Integrated Circuit
True RMS-to-DC Converter
AD536A
Data Sheet
FUNCTIONAL BLOCK DIAGRAM
FEATURES
AD536A
True rms-to-dc conversion
Laser trimmed to high accuracy
±0.2% maximum error (AD536AK)
±0.5% maximum error (AD536AJ)
Wide response capability
Computes rms of ac and dc signals
450 kHz bandwidth: V rms > 100 mV
2 MHz bandwidth: V rms > 1 V
Signal crest factor of 7 for 1% error
dB output with 60 dB range
Low power: 1.2 mA quiescent current
Single- or dual-supply operation
Monolithic integrated circuit
−55°C to +125°C operation (AD536AS)
+VS
ABSOLUTE
VALUE
COM
SQUARER/
DIVIDER
dB
+
CAV
CURRENT
MIRROR
25kΩ
RL
IOUT
+VS
BUFFER IN
25kΩ
The AD536A is a complete monolithic integrated circuit that
performs true rms-to-dc conversion. It offers performance
comparable or superior to that of hybrid or modular units costing
much more. The AD536A directly computes the true rms value of
any complex input waveform containing ac and dc components.
A crest factor compensation scheme allows measurements with 1%
error at crest factors up to 7. The wide bandwidth of the device
extends the measurement capability to 300 kHz with less than 3 dB
errors for signal levels greater than 100 mV.
An important feature of the AD536A, not previously available
in rms converters, is an auxiliary dB output pin. The logarithm
of the rms output signal is brought out to a separate pin to allow
the dB conversion, with a useful dynamic range of 60 dB. Using
an externally supplied reference current, the 0 dB level can be
conveniently set to correspond to any input level from 0.1 V to
2 V rms.
The AD536A is laser trimmed to minimize input and output offset
voltage, to optimize positive and negative waveform symmetry
(dc reversal error), and to provide full-scale accuracy at 7 V rms.
As a result, no external trims are required to achieve the rated
unit accuracy.
The input and output pins are fully protected. The input circuitry
can take overload voltages well beyond the supply levels. Loss of
supply voltage with the input connected to external circuitry does
not cause the device to fail. The output is short-circuit protected.
BUFFER
OUT
80kΩ
–VS
00504-001
BUF
GENERAL DESCRIPTION
Rev. G
VIN
Figure 1.
The AD536A is available in two accuracy grades (J and K) for
commercial temperature range (0°C to 70°C) applications, and
one grade (S) rated for the −55°C to +125°C extended range.
The AD536AK offers a maximum total error of ±2 mV ± 0.2%
of reading, while the AD536AJ and AD536AS have maximum
errors of ±5 mV ± 0.5% of reading. All three versions are available
in a hermetically sealed 14-lead DIP or a 10-pin TO-100 metal
header package. The AD536AS is also available in a 20-terminal
leadless hermetically sealed ceramic chip carrier.
The AD536A computes the true root-mean-square level of a
complex ac (or ac plus dc) input signal and provides an equivalent dc output level. The true rms value of a waveform is a more
useful quantity than the average rectified value because it relates
directly to the power of the signal. The rms value of a statistical
signal also relates to its standard deviation.
An external capacitor is required to perform measurements to the
fully specified accuracy. The value of this capacitor determines the
low frequency ac accuracy, ripple amplitude, and settling time.
The AD536A operates equally well from split supplies or a
single supply with total supply levels from 5 V to 36 V. With
1 mA quiescent supply current, the device is well suited for a
wide variety of remote controllers and battery-powered
instruments.
