to Figure 11.......................................................................11
Changes to Figure 13 and A Complete AC Digital Voltmeter
Section...............................................................................................12
Changes to Figure 17 and Figure 18..............................................13
Changes to Ordering Guide ...........................................................14
Changes to General Description .....................................................1
Changes to Table 1.............................................................................3
Changes to Ordering Guide.......................................................... 13
1/06—Rev B to Rev. C
Updated Format..................................................................Universal
Changes to Figure 1 and General Description ..............................1
Deleted Metallization Photograph ..................................................3
Added Pin Configuration and Function Description Section ....6
Updated Outline Dimensions....................................................... 14
Changes to Ordering Guide.......................................................... 14
8/99—Rev A to Rev. B
Rev. E | Page 2 of 16
Data Sheet
AD636
SPECIFICATIONS
@ 25°C, +VS = +3 V, and −VS = –5 V, unless otherwise noted.1
Table 1.
AD636J
Typ
AD636K
Typ
Model
Min
Max
Min
Max
Unit
TRANSFER FUNCTION
2
2
VOUT = avg ×
(
VIN
)
VOUT = avg ×
(
VIN
)
CONVERSION ACCURACY
Total Error, Internal Trim2, 3
0.5 1.0
0.2 0.5
mV % of
reading
vs. Temperature, 0°C to +70°C
vs. Supply Voltage
0.1 0.01
0.1 0.005 mV % of
reading/°C
0.1 0.01
0.1 0.01
mV % of
reading/V
DC Reversal Error at 200 mV
Total Error, External Trim2
0.2
0.3 0.3
0.1
0.1 0.2
% of reading
mV % of
reading
ERROR VS. CREST FACTOR4
Crest Factor 1 to 2
Specified Accuracy
Specified Accuracy
Crest Factor = 3
Crest Factor = 6
−0.2
−0.5
25
−0.2
−0.5
25
% of reading
% of reading
ms/μF of CAV
AVERAGING TIME CONSTANT
INPUT CHARACTERISTICS
Signal Range, All Supplies
Continuous RMS Level
Peak Transient Inputs
+3 V, −5 V Supply
0 to 200
0 to 200
mV rms
2.8
2.0
5.0
2.8
2.0
5.0
V p-p
V p-p
V p-p
2.5 V Supply
5 V Supply
Maximum Continuous
Nondestructive
Input Level (All Supply Voltages)
Input Resistance
12
12
V p-p
kΩ
5.33
6.67
8
5.33
6.67
8
Input Offset Voltage
0.5
0.2
mV
FREQUENCY RESPONSE3, 5
Bandwidth for 1% Additional
Error (0.09 dB)
VIN = 10 mV
VIN = 100 mV
VIN = 200 mV
14
90
130
14
90
130
kHz
kHz
kHz
3 dB Bandwidth
VIN = 10 mV
VIN = 100 mV
100
900
1.5
100
900
1.5
kHz
kHz
MHz
VIN = 200 mV
OUTPUT CHARACTERISTICS3
Offset Voltage, VIN = COM
vs. Temperature
vs. Supply
0.5
0.2
mV
μV/°C
mV/V
10
0.1
10
0.1
Voltage Swing
+3 V, −5 V Supply
5 V to 16.5 V Supply
Output Impedance
0.3
0.3
8
0 to 1.0
0 to 1.0
10
0.3
0.3
8
0 to 1.0
0 to 1.0
10
V
V
kΩ
12
12
Rev. E | Page 3 of 16
AD636
Data Sheet
AD636J
Typ
AD636K
Typ
Model
Min
Max
0.5
Min
Max
0.2
Unit
dB OUTPUT
Error, VIN = 7 mV to 300 mV rms
Scale Factor
0.3
−3.0
0.33
0.1
−3.0
0.33
dB
mV/dB
% of reading/°C
Scale Factor Temperature
Coefficient
−0.033
4
−0.033
4
dB/°C
μA
IREF for 0 dB = 0.1 V rms
IREF Range
2
1
8
2
8
50
1
50
μA
IOUT TERMINAL
IOUT Scale Factor
100
10
10
100
10
10
μA/V rms
IOUT Scale Factor Tolerance
Output Resistance
Voltage Compliance
−20
8
+20
12
−20
8
+20
12
%
kΩ
V
−VS to
−VS to
(+VS − 2 V)
(+VS − 2 V)
BUFFER AMPLIFIER
Input and Output Voltage Range
−VS to
−VS to
V
(+VS − 2 V)
(+VS − 2 V)
Input Offset Voltage, RS = 10 kΩ
Input Bias Current
Input Resistance
0.8
100
108
2
300
0.5
100
108
1
300
mV
nA
Ω
Output Current
(+5 mA,
(+5 mA,
−130 μA)
−130 μA)
Short-Circuit Current
Small Signal Bandwidth
Slew Rate6
20
1
5
20
1
5
mA
MHz
V/μs
POWER SUPPLY
Voltage, Rated Performance
Dual Supply
Single Supply
Quiescent Current7
TEMPERATURE RANGE
Rated Performance
Storage
+3, −5
0.80
+3, −5
0.80
V
V
V
mA
+2, −2.5
5
16.5
24
1.00
+2, −2.5
5
16.5
24
1.00
0
−55
+70
+150
0
−55
+70
+150
°C
°C
TRANSISTOR COUNT
62
62
1 All minimum and maximum specifications are guaranteed. Specifications shown in boldface are tested on all production units at final electrical test and are used to
calculate outgoing quality levels.
