Low Level, True RMS-to-DC Converter
Data Sheet AD636
Rev. E Document Feedback
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FEATURES
True rms-to-dc conversion
200 mV full scale
Laser-trimmed to high accuracy
0.5% maximum error (AD636K)
1.0% maximum error (AD636J)
Wide response capability
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
GENERAL DESCRIPTION
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.
The low power supply current requirement of the AD636,
typically 800 μA, is ideal for battery-powered portable
instruments. It operates from a wide range of dual and single
power supplies, from ±2.5 V to ±16.5 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 1.2 MHz at 0 dBm to greater than 10 kHz at −50 dBm.
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.
FUNCTIONAL BLOCK DIAGRAM
R
L
dB
BUFFER IN
BUFFER OUT
I
OUT
10kΩ
10kΩ
40kΩ
+V
S
+V
S
+V
S
–V
S
C
AV
V
IN
COM
CURRENT
MIRROR
SQUARER
DIVIDER
ABSOLUTE
VALUE
AD636
00787-001
–V
S
BUF
Figure 1.
The AD636 is available in two accuracy grades. The total error of the
J-version is typically less than ±0.5 mV ± 1.0% 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 14-lead SBDIP and 10-lead
TO-100 metal can.
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.
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.
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 10 MΩ input attenuators. As an
output buffer, it sources up to 5 mA.
AD636 Data Sheet
Rev. E | Page 2 of 16
TABLE OF CONTENTS
Features .............................................................................................. 1
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
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
REVISION HISTORY
5/13—Rev. D to Rev. E
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
11/06—Rev. C to Rev. D
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
Data Sheet AD636
Rev. E | Page 3 of 16
SPECIFICATIONS
@ 25°C, +VS = +3 V, and −VS = –5 V, unless otherwise noted.1
Table 1.
Model
AD636J AD636K
Unit Min Typ Max Min Typ Max
TRANSFER FUNCTION
( )
2
IN
OUT
VavgV ×=
( )
2
IN
OUT
VavgV ×=
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 ±0.1 ± 0.01 ±0.1 ± 0.005 mV ± % of
reading/°C
vs. Supply Voltage ±0.1 ± 0.01 ±0.1 ± 0.01 mV ± % of
reading/V
DC Reversal Error at 200 mV
±0.2
±0.1
% of reading
Total Error, External Trim2 ±0.3 ± 0.3 ± 0.1 ± 0.2 mV ± % of
reading
ERROR VS. CREST FACTOR
4
Crest Factor 1 to 2 Specified Accuracy Specified Accuracy
Crest Factor = 3 −0.2 −0.2 % of reading
Crest Factor = 6 −0.5 −0.5 % of reading
AVERAGING TIME CONSTANT 25 25 ms/μF of CAV
INPUT CHARACTERISTICS
Signal Range, All Supplies
Continuous RMS Level 0 to 200 0 to 200 mV rms
Peak Transient Inputs
+3 V, −5 V Supply ±2.8 ±2.8 V p-p
±2.5 V Supply ±2.0 ±2.0 V p-p
±5 V Supply
±5.0
±5.0
V p-p
Maximum Continuous
Nondestructive
Input Level (All Supply Voltages) ±12 ±12 V p-p
Input Resistance 5.33 6.67 8 5.33 6.67 8 kΩ
Input Offset Voltage ±0.5 ±0.2 mV
FREQUENCY RESPONSE3, 5
Bandwidth for 1% Additional
Error (0.09 dB)
VIN = 10 mV 14 14 kHz
VIN = 100 mV 90 90 kHz
VIN = 200 mV 130 130 kHz