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AD536A
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Frequency Response .....................................................................9
General Description ......................................................................... 1
AC Measurement Accuracy and Crest Factor ...........................9
Functional Block Diagram .............................................................. 1
Applications Information .............................................................. 11
Revision History ............................................................................... 2
Typical Connections .................................................................. 11
Specifications..................................................................................... 3
Optional External Trims For High Accuracy ......................... 11
Absolute Maximum Ratings............................................................ 5
Single-Supply Operation ........................................................... 12
ESD Caution .................................................................................. 5
Choosing the Averaging Time Constant ................................. 12
Pin Configurations and Function Descriptions ........................... 6
Outline Dimensions ....................................................................... 14
Theory of Operation ........................................................................ 8
Ordering Guide .......................................................................... 15
Connections for dB Operation ................................................... 8
REVISION HISTORY
3/2019—Rev. F to Rev. G
Changes to Figure 5 and Table 5 ..................................................... 7
Change to Figure 16 ....................................................................... 12
Changes to Ordering Guide .......................................................... 15
11/2014—Rev. E to Rev. F
Change to Figure 1 ........................................................................... 1
Changes to Table 1 ............................................................................ 3
Change to Figure 16 ....................................................................... 12
Changes to Ordering Guide .......................................................... 15
7/2012—Rev. D to Rev. E
Reorganized Layout ............................................................ Universal
Changes to Figure 1 .......................................................................... 1
Changes to Figure 6 .......................................................................... 8
Changes to Figure 7 .......................................................................... 9
Changes to Figure 13, Figure 14, and Figure 15 ......................... 11
Changes to Figure 16, Figure 17, and Single-Supply Operation
Section .............................................................................................. 12
Changes to Figure 21 ...................................................................... 13
Updated Outline Dimensions ....................................................... 14
8/2008—Rev. C to Rev. D
Changes to Features Section............................................................ 1
Changes to General Description Section ...................................... 1
Changes to Figure 1 .......................................................................... 1
Changes to Table 2 ............................................................................ 5
Change to Figure 2 ............................................................................5
Changes to Figure 15...................................................................... 10
Changes to Connections for dB Operation Section .................. 11
Changes to Figure 17...................................................................... 12
Changes to Frequency Response Section .................................... 12
Updated Outline Dimensions ....................................................... 14
Changes to Ordering Guide .......................................................... 15
3/2006—Rev. B to Rev. C
Updated Format .................................................................. Universal
Changed Product Description to General Description ................1
Changes to General Description .....................................................1
Changes to Table 1.............................................................................3
Changes to Table 2.............................................................................5
Added Pin Configurations and Function Descriptions ...............6
Changed Standard Connection to Typical Connections .............8
Changed Single Supply Connection to Single Supply
Operation............................................................................................9
