Datasheet AD536A Datasheet (Analog Devices)

Integrated Circuit

14

13

12

11

10

9

8

1

2

3

4

5

6

7

ABSOLUTE

VALUE

CURRENT

MIRROR

25k⍀

25k⍀

BUF

SQUARER

DIVIDER

AD536A

NC = NO CONNECT

V

IN

NC

–V

S

C

AV

dB

BUF
OUT

BUF

IN

+V

S

NC

NC

NC

COM

R

L

I

OUT

a

FEATURES
True RMS-to-DC Conversion
Laser-Trimmed to High Accuracy

0.2% Max Error (AD536AK)

0.5% Max 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
–55ⴗC to +125ⴗC Operation (AD536AS)

PRODUCT DESCRIPTION

The AD536A is a complete monolithic integrated circuit which
performs true rms-to-dc conversion. It offers performance which
is 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. It has a crest factor compensation scheme which
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 3 dB error for signal levels above 100 mV.

An important feature of the AD536A not previously available in
rms converters is an auxiliary dB output. 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 by the user to correspond to any input level from

0.1 to 2 volts rms.

The AD536A is laser trimmed at the wafer level for input and
output offset, positive and negative waveform symmetry (dc re­versal error), and full-scale accuracy at 7 V rms. As a result, no
external trims are required to achieve the rated unit accuracy.

There is full protection for both inputs and outputs. The input
circuitry can take overload voltages well beyond the supply lev­els. Loss of supply voltage with inputs connected will not cause
unit failure. The output is short-circuit protected.

The AD536A is available in two accuracy grades (J, 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, and the AD536AJ and AD536AS have maximum errors
of ±5 mV ± 0.5% of reading. All three versions are available in
either a hermetically sealed 14-lead DIP or 10-pin TO-100
metal can. The AD536AS is also available in a 20-leadless her­metically sealed ceramic chip carrier.

REV. B

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

True RMS-to-DC Converter

AD536A

PIN CONFIGURATIONS AND

FUNCTIONAL BLOCK DIAGRAMS

TO-116 (D-14) and

Q-14 Package

LCC (E-20A) Package

NC

NC NC

V

IN

3 2 1 20 19

4

–V

S

AD536A

5

NC

6

C

AV

7

NC

8

dB

BUF
OUT

ABSOLUTE

SQUARER

25k⍀

BUF

9 10 11 12 13

BUF

NC

IN

NC = NO CONNECT

PRODUCT HIGHLIGHTS

1. The AD536A computes the true root-mean-square level 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 since it
relates directly to the power of the signal. The rms value of a
statistical signal also relates to its standard deviation.

2. The crest factor of a waveform is the ratio of the peak signal
swing to the rms value. The crest factor compensation
scheme of the AD536A allows measurement of highly com­plex signals with wide dynamic range.

3. The only external component required to perform measure­ments to the fully specified accuracy is the capacitor which
sets the averaging period. The value of this capacitor determines
the low frequency ac accuracy, ripple level and settling time.

4. The AD536A will operate equally well from split supplies or
a single supply with total supply levels from 5 to 36 volts.
The one milliampere quiescent supply current makes the
device well-suited for a wide variety of remote controllers and
battery powered instruments.

5. The AD536A directly replaces the AD536 and provides im­proved bandwidth and temperature drift specifications.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] World Wide Web Site: http://www.analog.com
Fax: [phone] © Analog Devices, Inc., 1999

TO-100 (H-10A)

R

L

+V

S

+V

S

VALUE

MIRROR

25k⍀

I

OUT

V

IN

R

L

AD536A

COM

DIVIDER

CURRENT

Package

25k⍀

CURRENT

MIRROR

SQUARER

DIVIDER

ABSOLUTE

VALUE

18

NC

17

NC

16

NC

15

NC

14

COM

I

–V

OUT

BUF IN

25k⍀

BUF
OUT

BUF

dB

C

AV

S

AD536A–SPECIFICATIONS

Model AD536AJ AD536AK AD536AS

TRANSFER FUNCTION

CONVERSION ACCURACY

Total Error, Internal Trim1 (Figure 1) ⴞ5 ⴞ0.5 ⴞ2 ⴞ0.2 ⴞ5 ⴞ0.5 mV ± % of Reading

vs. Temperature, T

+70°C to +125°C ⴞ0.3 ⴞ0.005 mV ± % of Reading/°C

vs. Supply Voltage ±0.1 ±0.01 ±0.1 ±0.01 ±0.1 ±0.01 mV ± % of Reading/V
dc Reversal Error ±0.2 ±0.1 ±0.2 ± % of Reading

Total Error, External Trim1 (Figure 2) ±3 ±0.3 ±2 ±0.1 ±3 ±0.3 mV ± % of Reading

ERROR VS. CREST FACTOR

Crest Factor 1 to 2 Specified Accuracy Specified Accuracy Specified Accuracy
Crest Factor = 3 –0.1 –0.1 – 0.1 % of Reading
Crest Factor = 7 –1.0 –1.0 – 1.0 % of Reading