2 Accuracy specified for 0 mV to 200 mV rms, dc or 1 kHz sine wave input. Accuracy is degraded at higher rms signal levels.
3 Measured at Pin 8 of PDIP (IOUT), with Pin 9 tied to common.
4 Error vs. crest factor is specified as additional error for a 200 mV rms rectangular pulse train, pulse width = 200 µs.
5 Input voltages are expressed in V rms.
6 With 10 kΩ pull-down resistor from Pin 6 (BUF OUT) to −VS.
7 With BUF IN tied to COMMON.
Rev. E | Page 4 of 16
Data Sheet
AD636
ABSOLUTE MAXIMUM RATINGS
Table 2.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent 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.
Parameter
Ratings
Supply Voltage
Dual Supply
16.5 V
Single Supply
24 V
Internal Power Dissipation1
Maximum Input Voltage
Storage Temperature Range
Operating Temperature Range
Lead Temperature Range (Soldering 60 sec)
ESD Rating
500 mW
12 VPEAK
−55°C to +150°C
0°C to 70°C
300°C
ESD CAUTION
1000 V
1 10-Lead TO: θJA = 150°C/W.
14-Lead PDIP: θJA = 95°C/W.
Rev. E | Page 5 of 16
AD636
Data Sheet
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
BUF IN
9
BUF OUT
V
1
2
3
4
5
6
7
14 +V
S
I
IN
OUT
8
NC
13 NC
12 NC
11 NC
10 COM
10
dB
7
–V
S
AD636
R
L
1
C
TOP VIEW
AD636
AV
6
C
AV
(Not to Scale)
dB
BUF OUT
BUF IN
2
COM
5
9
8
R
I
3
4
L
–V
S
+V
OUT
S
V
IN
NC = NO CONNECT
Figure 3. 10-Pin TO-100 Pin Configuration
Figure 2. 14-Lead SBDIP Pin Configuration
Table 3. Pin Function Descriptions—14-Lead SBDIP
Table 4. Pin Function Descriptions—10-Pin TO-100
Pin No.
Mnemonic Description
Pin No.
Mnemonic Description
1
2
3
4
5
VIN
Input Voltage.
No Connection.
Negative Supply Voltage.
Averaging Capacitor.
Log (dB) Value of the RMS Output
Voltage.
Buffer Output.
Buffer Input.
RMS Output Current.
Load Resistor.
1
2
3
4
5
6
7
8
RL
COM
+VS
VIN
−VS
CAV
dB
BUF OUT
BUF IN
IOUT
Load Resistor.
Common.
NC
−VS
CAV
dB
Positive Supply Voltage.
Input Voltage.
Negative Supply Voltage.
Averaging Capacitor.
Log (dB) Value of the RMS Output Voltage.
Buffer Output.
6
7
8
9
BUF OUT
BUF IN
IOUT
9
10
Buffer Input.
RMS Output Current.
RL
10
11, 12, 13
14
COM
NC
+VS
Common.
No Connection.
Positive Supply Voltage.