Changes to Connections for dB Operation................................. 11
Changes to Figure 17...................................................................... 12
Updated Outline Dimensions ....................................................... 14
Changes to Ordering Guide .......................................................... 15
6/1999—Rev. A to Rev. B
1/1976—Revision 0: Initial Version
Rev. G | Page 2 of 15
Data Sheet
AD536A
SPECIFICATIONS
TA = +25°C and ±15 V dc, unless otherwise noted.
Table 1.
AD536AJ
Parameter
TRANSFER FUNCTION
CONVERSION ACCURACY
Total Error, Internal Trim1
(See Figure 13)
vs. Temperature
TMIN to +70°C
Min
AD536AK
Typ
Max
VOUT = √Avg(VIN)2
Min
AD536AS
Typ
Max
VOUT = √Avg(VIN)2
Min
Typ
Max
VOUT = √Avg(VIN)2
±5 ± 0.5
±2 ± 0.2
±5 ± 0.5
mV ± % of rdg
±0.1 ± 0.01
±0.05 ±
0.005
±0.1 ±
0.005
±0.3 ±
0.005
mV ± % of rdg/°C
+70°C to +125°C
vs. Supply Voltage
DC Reversal Error
Total Error, External Trim1
(See Figure 16)
ERROR VS. CREST FACTOR2
Crest Factor 1 to Crest Factor 2
Crest Factor = 3
Crest Factor = 7
FREQUENCY RESPONSE3
Bandwidth for 1% Additional
Error (0.09 dB)
VIN = 10 mV
VIN = 100 mV
VIN = 1 V
±3 dB Bandwidth
VIN = 10 mV
VIN = 100 mV
VIN = 1 V
AVERAGING TIME CONSTANT
(See Figure 19)
INPUT CHARACTERISTICS
Signal Range, ±15 V Supplies
Continuous RMS Level
Peak Transient Input
Continuous RMS Level,
VS = ±5 V
Peak Transient Input,
VS = ±5 V
Maximum Continuous
Nondestructive Input Level
(All Supply Voltages)
Input Resistance
Input Offset Voltage
OUTPUT CHARACTERISTICS
Offset Voltage, VIN = COM
(See Figure 13)
vs. Temperature
vs. Supply Voltage
Voltage Swing, ±15 V Supplies
± 5 V Supply
dB OUTPUT, 0 dB = 1 V rms
(See Figure 7)
Error, 7 mV < VIN < 7 V rms
Scale Factor
Scale Factor Temperature
Coefficient
Uncompensated
IREF for 0 dB = 1 V rms
IREF Range
±0.1 ± 0.01
±0.2
±3 ± 0.3
±0.1 ± 0.01
±0.1
±2 ± 0.1
Specified accuracy
−0.1
−1.0
mV ± % of rdg/V
mV ± % of rdg
mV ± % of rdg
Specified accuracy
−0.1
−1.0
% of rdg
% of rdg
5
45
120
5
45
120
kHz
kHz
kHz
90
450
2.3
25
90
450
2.3
25
90
450
2.3
25
kHz
kHz
MHz
ms/μF
0 to 7
0 to 2
V rms
V peak
V rms
0 to 2
±7
±7
V peak
±25
±25
±25
V peak
20
±2
kΩ
mV
±2
mV
±0.2
mV/°C
mV/V
V
V
±0.6
dB
mV/dB
dB/°C
20
±2
±1
±2
±0.1
±0.1
+12.5
+0.33
20
±20
±7
16.67
0.8
±0.4
−3
−0.033
0 to 7
±20
0 to 2
5
1
mV ± % of rdg/°C
5
45
120
±20
0 to +11
0 to +2
±0.1 ± 0.01
±0.2
±3 ± 0.3
Specified accuracy
−0.1
−1.0
0 to 7
13.33
Unit
13.33
0 to +11
0 to +2
±0.6
80
100
16.67
0.5
20
±1
±0.5
±1
±0.1
±0.1
+12.5
±0.2
−3
−0.033
5
1
+0.33
20
Rev. G | Page 3 of 15
13.33
0 to +11
0 to +2
±0.3
80
100
16.67
0.8
±0.2
+12.5
±0.5
−3
−0.033
5
1
+0.33
20
80
100
% of rdg/°C
μA
μA
AD536A
Data Sheet
AD536AJ
Parameter
IOUT TERMINAL
IOUT Scale Factor
IOUT Scale Factor Tolerance
Output Resistance
Voltage Compliance
BUFFER AMPLIFIER
Input and Output Voltage
Range
Input Offset Voltage, RS = 25 kΩ
Input Bias Current
Input Resistance
Output Current
Short-Circuit Current
Output Resistance
Small-Signal Bandwidth
Slew Rate4
POWER SUPPLY
Voltage Rated Performance
Dual Supply
Single Supply
Quiescent Current
Total VS, 5 V to 36 V, TMIN to TMAX
TEMPERATURE RANGE
Rated Performance
Storage
NUMBER OF TRANSISTORS
Min
Typ
20
40
±10
25
−VS to
(+VS − 2.5 V)
AD536AK
Max
±20
30
−VS to
(+VS − 2.5V)
Min
Typ
20
40
±10
25
−VS to
(+VS − 2.5 V)
AD536AS
Max
±20
30
−VS to
(+VS − 2.5V)
±0.5
20
108
±4
60
(+5 mA,
−130 μA)
0
−55
1
5
±15
±3.0
+5
2
+70
+150
65
0.5
1
5
±18
+36
±4
60
20
0.5
±15
±20
30
Unit
μA/V rms
%
kΩ
V
V
±0.5
20
108
20
1
5
Max
mV
nA
Ω
(+5 mA,
−130 μA)
0.5
1.2
20
40
±10
25
−VS to
(+VS − 2.5 V)
±4
60
(+5 mA,
−130 μA)
±3.0
+5
Typ
−VS to
(+VS − 2.5V)
±0.5
20
108
20
Min
±15
±18
+36
1.2
0
−55
2
+70
+150
65
1
±3.0
+5
1.2
−55
−55
65
Accuracy is specified for 0 V to 7 V rms, dc or 1 kHz sine wave input with the AD536A connected as in the figure referenced.
Error vs. crest factor is specified as an additional error for 1 V rms rectangular pulse input, pulse width = 200 μs.
3
Input voltages are expressed in volts rms, and error is expressed as a percentage of the reading.
4
With 2 kΩ external pull-down resistor.
2
Rev. G | Page 4 of 15
mA
Ω
MHz
V/μs
±18
+36
V
V
V
2
mA
+125
+150
°C
°C
Data Sheet
AD536A
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter
Supply Voltage
Dual Supply
Single Supply
Internal Power Dissipation
Maximum Input Voltage
Buffer Maximum Input Voltage
Maximum Input Voltage
Storage Temperature Range
Operating Temperature Range
AD536AJ/AD536AK
AD536AS
Lead Temperature (Soldering, 60 sec)
ESD Rating
Thermal Resistance θJA1
10-Pin Header (H-10 Package)
20-Terminal LCC (E-20 Package)
14-Lead SBDIP (D-14 Package)
14-Lead CERDIP (Q-14 Package)
±18 V
+36 V
500 mW
±25 V peak
±VS
±25 V peak
−55°C to +150°C
ESD CAUTION
0°C to +70°C
−55°C to +125°C
300°C
1000 V
150°C/W
95°C/W
95°C/W
95°C/W
θJA is specified for the worst-case conditions, that is, a device soldered in a
circuit board for surface-mount packages.