FREQUENCY RESPONSE

Bandwidth for 1% Additional Error (0.09 dB)

VIN = 10 mV 5 5 5 kHz
VIN = 100 mV 45 45 45 kHz
VIN = 1 V 120 120 120 kHz

±3 dB Bandwidth

VIN = 10 mV 90 90 90 kHz
VIN = 100 mV 450 450 450 kHz
VIN = 1 V 2.3 2.3 2.3 MHz

to +70°C ±0.1 ±0.01 ±0.05 ±0.005 ⴞ0.1 ⴞ0.005 mV ± % of Reading/°C

MIN

2

3

Min Typ Max Min Typ Max Min Typ Max Units

V

= avg .(VIN)

OUT

(@ +25ⴗC, and ⴞ15 V dc unless otherwise noted)

2

V

OUT

= avg .(VIN)

2

V

OUT

= avg .(VIN)

2

AVERAGlNG TlME CONSTANT (Figure 5) 25 25 25 ms/µF CAV

INPUT CHARACTERISTICS

Signal Range, ±15 V Supplies

Continuous rms Level 0 to 7 0 to 7 0 to 7 V rms
Peak Transient Input ±20 ±20 ± 20 V peak
Continuous rms Level, ±5 V Supplies 0 to 2 0 to 2 0 to 2 V rms
Peak Transient Input, ±5 V Supplies ± 7 ±7 ±7 V peak

Maximum Continuous Nondestructive

Input Level (All Supply Voltages) ±25 ±25 ± 25 V peak
Input Resistance 13.33 16.67 20 13.33 16.67 20 13.33 16.67 20 kΩ
Input Offset Voltage 0.8 ±2 0.5 ±1 0.8 ±2mV

OUTPUT CHARACTERISTICS

Offset Voltage, VIN = COM (Figure 1) ± 1 ±2 ± 0.5 ±1 ⴞ2 mV

vs. Temperature ±0.1 ±0.1 ⴞ0.2 mV/°C

vs. Supply Voltage ±0.1 ±0.1 ±0.2 mV/V
Voltage Swing, ±15 V Supplies 0 to +11 +12.5 0 to +11 +12.5 0 to +11 +12.5 V
±5 V Supply 0 to +2 0 to +2 0 to +2 V

dB OUTPUT (Figure 13)

Error, VlN 7 mV to 7 V rms, 0 dB = 1 V rms ±0.4 ⴞ0.6 ±0.2 ⴞ0.3 ±0.5 ⴞ0.6 dB
Scale Factor –3 –3 –3 mV/dB
Scale Factor TC (Uncompensated, see Fig-

ure 1 for Temperature Compensation) – 0.033 –0.033 –0.033 dB/°C

I

for 0 dB = 1 V rms 5 20 80 5 20 80 5 20 80 µA

REF

I

Range 1 100 1 100 1 100 µA

REF

I

TERMINAL

OUT

I

Scale Factor 40 40 40 µA/V rms

OUT

I

Scale Factor Tolerance ±10 ±20 ±10 ±20 ± 10 ± 20 %

OUT

Output Resistance 20 25 30 20 25 30 20 25 30 kΩ
Voltage Compliance –VS to (+V

BUFFER AMPLIFIER

Input and Output Voltage Range –VS to (+V

Input Offset Voltage, RS = 25 k ±0.5 ⴞ4 ±0.5 ⴞ4 ± 0.5 ⴞ4 mV
Input Bias Current 20 60 20 60 20 60 nA
Input Resistance 10
Output Current (+5 mA, (+5 mA, (+5 mA,

Short Circuit Current 20 20 20 mA
Output Resistance 0.5 0.5 0.5 Ω
Small Signal Bandwidth 1 1 1 MHz

4

Slew Rate

POWER SUPPLY

Voltage Rated Performance ± 15 ± 15 ± 15 V

Dual Supply ±3.0 ±18 ±3.0 ±18 ± 3.0 ±18 V

Single Supply +5 +36 +5 +36 +5 +36 V
Quiescent Current

Total VS, 5 V to 36 V, T

TEMPERATURE RANGE

Rated Performance 0 +70 0 +70 –55 +125 °C
Storage –55 +150 –55 +150 –55 +150 °C

MIN

to T

MAX

–2.5 V) –2.5 V) –2.5 V)

–130 µA) –130 µA) –130 µA)

+0.33 +0.33 +0.33 % of Reading/°C

S

–2.5 V) –2.5 V) –2.5 V) V

S

8

–VS to (+V

–VS to (+V

S

10

S

–VS to (+V

8

–VS to (+V

S

8

10

S

V

Ω

555V/µs

1.2 2 1.2 2 1.2 2 mA

NUMBER OF TRANSISTORS 65 65 65

NOTES

1

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.

2

Error vs. crest factor is specified as an additional error for 1 V rms rectangular pulse input, pulsewidth = 200 µs.