±3 dB Bandwidth
VIN = 10 mV 100 100 kHz
VIN = 100 mV 900 900 kHz
VIN = 200 mV 1.5 1.5 MHz
OUTPUT CHARACTERISTICS3
Offset Voltage, V
IN
= COM
±0.5
±0.2
mV
vs. Temperature ±10 ±10 μV/°C
vs. Supply ±0.1 ±0.1 mV/V
Voltage Swing
+3 V, −5 V Supply 0.3 0 to 1.0 0.3 0 to 1.0 V
±5 V to ±16.5 V Supply 0.3 0 to 1.0 0.3 0 to 1.0 V
Output Impedance 8 10 12 8 10 12 kΩ
AD636 Data Sheet
Rev. E | Page 4 of 16
Model
AD636J AD636K
Unit Min Typ Max Min Typ Max
dB OUTPUT
Error, VIN = 7 mV to 300 mV rms ±0.3 ±0.5 ±0.1 ±0.2 dB
Scale Factor −3.0 −3.0 mV/dB
Scale Factor Temperature
Coefficient
0.33 0.33 % of reading/°C
−0.033 −0.033 dB/°C
IREF for 0 dB = 0.1 V rms 2 4 8 2 4 8 μA
IREF Range 1 50 1 50 μA
IOUT TERMINAL
I
OUT
Scale Factor
100
100
μA/V rms
IOUT Scale Factor Tolerance −20 ±10 +20 −20 ±10 +20 %
Output Resistance 8 10 12 8 10 12 kΩ
Voltage Compliance −VS to
(+VS − 2 V)
−VS to
(+VS − 2 V)
V
BUFFER AMPLIFIER
Input and Output Voltage Range −VS to
(+VS − 2 V)
−VS to
(+VS − 2 V)
V
Input Offset Voltage, RS = 10 kΩ ±0.8 ±2 ±0.5 ±1 mV
Input Bias Current 100 300 100 300 nA
Input Resistance 108 108 Ω
Output Current (+5 mA,
−130 μA)
(+5 mA,
−130 μA)
Short-Circuit Current 20 20 mA
Small Signal Bandwidth 1 1 MHz
Slew Rate6 5 5 V/μs
POWER SUPPLY
Voltage, Rated Performance +3, −5 +3, −5 V
Dual Supply +2, −2.5 ±16.5 +2, −2.5 ±16.5 V
Single Supply 5 24 5 24 V
Quiescent Current7 0.80 1.00 0.80 1.00 mA
TEMPERATURE RANGE
Rated Performance
0
+70
0
+70
°C
Storage −55 +150 −55 +150 °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.
Data Sheet AD636
Rev. E | Page 5 of 16
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter Ratings
Supply Voltage
Dual Supply ±16.5 V
Single Supply 24 V
Internal Power Dissipation1 500 mW
Maximum Input Voltage ±12 VPEAK
Storage Temperature Range −55°C to +150°C
Operating Temperature Range 0°C to 70°C
Lead Temperature Range (Soldering 60 sec)
300°C
ESD Rating 1000 V
1 10-Lead TO: θJA = 150°C/W.
14-Lead PDIP: θJA = 95°C/W.
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.
ESD CAUTION
AD636 Data Sheet
Rev. E | Page 6 of 16
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
V
IN 1
NC
2
–V
S3
C
AV 4
+V
S
14
NC
13
NC
12
NC
11
dB
5
COM
10
BUF OUT
6
R
L
9
BUF IN
7
I
OUT
8
NC = NO CONNECT
AD636
TOP VIEW
(Not to Scale)
0
0787-003
Figure 2. 14-Lead SBDIP Pin Configuration
BUF IN BUF OUT
I
OUT
–V
S
+V
S
V
IN
COM
R
L
dB
C
AV
6
7
8
9
10
34
2
1
5
AD636
00787-004
Figure 3. 10-Pin TO-100 Pin Configuration
Table 3. Pin Function Descriptions—14-Lead SBDIP
Pin No. Mnemonic Description
1 VIN Input Voltage.
2 NC No Connection.
3 −VS Negative Supply Voltage.
4 CAV Averaging Capacitor.
5 dB Log (dB) Value of the RMS Output
Voltage.
6 BUF OUT Buffer Output.
7 BUF IN Buffer Input.
8 IOUT RMS Output Current.
9 RL Load Resistor.
10 COM Common.
11, 12, 13 NC No Connection.
14 +VS Positive Supply Voltage.
Table 4. Pin Function Descriptions—10-Pin TO-100
Pin No. Mnemonic Description
1 RL Load Resistor.
2 COM Common.
3 +VS Positive Supply Voltage.
4 VIN Input Voltage.
5 −VS Negative Supply Voltage.
6 CAV Averaging Capacitor.
7 dB Log (dB) Value of the RMS Output Voltage.