Rev. E | Page 6 of 16
Data Sheet
AD636
TYPICAL PERFORMANCE CHARACTERISTICS
0.5
200µs
ŋ
= DUTY CYCLE =
1.0
T
T
CF = 1/
ŋ
V
P
0
E
(rms) = 200mV
IN
E
O
0
200µs
R
= 50kΩ
L
0.5
R
= 16.7kΩ
L
–0.5
–1.0
R
= 6.7kΩ
(Ω)
L
0
0
1k
10k
EXTERNAL
100k
1M
1
2
3
4
5
6
7
R
CREST FACTOR
Figure 4. Ratio of Peak Negative Swing to −VS vs. REXTERNAL
for Several Load Resistances
Figure 6. Error vs. Crest Factor
1V rms INPUT
1
1%
10%
±3dB
200mV rms INPUT
100mV rms INPUT
200m
100m
30mV rms INPUT
30m
10m
10mV rms
INPUT
1m
1mV rms INPUT
0.1m
1k
10k
100k
FREQUENCY (Hz)
1M
10M
Figure 5. AD636 Frequency Response
Rev. E | Page 7 of 16
AD636
Data Sheet
THEORY OF OPERATION
RMS MEASUREMENTS
CURRENT MIRROR
14 +V
S
The AD636 embodies an implicit solution of the rms equation
that overcomes the dynamic range as well as other limitations
inherent in a straightforward computation of rms. The actual
computation performed by the AD636 follows the equation:
10 COM
20µA
FS
R1
25kΩ
4
8
9
R
I3
L
R2
10kΩ
C
I
AV OUT
10µA
FS
2
VIN
C
ABSOLUTE VALUE/
VOLTAGE–CURRENT
CONVERTER
AV
V rms Avg
I4
+V
S
V rms
I
REF
I1
dB
5
6
A3
OUT
BUF
IN
Q1
R4
20kΩ
BUFFER
A4
The AD636 is comprised of four major sections: absolute value
circuit (active rectifier), squarer/divider, current mirror, and
buffer amplifier (see Figure 7, for a simplified schematic). 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:
|V
|
IN
Q3
7
BUF
OUT
+
R4
V
1
IN
Q5
8kΩ
A1
Q2 Q4
10kΩ
A2
R3
10kΩ
8kΩ
ONE-QUADRANT
SQUARER/
DIVIDER
–V
S
3
Figure 7. Simplified Schematic
I12
I4
I3
THE AD636 BUFFER AMPLIFIER
The buffer amplifier included in the AD636 offers the user
The output current, I4, of the squarer/divider drives the current
mirror through a low-pass filter formed by R1 and the externally
connected capacitor, CAV. If the R1, CAV time constant is much
greater than the longest period of the input signal, then I4 is
effectively averaged. The current mirror returns a current I3,
which equals Avg. [I4], back to the squarer/divider to complete
the implicit rms computation. Therefore,
additional application flexibility. It is important to understand
some of the characteristics of this amplifier to obtain optimum
performance. Figure 8 shows a simplified schematic of the buffer.
Because the output of an rms-to-dc converter is always positive,
it is not necessary to use a traditional complementary Class AB
output stage. In the AD636 buffer, a Class A emitter follower is
used instead. In addition to excellent positive output voltage
swing, this configuration allows the output to swing fully down
to ground in single-supply applications without the problems
associated with most IC operational amplifiers.
2
I2
I4
I4 Avg
I1 rms
The current mirror also produces the output current, IOUT, which
equals 2I4. IOUT can be used directly or converted to a voltage
with R2 and buffered by A4 to provide a low impedance voltage
output. The transfer function of the AD636 thus results
+V
S
CURRENT
MIRROR
V
OUT = 2 R2 I rms = VIN rms
BUFFER
OUTPUT
5µA 5µA
The dB output is derived from the emitter of Q3, because the
voltage at this point is proportional to –log VIN. Emitter follower,
Q5, buffers and level shifts this voltage, so that the dB output
voltage is zero when the externally supplied emitter current
(IREF) to Q5 approximates I3.
10kΩ
BUFFER
INPUT
R
R
LOAD
E
40kΩ
R
EXTERNAL
(OPTIONAL, SEE TEXT)
–V
S
Figure 8. Buffer Amplifier Simplified Schematic
When this amplifier is used in dual-supply applications as an
input buffer amplifier driving a load resistance referred to
ground, steps must be taken to ensure an adequate negative
voltage swing. For negative outputs, current flows from the load
resistor through the 40 kΩ emitter resistor, setting up a voltage
divider between −VS and ground. This reduced effective −VS,
limits the available negative output swing of the buffer. The
addition of an external resistor in parallel with RE alters this
voltage divider such that increased negative swing is possible.