+VS
14
0.1315 (3.340)
COM
10
RL
9
IOUT
8
0.0807
(2.050)
VIN
1A1
VIN
1B1
BUF IN
7
CAV dB
BUF OUT
–VS
6
3
4 5
PAD NUMBERS CORRESPOND TO PIN NUMBERS FOR THE
TO-100 14-LEAD CERAMIC DIP PACKAGE.
1BOTH
PADS SHOWN MUST BE CONNECTED TO VIN.
THE AD536A IS AVAILABLE IN LASER-TRIMMED CHIP FORM.
SUBSTRATE CONNECTED TO –VS.
Figure 2. Die Dimensions and Pad Layout
Dimensions shown in inches and (millimeters)
Rev. G | Page 5 of 15
00504-002
1
Rating
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
AD536A
Data Sheet
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
VIN 1
14
+VS
NC 2
13
NC
12
NC
–VS 3
AD536A
TOP VIEW 11 NC
(Not to Scale)
dB 5
10 COM
BUF OUT 6
9
RL
BUF IN 7
8
IOUT
NC = NO CONNECT
00504-003
CAV 4
Figure 3. D-14 and Q-14 Packages Pin Configuration
Table 3. D-14 and Q-14 Packages Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Mnemonic
VIN
NC
−VS
CAV
dB
BUF OUT
BUF IN
IOUT
RL
COM
NC
NC
NC
+VS
Description
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
Common
No Connection
No Connection
No Connection
Positive Supply Voltage
IOUT
BUF IN
10
1
COM 2
9
AD536A
TOP VIEW
(Not to Scale)
+VS 3
4
VIN
8
BUF OUT
7
dB
6
5
CAV
–VS
00504-004
RL
Figure 4. H-10 Package Pin Configuration
Table 4. H-10 Package Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
9
10
Mnemonic
RL
COM
+VS
VIN
−VS
CAV
dB
BUF OUT
BUF IN
IOUT
Description
Load Resistor
Common
Positive Supply Voltage
Input Voltage
Negative Supply Voltage
Averaging Capacitor
Log (dB) Value of the RMS Output Voltage
Buffer Output
Buffer Input
RMS Output Current
Rev. G | Page 6 of 15
VIN
NC
+VS
NC
AD536A
NC
Data Sheet
3
2
1
20
19
–VS 4
18 NC
NC 5
AD536A
17 NC
CAV 6
TOP VIEW
(Not to Scale)
16 NC
15 NC
dB 8
9
10
11
12
13
BUF OUT
BUF IN
NC
IOUT
RL
14 COM
NC = NO CONNECT
00504-005
NC 7
Figure 5. E-20-1 Package Pin Configuration
Table 5. E-20-1 Package Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Mnemonic
NC
VIN
NC
−VS
NC
CAV
NC
dB
BUF OUT
BUF IN
NC
IOUT
RL
COM
NC
NC
NC
NC
NC
+VS
Description
No Connection
Input Voltage
No Connection
Negative Supply Voltage
No Connection
Averaging Capacitor
No Connection
Log (dB) Value of the RMS Output Voltage
Buffer Output
Buffer Input
No Connection
RMS Output Current
Load Resistor
Common
No Connection
No Connection
No Connection
No Connection
No Connection
Positive Supply Voltage
Rev. G | Page 7 of 15
AD536A
Data Sheet
THEORY OF OPERATION
The AD536A 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 AD536A follows the equation
V 2
V rms Avg IN
V rms
VOUT = 2R2 × I rms = VIN rms
Figure 6 is a simplified schematic of the AD536A. Note that it is
subdivided into four major sections: absolute value circuit
(active rectifier), squarer/divider, current mirror, and buffer
amplifier. The input voltage (VIN), which can be ac or dc, is
converted to a unipolar current (I1) by the active rectifiers
(A1, A2). I1 drives one input of the squarer/divider, which has
the transfer function
2
I
I4 = I /I3
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. Thus,
CURRENT MIRROR
14
+VS
10
4
A3
I2
I1
VIN
Q1
|VIN|R–1
Q3
1
Q2
A1
12kΩ
R3
25kΩ
R1
0.4mA
25kΩ FS
I3
Q4
Q5
A2
R2
25kΩ
NOTES
1. PINOUTS ARE FOR 14-LEAD DIP.
Figure 6. Simplified Schematic
9
RL
80kΩ
CONNECTIONS FOR dB OPERATION
The logarithmic (or decibel) output of the AD536A is one of
its most powerful features. The internal circuit computing dB
works accurately over a 60 dB range. The connections for dB
measurements are shown in Figure 7.