8 BUF OUT Buffer Output.
9 BUF IN Buffer Input.
10 IOUT RMS Output Current.
Data Sheet AD636
Rev. E | Page 7 of 16
TYPICAL PERFORMANCE CHARACTERISTICS
1.0
0.5
00 1k 10k100k 1M
REXTERNAL(Ω)
RATIO OF VPEAK/VSUPPLY
RL=50kΩ
RL=16.7kΩ
RL= 6.7kΩ
00787-015
Figure 4. Ratio of Peak Negative Swing to −VS vs. REXTERNAL
for Several Load Resistances
FREQUENCY(Hz)
1Vrms INPUT
200mVrms INPUT
100mVrms INPUT
30mVrms INPUT
1mVrms INPUT
10%±3dB
1%
10mVrms
INPUT
1k 10k100k 1M10M
VOUT (V)
1
200m
100m
10m
1m
30m
0.1m
00787-016
Figure 5. AD636 Frequency Response
CREST FACTOR
0.5
0
–1.0
INCRE AS E IN ERRO R ( % of Reading )
–0.5
T
V
P
0
200µs E
O
=DUTY CYCLE =
CF = 1/
E
IN
(rms) = 200mV
200µs
T
ŋ
ŋ
12345 6 7
00787-017
Figure 6. Error vs. Crest Factor
AD636 Data Sheet
Rev. E | Page 8 of 16
THEORY OF OPERATION
RMS MEASUREMENTS
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:
rmsV
V
AvgrmsV IN
2
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:
I3
I1
I4
2
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,
rmsI1
I4
I2
AvgI4
2
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
VOUT = 2 R2 I rms = VIN rms
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.
ABSOLUTE VALUE/
VOLTAGE–CURRENT
CONVERTER
A4
6
7
5
3
984
10
14
A1
A2
A3
1
COM
BUFFER
BUF
IN
10kΩ
Q5
Q4Q2
Q1
Q3
C
AV
I
OUT
8kΩ
8kΩ
+
|V
IN
|
R4
I1
I3
I4
I
REF
CURRENT MIRRO
R
V
IN
R4
20kΩ
R3
10kΩONE-QUADRANT
SQUARER/
DIVIDER –V
S
+V
S
R
L
dB
OUT
BUF
OUT
R2
10kΩ
20µA
FS
R1
25kΩ
10µA
FS
00787-013
+V
S
C
AV
Figure 7. Simplified Schematic
THE AD636 BUFFER AMPLIFIER
The buffer amplifier included in the AD636 offers the user
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.
BUFFER
OUTPUT
10kΩ
R
EXTERNA L
(OPTIONAL, SEE TEXT)
–V
S
+
V
S
BUFFER
INPUT
CURRENT
MIRROR
R
LOAD
R
E
40kΩ
5µA5µA
00787-014
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.
Data Sheet AD636
Rev. E | Page 9 of 16
Figure 4 shows the value of REXTERNAL for a particular ratio of
VPEAK to −VS for several values of RLOAD. The addition of
REXTERNAL increases the quiescent current of the buffer amplifier
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.
AC MEASUREMENT ACCURACY AND CREST
FACTOR (CF)
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
cycle has a crest factor of 10 (CF = 1/√η).
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.
AD636 Data Sheet
Rev. E | Page 10 of 16
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.
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.
VIN
AD636
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
BUF
+
–
CURRENT
MIRROR
10kΩ
10kΩ
+VS
CF
(OPTIONAL)
SQUARER
DIVIDER
ABSOLUTE
VALUE
AD636 CURRENT
MIRROR
+
BUF
10kΩ
10kΩ
+–
1
2
10
9
4
5
6
8
37
VIN
–VS
CF
(OPTIONAL)
VOUT
–VS
CAV
CAV
+VS
–
00787-005
BUF O UT
BUF I N
IOUT
RL
COM
+V
erms
–V
dB
+V
NC
NC
NC
COM
RL
IOUT
BUF I N
BUF O UT
dB
CAV
+ –
C
+V
–V
NC
erms
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%.
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.
R2
154Ω
1
2
3
4
5
6
7
AD636
14
13
12
11
10
9
8
ABSOLUTE
VALUE
SQUARER
DIVIDER
10kΩ
10kΩ
CURRENT
MIRROR
VIN
–VS
–V
SCALE
FACTOR
ADJUST
–C
AV
BUF
+
R1
200Ω
±1.5%
+V
S
+V
S
R4
500kΩ
–V
S
OFFSET
ADJUST
R3
470kΩ
V
OUT
00787-006
+V
NC
NC
NC
COM
R
L
I
OUT
erms
NC
C
AV
dB
BUF OUT
BUF IN
NC = NO CONNECT
+
–
Figure 10. Optional External Gain and Output Offset Trims
SINGLE-SUPPLY CONNECTION
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).
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
Data Sheet AD636
Rev. E | Page 11 of 16
slightly more restricted than in the dual supply connection. The
load resistor, RL, is necessary to provide current sinking capability.