Rev. E | Page 8 of 16
Data Sheet
AD636
Figure 4 shows the value of REXTERNAL for a particular ratio of
AC MEASUREMENT ACCURACY AND CREST
FACTOR (CF)
V
PEAK to −VS for several values of RLOAD. The addition of
REXTERNAL increases the quiescent current of the buffer amplifier
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 (CF = VP/V
rms). Most common waveforms, such as sine and triangle
waves, have relatively low crest factors (<2). Waveforms that
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
by an amount equal to REXT/−VS. Nominal buffer quiescent
current with no REXTERNAL is 30 µA at −VS = −5 V.
FREQUENCY RESPONSE
The AD636 uses a logarithmic circuit to perform the implicit rms
computation. As with any log circuit, bandwidth is proportional to
signal level. The solid lines in Figure 5 represent the frequency
response of the AD636 at input levels from 1 mV to 1 V rms.
The dashed lines indicate the upper frequency limits for 1%,
10%, and 3 dB of reading additional error. For example, note
that a 1 V rms signal produces less than 1% of reading additional
error up to 220 kHz. A 10 mV signal can be measured with 1%
of reading additional error (100 µV) up to 14 kHz.
η
cycle has a crest factor of 10 (CF = 1/√ ).
Figure 6 is a curve of reading error for the AD636 for a
200 mV rms input signal with crest factors from 1 to 7. A
rectangular pulse train (pulse width 200 μs) was used for this
test because it is the worst-case waveform for rms measurement
(all the energy is contained in the peaks). The duty cycle and
peak amplitude were varied to produce crest factors from 1 to 7
while maintaining a constant 200 mV rms input amplitude.
Rev. E | Page 9 of 16
AD636
Data Sheet
APPLICATIONS
The input and output signal ranges are a function of the supply
voltages as detailed in the specifications. The AD636 can also be
used in an unbuffered voltage output mode by disconnecting the
input to the buffer. The output then appears unbuffered across
the 10 kΩ resistor. The buffer amplifier can then be used for
other purposes. Further, the AD636 can be used in a current
output mode by disconnecting the 10 kΩ resistor from the ground.
The output current is available at Pin 8 (Pin 10 on the H package)
with a nominal scale of 100 μA per volt rms input, positive out.
The trimming procedure is as follows:
•
Ground the input signal, VIN, and adjust R4 to give 0 V
output from Pin 6. Alternatively, R4 can be adjusted to give
the correct output with the lowest expected value of VIN.
•
Connect the desired full-scale input level to VIN, either dc or a
calibrated ac signal (1 kHz is the optimum frequency); then
trim R1 to give the correct output from Pin 6, that is, 200 mV
dc input should give 200 mV dc output. Of course, a 200 mV
peak-to-peak sine wave should give a 141.4 mV dc output.
The remaining errors, as given in the specifications, are due to
the nonlinearity.
STANDARD CONNECTION
The AD636 is simple to connect for the majority of high accuracy
rms measurements, requiring only an external capacitor to set
the averaging time constant. The standard connection is shown
in Figure 9 In this configuration, the AD636 measures the rms
of the ac and dc level present at the input but shows an error for
low frequency inputs as a function of the filter capacitor, CAV, as
shown in Figure 13. Therefore, if a 4 μF capacitor is used, the
additional average error at 10 Hz is 0.1%, and at 3 Hz it is 1%.
The accuracy at higher frequencies is according to specification.
If it is desired to reject the dc input, a capacitor is added in
series with the input, as shown in Figure 11; the capacitor must
be nonpolar. If the AD636 is driven with power supplies with a
considerable amount of high frequency ripple, it is advisable to
bypass both supplies to ground with 0.1 μF ceramic discs as near
the device as possible. CF is an optional output ripple filter.