Select the 0 dB level by adjusting R1 for the proper 0 dB reference
current (which is set to cancel the log output current from the
squarer/divider at the desired 0 dB point). The external op amp
provides a more convenient scale and allows compensation of
the +0.33%/°C scale factor drift of the dB output pin.
5
For dB calibration,
6
1.
2.
3.
4.
BUF
IN BUFFER dB
OUT
7
A4
25kΩ
ONE-QUADRANT
SQUARER/DIVIDER
12kΩ
8
IOUT
COM
BUF
OUT
3
–VS
00504-106
0.2mA
FS
ABSOLUTE VALUE;
VOLTAGE-CURRENT
CONVERTER
The dB output is derived from the emitter of Q3 because the
voltage at this point is proportional to –log VIN. The 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.
The temperature-compensating resistor, R2, is available online
in several styles from Precision Resistor Company, Inc., (Part
Number AT35 and Part Number ST35). The average temperature
coefficients of R2 and R3 result in the +3300 ppm required to
compensate for the dB output. The linear rms output is available
at Pin 8 on the DIP or Pin 10 on the header device with an output
impedance of 25 kΩ. Some applications require an additional
buffer amplifier if this output is desired.
I4 = Avg[II2/I4] = II rms
R4
50kΩ
The current mirror also produces the output current, IOUT, which
equals 2I4. IOUT can be used directly or can be converted to a
voltage with R2 and buffered by A4 to provide a low impedance
voltage output. The transfer function of the AD536A results in
the following:
Set VIN = 1.00 V dc or 1.00 V rms.
Adjust R1 for dB output = 0.00 V.
Set VIN = +0.1 V dc or 0.10 V rms.
Adjust R5 for dB output = −2.00 V.
Any other desired 0 dB reference level can be used by setting
VIN and adjusting R1 accordingly. Note that adjusting R5 for the
proper gain automatically provides the correct temperature
compensation.
Rev. G | Page 8 of 15
Data Sheet
AD536A
VIN
NC 2
–VS
–VS
+
C1, CAV
+VS
C2
CAV
dB
0.1µF
BUF OUT
dB OUT
3mV/dB
BUF IN
ABSOLUTE
VALUE
1
14
AD536A
12 NC
11 NC
4
CURRENT
MIRROR
5
10
6
9
COM
RL
25kΩ
8
BUF
7
+VS
4.6V TO 18V
+E
13 NC
SQUARER/
DIVIDER
3
+VS
IOUT
EOUT
AD580J
2.5V
–E
R1
500kΩ
0dB
REF
ADJUST
R4
33.2kΩ
dB SCALE
FACTOR ADJUST
+VS
R6
24.9kΩ
LINEAR
rms
OUTPUT
R5
5kΩ
7
2
R3
60.4Ω
OP77
3
6
TEMPERATURE
COMPENSATED
dB OUTPUT
+100mV/dB
4
R21
1kΩ
00504-107
–VS
1SPECIAL
TC COMPENSATION RESISTOR, +3300ppm/°C,
PRECISION RESISTOR COMPANY PART NUMBER AT 35 OR PART NUMBER ST35.
Figure 7. dB Connection
FREQUENCY RESPONSE
The AD536A utilizes a logarithmic circuit in performing the
implicit rms computation. As with any log circuit, bandwidth
is proportional to signal level. The solid lines in the graph of
Figure 8 represent the frequency response of the AD536A at
input levels from 10 mV rms to 7 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 120 kHz. A 10 mV
signal can be measured with 1% of reading additional error
(100 μV) up to only 5 kHz.
Figure 9 illustrates a curve of reading error for the AD536A for
a 1 V rms input signal with crest factors from 1 to 11. A rectangular pulse train (pulse width = 100 μs) was used for this test
because it is the worst-case waveform for rms measurement (all
of the energy is contained in the peaks). The duty cycle and
peak amplitude were varied to produce crest factors from 1 to
11 while maintaining a constant 1 V rms input amplitude.
T
өO
0
η = DUTY CYCLE =
CF = 1/√η
өIN (rms) = 1 V rms
VP
100µs
T
100µs
1
VOUT (V)
10%
1
±3dB
1V rms INPUT
0.1
100mV rms INPUT
0.01
1k
10k
100k
FREQUENCY (Hz)
1M
10M
00504-016
10mV rms INPUT
0
–1
–2
–3
–4
1
2
3
4
Figure 8. High Frequency Response
5
6
7
CREST FACTOR
8
9
10
Figure 9. Error vs. Crest Factor
Crest factor is often overlooked when determining the accuracy
of an ac measurement. The definition of crest factor is 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 (
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