C2
3.3µF
AD636
ABSOLUTE
VALUE
SQUARER
DIVIDER
10kΩ
10kΩ
CURRENT
MIRROR
–CAV
BUF
+
20kΩ
NONPOLARIZED
39kΩ
0.1µF
0.1µF
+VS
VOUT
RL
1kΩ TO 10kΩ
VIN
00787-007
1
2
3
4
5
6
7
14
13
12
11
10
9
8
VIN
NC
–VS
CAV
dB
BUF OUT
BUF IN
NC
NC
NC
COM
RL
IOUT
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.
TIME
IDEAL
E
O
DC ERROR = E
O
– E
O
(IDEAL)
AVERAGE E
O
= E
O
DOUBLE-FREQUENCY
RIPPLE
E
O
00787-008
Figure 12. Typical Output Waveform for Sinusoidal Input
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.
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).
INP UT FRE QUENCY ( Hz )
100
0.01
1
10
0.1
1
10
100
0.1
0.01
0.01% E RROR
0.1% E RROR
*% dc E RROR + % RI P P LE ( P E AK)
1% ERRO R
FOR 1% SETTLING TIME IN SECONDS
MUL TIP LY RE ADING BY 0.115
REQ UIRED C AV (µF )
110 100 1k 10k 100k
VAL UE S FOR CAV AND
1% SETTLING TIME FOR
ST ATED % O F READING
AVERAGING ERROR*
ACCURACY ±20% DUE TO
COM P ONENT TO LERANCE
10% ERRO R
00787-009
Figure 13. Error/Settling Time Graph for Use with the Standard RMS
Connection
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.
rms INPUT LEVEL
10.0
7.5
010mV100mV
1.0
5.0
2.5
1V1mV
SETTLING TIME RELATIVE TO
SETTLI NG TIME @ 200mV rms
00787-010
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
CAV = 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.
AD636 Data Sheet
Rev. E | Page 12 of 16
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.
Rx
10kΩ
1
2
3
4
5
6
7
AD636
14
13
12
11
10
9
8
ABSOLUTE
VALUE
SQUARER
DIVIDER
BUF
CURRENT
MIRROR
+–
+
–
+
–
+
–
C2 C3
(FOR SINGLE POLE, SHORT Rx,
REMOVE C3)
C
–V
V
IN
V
IN
+V
S
+V
V
rms
OUT
10kΩ
10kΩ
00787-011
NC
NC
NC
COM
R
L
I
OUT
NC
–V
S
C
AV
dB
BUF O UT
BUF I N
NC = NO CONNECT
+V
S
Figure 15. 2-Pole Post Filter
FREQUENCY (Hz)
10
0.1
DC ERRO R OR RIP P LE ( % of Read in g)
1
10 100 1k 10k
p-p RIPPLE
(ONE POLE)
CAV = 1µF
C2 = 4.7µF
DC ERROR
CAV = 1µF
(ALL FILTERS)
p-p RIPPLE
(TWO POLE)
CAV = 1µF, C2 = C3 = 4.7µF
00787-012
p-p RIPPLE
CAV = 1µF
(STANDARD CONNECTION)
Figure 16. Performance Features of Various Filter Types
A COMPLETE AC DIGITAL VOLTMETER
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.
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).
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 A776 low 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.
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.
Circuit Description
The input voltage, VIN, is ac-coupled by C4 while R8, together
with D1 and D2, provide high input voltage protection.
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.
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
current, IREF.
Performance Data
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
VSUPPLY Operating Range = +5 V dc to +20 V dc
IQUIESCENT = 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
Data Sheet AD636
Rev. E | Page 13 of 16
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
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.
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.
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.
R2
900kΩ
1
2
3
4
5
6
7
AD636
14
13
12
11
10
9
8
BUF
+
–
+
–
+
+
–
+V
DD
REF HI
REF LO
COM
HI
LO
+
–+
ON
OFF
LX D 7543
LIN
dB
LIN
dB
LIN
dB
200mV
2V
20V
200V
COM
V
IN
R1
9MΩ
R3
90kΩ
R4
10kΩ
C3
0.02µF
R5
47kΩ
1W
10%
D1
1N4148
C4
2.2µF
R6
1MΩ ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
6.8µF
R7
20kΩ
D4
1N4148
10kΩ
10kΩ
C7
6.8µF
D2
1N4148
R8
2.49kΩ
+V
S
R9
100kΩ
0dB S E T
R10
20kΩ
D3
1.2V
AD589
LIN
SCALE
R15
1MΩ C6
0.01µF
–V
S
–V
SS
ANALOG
IN
3-1/ 2 DIGIT
7106 TY P E
A/D
CONVERTER
+V
DD
–V
SS
9V
BATTERY
1µF
R11
10kΩ
R12
1kΩ
R13
500Ω
R14
10kΩ
dB
SCALE
3-1/2
DIGIT
LCD
DISPLAY
00787-018
V
IN
NC
–V
S
C
AV
dB
BUF OUT
BUF IN
+V
S
NC
NC
NC
COM
R
L
I
OUT
NC = NO CONNECT
Figure 17. Portable, High-Z Input, RMS DPM and dB Meter Circuit
ALL RESISTORS 1/ 4W 1% METAL FILM UNLESS OTHERWISE STATED EXCEPT
*WHICH IS 2kΩ +3500ppm 1% TC RESISTOR.