C
AV
–
+
SCALE
FACTOR
ADJUST
V
+V
IN
S
1
2
3
4
5
6
7
14
+V
erms
ABSOLUTE
VALUE
R1
200Ω
±1.5%
NC
13 NC
NC
11 NC
AD636
–V
S
12
–V
SQUARER
DIVIDER
C
AV
COM
CURRENT
MIRROR
dB
10
9
R2
+V
–V
S
R
154Ω
BUF OUT
L
V
R4
500kΩ
OUT
+
10kΩ
BUF IN
I
BUF
OUT R3
8
–
470kΩ
10kΩ
S
OFFSET
ADJUST
C
F
(OPTIONAL)
I
OUT
10
NC = NO CONNECT
Figure 10. Optional External Gain and Output Offset Trims
R
L
+V
14
BUF IN
1
9
S
V
IN
10kΩ
erms
–V
1
2
3
4
5
6
7
+V
ABSOLUTE
VALUE
–
+
BUF
AD636
BUF OUT
COM
SINGLE-SUPPLY CONNECTION
13
12
11
10
NC
NC
NC
NC
2
CURRENT
MIRROR
8
AD636
V
–V
OUT
S
Although the applications illustrated in Figure 9 and Figure 10
assume the use of dual power supplies, three external bias
components connected to the COM pin enable powering the
AD636 with unipolar supplies as low as 5 V. The two resistors
and capacitor network shown connected to Pin 10 in Figure 11
are satisfactory over the same range of voltages permissible with
dual supply operation. Any external bias voltage applied to Pin 10 is
internally reflected to the VIN pin, rendering the same ac operation
as with a dual supply. DC or ac + dc conversion is impractical,
due to the resultant dc level shift at the input. The capacitor
insures that no extraneous signals are coupled into the COM
pin. The values of the resistors are relatively high to minimize
power consumption because only 1 µA of bias current flows
into Pin 10 (Pin 2 on the H package).
SQUARER
DIVIDER
10kΩ
C
AV
+V
S
SQUARER
DIVIDER
+V
+
–
+V
7
3
dB
C
COM
CURRENT
MIRROR
dB
ABSOLUTE
VALUE
R
BUF OUT
L
V
9
8
4
6
IN
10kΩ
+
I
C
AV
BUF IN
OUT
5
BUF
erms
–V
S
–
C
10kΩ
F
(OPTIONAL)
–
–V
+
C
AV
NC = NO CONNECT
Figure 9. Standard RMS Connection
OPTIONAL TRIMS FOR HIGH ACCURACY
If it is desired to improve the accuracy of the AD636, the
external trims shown in Figure 10 can be added. R4 is used to
trim the offset. The scale factor is trimmed by using R1 as
shown. The insertion of R2 allows R1 to either increase or
decrease the scale factor by 1.5%.
Alternately, the COM pin of some CMOS ADCs provides a suitable
artificial ground for the AD636. AC input coupling requires only
Capacitor C2 as shown; a dc return is not necessary because it is
provided internally. C2 is selected for the proper low frequency
break point with the input resistance of 6.7 kΩ; for a cut-off at
10 Hz, C2 should be 3.3 μF. The signal ranges in this connection are
Rev. E | Page 10 of 16
Data Sheet
AD636
100
10
100
slightly more restricted than in the dual supply connection. The
load resistor, RL, is necessary to provide current sinking capability.
0.
01% E
C
AV
RRO
10
0.
–
+
1% E
R
C2
3.3µF
RRO
1% E
+V
S
R
V
IN
V
IN
1
2
3
4
5
6
7
14
RRO
ABSOLUTE
VALUE
1
1
0.1µF
10% E
NONPOLARIZED
R
NC
13 NC
RRO
AD636
VALUES FOR C AND
1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
ACCURACY ±20% DUE TO
COMPONENT TOLERANCE
–V
R
AV
S
12
11
10
9
NC
NC
SQUARER
DIVIDER
0.1
0.01
0.1
0.01
20kΩ
C
AV
COM
CURRENT
MIRROR
dB
*% dc ERROR + % RIPPLE (PEAK)
0.1µF
V
R
L
OUT
1
10 100
1k
10k
100k
BUF OUT
BUF IN
INPUT FREQUENCY (Hz)
+
10kΩ
R
I
OUT
L
BUF
Figure 13. Error/Settling Time Graph for Use with the Standard RMS
Connection
8
–
1kΩ TO 10kΩ
39kΩ
10kΩ
The primary disadvantage in using a large CAV to remove ripple
is that the settling time for a step change in input level is
increased proportionately. Figure 13 shows the relationship
between CAV and 1% settling time is 115 ms for each microfarad
of CAV. The settling time is twice as great for decreasing signals
as for increasing signals (the values in Figure 13 are for decreasing
signals). Settling time also increases for low signal levels, as
shown in Figure 14.
NC = NO CONNECT
Figure 11. Single-Supply Connection (See Text)
CHOOSING THE AVERAGING TIME CONSTANT
The AD636 computes the rms of both ac and dc signals. If the
input is a slowly varying dc voltage, the output of the AD636
tracks the input exactly. At higher frequencies, the average
output of the AD636 approaches the rms value of the input
signal. The actual output of the AD636 differs from the ideal
output by a dc (or average) error and some amount of ripple, as
demonstrated in Figure 12.
10.0
E
O
IDEAL
O
7.5
5.0
2.5
E
DC ERROR = E – E (IDEAL)
O
O
AVERAGE E = E
O
O
DOUBLE-FREQUENCY
RIPPLE
TIME
1.0
0
Figure 12. Typical Output Waveform for Sinusoidal Input
1mV
10mV
100mV
1V
The dc error is dependent on the input signal frequency and the
value of CAV. Figure 13 can be used to determine the minimum
value of CAV, which yields a given % dc error above a given
frequency using the standard rms connection.
rms INPUT LEVEL
Figure 14. Settling Time vs. Input Level
A better method for reducing output ripple is the use of a post-
filter. Figure 15 shows a suggested circuit. If a single-pole filter
is used (C3 removed, RX shorted), and C2 is approximately
5 times the value of CAV, the ripple is reduced, as shown in
Figure 16, and the settling time is increased. For example, with
The ac component of the output signal is the ripple. There are
two ways to reduce the ripple. The first method involves using a
large value of CAV. Because the ripple is inversely proportional
to CAV, a tenfold increase in this capacitance effects a tenfold
reduction in ripple. When measuring waveforms with high crest
factors (such as low duty cycle pulse trains), the averaging time
constant should be at least ten times the signal period. For example,
a 100 Hz pulse rate requires a 100 ms time constant, which
corresponds to a 4 μF capacitor (time constant = 25 ms per μF).
C
AV = 1 µF and C2 = 4.7 μF, the ripple for a 60 Hz input is
reduced from 10% of reading to approximately 0.3% of reading.
The settling time, however, is increased by approximately a
factor of 3. The values of CAV and C2 can therefore be reduced
to permit faster settling times while still providing substantial
ripple reduction.
Rev. E | Page 11 of 16
AD636
Data Sheet
Calibration of the dB range is accomplished by adjusting R9
for the desired 0 dB reference point, and then adjusting R14 for the
desired dB scale factor (a scale of 10 counts per dB is convenient).
The 2-pole post filter uses an active filter stage to provide even
greater ripple reduction without substantially increasing the
settling times over a circuit with a 1-pole filter. The values of
CAV, C2, and C3 can then be reduced to allow extremely fast
settling times for a constant amount of ripple. Caution should
be exercised in choosing the value of CAV, because the dc error
is dependent upon this value and is independent of the post
filter. For a more detailed explanation of these topics, refer to
the RMS-to-DC Conversion Application Guide, 2nd Edition.
Total power supply current for this circuit is typically 2.8 mA
using a 7106-type ADC.
A LOW POWER, HIGH INPUT, IMPEDANCE dB METER
The portable dB meter circuit combines the functions of the
AD636 rms converter, the AD589 voltage reference, and a
μ A67l7ow power operational amplifier (see Figure 18). This
meter offers excellent bandwidth and superior high and low
level accuracy while consuming minimal power from a
standard 9 V transistor radio battery.
V
IN
+V
S
V
1
2
3
4
14
IN
+V
ABSOLUTE
VALUE
13
12
11
NC
NC
NC
AD636
–V
S
–V
SQUARER
DIVIDER
C
AV
+
–
+V
NC
S
In this circuit, the built-in buffer amplifier of the AD636 is
used as a bootstrapped input stage increasing the normal 6.7 kΩ
input Z to an input impedance of approximately 1010 Ω.
C
COM
CURRENT
MIRROR
dB
5
6
10
9
R
L
BUF OUT
+
10kΩ
I
BUF IN
OUT
BUF
8
7
(FOR SINGLE POLE, SHORT Rx,
REMOVE C3)
–
Circuit Description
10kΩ
The input voltage, VIN, is ac-coupled by C4 while R8, together
with D1 and D2, provide high input voltage protection.
+
Rx
10kΩ
–
+
C2
–
C3
V
OUT
rms
NC = NO CONNECT
The buffer’s output, Pin 6, is ac-coupled to the rms converter’s
input (Pin 1) by capacitor C2. Resistor R9 is connected between
the buffer’s output, a Class A output stage, and the negative output
swing. Resistor R1 is the amplifier’s bootstrapping resistor.