1
2
3
4
5
6
7
AD636
14
13
12
11
10
9
8
BUF
+
–
+ –
+
–
µA776
+ –
+
+
+
+
ON/OFF
9V
+1.2V
AD589J
250µA100µA
+
+4.2V
–4.8V
D1
1N6263
SIGNAL
INPUT
C4
0.1µF
R8
47kΩ
1W
D2
1N6263
C1
3.3µF
R1
1MΩ
C2
6.8µF
10kΩ
10kΩ
R9
10kΩ
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
R2
12.7kΩ
C3
10µF
C5
10µF
R10
20kΩ
C6
0.1µF
*R7
2kΩ
R6
100Ω
R3
5kΩ
R4
500kΩ
IREF
ADJUST
R11
820kΩ
5%
0–50µA
R5
10kΩ
SCALE FACTOR
ADJUST
2
348
7
6
00787-019
NC = NO CONNECT
VIN
NC
–VS
CAV
dB
BUF OUT
BUF IN
+VS
NC
NC
NC
COM
RL
IOUT
Figure 18. Low Power, High Input Impedance dB Meter
AD636 Data Sheet
Rev. E | Page 14 of 16
OUTLINE DIMENSIONS
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.
14
17
80.310 (7.87)
0.220 (5.59)
PIN 1
0.080 (2.03) MAX
0.005 (0.13) MIN
SEATING
PLANE
0.023 (0.58)
0.014 (0.36)
0.060 (1.52)
0.015 (0.38)
0.200 (5.08)
MAX
0.200 (5.08)
0.125 (3.18)
0.070 (1.78)
0.030 (0.76)
0.100 (2.54)
BSC
0.150
(3.81)
MIN
0.765 (19.43) MAX
0.320 (8.13)
0.290 (7.37)
0.015 (0.38)
0.008 (0.20)
Figure 19. 14-Lead Side-Brazed Ceramic Dual In-Line Package [SBDIP]
(D-14)
Dimensions shown in inches and (millimeters)
CONTROLLING DIMENSIONSARE IN INCHES; MILL I MET ER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED- OF F INCH E QUIVALENTS FOR
REF E RE NCE ONLY AND ARE NO T APPROPRIATE FOR USE IN DESIGN.
DIMENSIONS PER JEDEC STANDARDS MO-006-AF
0.500 (12.70)
MIN
0.185 ( 4.70)
0.165 ( 4.19)
REF E RE NCE P LANE
0.050 ( 1.27) MAX
0.040 ( 1.02) MAX
0.335 ( 8.51)
0.305 ( 7.75)
0.370 ( 9.40)
0.335 ( 8.51)
0.021 ( 0.53)
0.016 ( 0.40)
10.034 ( 0.86)
0.025 ( 0.64)
0.045 ( 1.14)
0.025 ( 0.65)
0.160 ( 4.06)
0.110 (2.79)
6
2
8
7
5
4
3
0.115
(2.92)
BSC 9
10
0.230 ( 5.84)
BSC
BASE & S EATING PLANE
36° BSC
022306-A
Figure 20. 10-Pin Metal Header Package [TO-100]
(H-10)
Dimensions shown in inches and (millimeters)
ORDERING GUIDE
Model1 Temperature Range Package Description Package Option
AD636JDZ 0°C to +70°C 14-Lead SBDIP D-14
AD636KDZ 0°C to +70°C 14-Lead SBDIP D-14
AD636JH 0°C to +70°C 10-Pin TO-100 H-10
AD636JHZ 0°C to +70°C 10-Pin TO-100 H-10
AD636KH 0°C to +70°C 10-Pin TO-100 H-10
AD636KHZ 0°C to +70°C 10-Pin TO-100 H-10
1 Z = RoHS-Compliant Part.
Data Sheet AD636
Rev. E | Page 15 of 16
NOTES
AD636 Data Sheet
Rev. E | Page 16 of 16
NOTES
©2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D00787-0-5/13(E)