Figure 15. 2-Pole Post Filter
10
With this circuit, single-supply operation is made possible by
setting ground at a point between the positive and negative
sides of the battery. This is accomplished by sending 250 μA
from the positive battery terminal through R2, then through the
1.2 V AD589 band gap reference, and finally back to the negative
side of the battery via R10. This sets ground at 1.2 V + 3.18 V
(250 μA × 12.7 kΩ) = 4.4 V below the positive battery terminal and
5.0 V (250 μA × 20 kΩ) above the negative battery terminal.
Bypass capacitors, C3 and C5, keep both sides of the battery at a
low ac impedance to ground. The AD589 band gap reference
establishes the 1.2 V regulated reference voltage, which together
with R3 and trimming Potentiometer R4, sets the 0 dB reference
p-p RIPPLE
(ONE POLE)
= 1µF
p-p RIPPLE
= 1µF
C
AV
(STANDARD CONNECTION)
C
AV
C2 = 4.7µF
DC ERROR
= 1µF
1
C
AV
(ALL FILTERS)
p-p RIPPLE
(TWO POLE)
C
= 1µF, C2 = C3 = 4.7µF
AV
0.1
10
100
1k
10k
FREQUENCY (Hz)
Figure 16. Performance Features of Various Filter Types
current, IREF
.
A COMPLETE AC DIGITAL VOLTMETER
Performance Data
Figure 17 shows a design for a complete low power ac digital
voltmeter circuit based on the AD636. The 10 MΩ input
attenuator allows full-scale ranges of 200 mV, 2 V, 20 V, and
200 V rms. Signals are capacitively coupled to the AD636 buffer
amplifier, which is connected in an ac bootstrapped configuration
to minimize loading. The buffer then drives the 6.7 kΩ input
impedance of the AD636. The COM terminal of the ADC
provides the false ground required by the AD636 for single-
supply operation. An AD589 1.2 V reference diode is used to
provide a stable 100 mV reference for the ADC in the linear
rms mode; in the dB mode, a 1N4148 diode is inserted in series
to provide correction for the temperature coefficient of the dB
scale factor. Adjust R13 to calibrate the meter for an accurate
readout at full scale.
0 dB Reference Range = 0 dBm (770 mV) to −20 dBm (77 mV) rms
0 dBm = 1 mW in 600 Ω
Input Range (at IREF = 770 mV) = 50 dBm
Input Impedance = approximately 1010
V
SUPPLY Operating Range = +5 V dc to +20 V dc
I
QUIESCENT = 1. 8 mA typical
Accuracy with 1 kHz sine wave and 9 V dc supply:
0 dB to −40 dBm 0.1 dBm
0 dBm to −50 dBm 0.15 dBm
+10 dBm to −50 dBm 0.5 dBm
Rev. E | Page 12 of 16
Data Sheet
AD636
This can be anywhere from 0 dBm (770 mV rms − 2.2 V p-p)
to −20 dBm (77 mV rms − 220 mV p-p). Adjust the IREF
calibration trimmer for a zero indication on the analog meter.
Frequency Response 3 dBm
Input
0 dBm = 5 Hz to 380 kHz
−10 dBm = 5 Hz to 370 kHz
−20 dBm = 5 Hz to 240 kHz
−30 dBm = 5 Hz to 100 kHz
−40 dBm = 5 Hz to 45 kHz
−50 dBm = 5 Hz to 17 kHz
Then, calibrate the meter scale factor or gain. Apply an input
signal −40 dB below the set 0 dB reference and adjust the scale
factor calibration trimmer for a 40 μA reading on the analog meter.
The temperature compensation resistors for this circuit can be
purchased from Micro-Ohm Corporation, 1088 Hamilton Rd.,
Duarte, CA 91010, Part #Type 401F, 2 kΩ ,1% + 3500 ppm/°C.
Calibration
First, calibrate the 0 dB reference level by applying a 1 kHz sine
wave from an audio oscillator at the desired 0 dB amplitude.
D1
1N4148
+
R5
47kΩ
1W
C4
2.2µF
–
R6
1MΩ
200mV
V
10%
+V
+V
OFF
+V
IN
S
DD
S
V
+V
1
2
3
4
5
6
14
13
12
11
10
9
IN
DD
ABSOLUTE
VALUE
ON
–
R8
D2
C3
0.02µF
+
R1
2.49kΩ
1N4148
1µF
–V
3-1/2 DIGIT
7106 TYPE
A/D
9MΩ
NC
NC
NC
AD636
R9
2V
–V
R11
10kΩ
S
+
SS
CONVERTER
100kΩ
0dB SET
SQUARER
DIVIDER
LIN
dB
R2
C
900kΩ
REF HI
AV
9V
R12
1kΩ
R10
20kΩ
NC
–
BATTERY
20V
R14
+
6.8µF
dB
D3
1.2V
AD589
COM
REF LO
10kΩ
dB
CURRENT
MIRROR
R13
500Ω
R3
90kΩ
SCALE
R
L
BUF OUT
BUF IN
COM
200V
LIN
+
10kΩ
I
LIN
dB
SCALE
OUT
BUF
R4
10kΩ
3-1/2
DIGIT
8
7
–
HI
10kΩ
NC = NO CONNECT
R7
20kΩ
LCD
R15
1MΩ
+
DISPLAY
C6
0.01µF
ANALOG
IN
COM
LIN
dB
C7
6.8µF
LO
D4
1N4148
–V
S
LXD 7543
–V
SS
Figure 17. Portable, High-Z Input, RMS DPM and dB Meter Circuit
+
C1
3.3µF
D1
1N6263
R1
1MΩ
ON/OFF
V
IN
+4.2V
+1.2V
+V
+
–
S
1
2
3
4
5
6
7
14
13
ABSOLUTE
VALUE
R2
9V
12.7kΩ
C2
6.8µF
NC
NC
SCALE FACTOR
ADJUST
AD636
+
+
R4
500kΩ
C3
–V
S
R3
5kΩ
10µF
12 NC
11 NC
SQUARER
DIVIDER
I
+
REF
SIGNAL
INPUT
R5
10kΩ
C
AD589J
ADJUST
AV
*R7
250µA
100µA
2kΩ
C4
0.1µF
+
–
COM
dB
BUF OUT
BUF IN
CURRENT
MIRROR
10
9
R6
100Ω
R
7
R8
L
–
0–50µA
6
2
3
C6
0.1µF
47kΩ
+
C5
µA776
1W
+
10kΩ
10µF
8
+
4
BUF
I
8
OUT
–
R10
20kΩ
10kΩ
R11
R9
10kΩ
D2
1N6263
820kΩ
5%
NC = NO CONNECT
–4.8V
ALL RESISTORS 1/4W 1% METAL FILM UNLESS OTHERWISE STATED EXCEPT
*WHICH IS 2kΩ +3500ppm 1% TC RESISTOR.
Figure 18. Low Power, High Input Impedance dB Meter
Rev. E | Page 13 of 16
AD636
Data Sheet
OUTLINE DIMENSIONS
0.005 (0.13) MIN
0.080 (2.03) MAX
8
14
0.310 (7.87)
1
0.220 (5.59)
7
PIN 1
0.100 (2.54)
BSC
0.320 (8.13)
0.290 (7.37)
0.765 (19.43) MAX
0.060 (1.52)
0.015 (0.38)
0.200 (5.08)
MAX
0.150
(3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.015 (0.38)
0.008 (0.20)
SEATING
PLANE
0.070 (1.78)
0.030 (0.76)
0.023 (0.58)
0.014 (0.36)
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
Figure 19. 14-Lead Side-Brazed Ceramic Dual In-Line Package [SBDIP]
(D-14)
Dimensions shown in inches and (millimeters)
REFERENCE PLANE
0.500 (12.70)
0.160 (4.06)
MIN
0.185 (4.70)
0.165 (4.19)
0.110 (2.79)
6
7
5
8
0.021 (0.53)
0.016 (0.40)
0.115
(2.92)
BSC
4
0.045 (1.14)
0.025 (0.65)
9
3
10
0.034 (0.86)
0.025 (0.64)
2
1
0.230 (5.84)
BSC
BASE & SEATING PLANE
0.040 (1.02) MAX
0.050 (1.27) MAX
36° BSC
DIMENSIONS PER JEDEC STANDARDS MO-006-AF
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
Figure 20. 10-Pin Metal Header Package [TO-100]
(H-10)
Dimensions shown in inches and (millimeters)
ORDERING GUIDE
Model1
AD636JDZ
AD636KDZ
AD636JH
AD636JHZ
AD636KH
AD636KHZ
Temperature Range
Package Description
14-Lead SBDIP
14-Lead SBDIP
10-Pin TO-100
10-Pin TO-100
10-Pin TO-100
10-Pin TO-100
Package Option
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
D-14
D-14
H-10
H-10
H-10
H-10
1 Z = RoHS-Compliant Part.
Rev. E | Page 14 of 16
Data Sheet
NOTES
AD636
Rev. E | Page 15 of 16
AD636
NOTES
Data Sheet
©2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D00787-0-5/13(E)
Rev. E | Page 16 of 16
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