3

Input voltages are expressed in volts rms, and error is percent of reading.

4

With 2k external pull-down resistor.

Specifications subject to change without notice.

Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min and max specifications are guaranteed,

although only those shown in boldface are tested on all production units.

–2–

REV. B

AD536A

ABSOLUTE MAXIMUM RATINGS

1

Supply Voltage

Dual Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±18 V

Single Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +36 V

Internal Power Dissipation

2

. . . . . . . . . . . . . . . . . . . . 500 mW

Maximum Input Voltage . . . . . . . . . . . . . . . . . . . . ±25 V Peak

Buffer Maximum Input Voltage . . . . . . . . . . . . . . . . . . . . . ±V

S

Maximum Input Voltage . . . . . . . . . . . . . . . . . . . . ± 25 V Peak

Storage Temperature Range . . . . . . . . . . . . –55°C to +150°C

Operating Temperature Range

AD536AJ/K . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C

AD536AS . . . . . . . . . . . . . . . . . . . . . . . . –55°C to +125°C

Lead Temperature Range

(Soldering 60 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . +300°C

ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000 V

NOTES

1

Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent 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.

2

10-Pin Header: θJA = 150°C/W; 20-Leadless LCC: θJA = 95°C/W; 14-Lead Size

Brazed Ceramic DIP: θJA = 95°C/W.

CHIP DIMENSIONS AND PAD LAYOUT

Dimensions shown in inches and (mm).

STANDARD CONNECTION

The AD536A 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 1. In this configuration, the AD536A will mea­sure the rms of the ac and dc level present at the input, but will
show an error for low frequency inputs as a function of the filter
capacitor, C

, as shown in Figure 5. Thus, if a 4 µF capacitor

AV

is used, the additional average error at 10 Hz will be 0.1%, at
3 Hz it will be 1%. The accuracy at higher frequencies will be
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 3,
the capacitor must be nonpolar. If the AD536A 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.

C

AV

V

IN

–V

S

V

OUT

1

AD536A

2

3

4

5

6

7

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

BUF

25k⍀

25k⍀

+V

14

13

12

11

10

9

8

S

ORDERING GUIDE

Temperature Package Package

Model Range Description Option

AD536AJD 0°C to +70°C Side Brazed Ceramic DIP D-14
AD536AKD 0°C to +70°C Side Brazed Ceramic DIP D-14
AD536AJH 0°C to +70°C Header H-10A
AD536AKH 0°C to +70°C Header H-10A
AD536AJQ 0°C to +70°C Cerdip Q-14
AD536AKQ 0°C to +70°C Cerdip Q-14
AD536ASD –55°C to +125°C Side Brazed Ceramic DIP D-14
AD536ASD/883B –55°C to +125°C Side Brazed Ceramic DIP D-14
AD536ASE/883B –55°C to +125°C LCC E-20A
AD536ASH –55°C to +125°C Header H-10A
AD536ASH/883B –55°C to +125°C Header H-10A
AD536AJCHIPS 0°C to +70°CDie
AD536AKH/+ 0°C to +70°C Header H-10A
AD536ASCHIPS –55°C to +125°CDie
5962-89805012A –55°C to +125°C LCC E-20A
5962-8980501CA –55°C to +125°C Side Brazed Ceramic DIP D-14
5962-8980501IA –55°C to +125°C Header H-10A

25k⍀

AD536A

CURRENT

MIRROR

+V

S

–V

S

dB

V

OUT

SQUARER

V

IN

C

AV

C

AV

3 2 1 20 19

4

AD536A

5

6

25k⍀

7

8

BUF

9 10 11 12 13

DIVIDER

V

ABSOLUTE

VALUE

–V

S

IN

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

25k⍀

V

BUF

+V

S

25k⍀

OUT

18

17

16

15

14

Figure 1. Standard RMS Connection

REV. B

–3–

AD536A

The input and output signal ranges are a function of the supply
voltages; these ranges are shown in Figure 14. The AD536A can
also be used in an unbuffered voltage output mode by discon­necting the input to the buffer. The output then appears unbuf­fered across the 25 kΩ resistor. The buffer amplifier can then be
used for other purposes. Further the AD536A can be used in a
current output mode by disconnecting the 25 kΩ resistor from
ground. The output current is available at Pin 8 (Pin 10 on the
“H” package) with a nominal scale of 40 µA per volt rms input
positive out.

OPTIONAL EXTERNAL TRIMS FOR HIGH ACCURACY

If it is desired to improve the accuracy of the AD536A, the
external trims shown in Figure 2 can be added. R4 is used to
trim the offset. Note that the offset trim circuit adds 365 Ω in
series with the internal 25 kΩ resistor. This will cause a 1.5%
increase in scale factor, which is trimmed out by using R1 as
shown. Range of scale factor adjustment is ±1.5%.

The trimming procedure is as follows:

1. Ground the input signal, V

, and adjust R4 to give zero

IN

volts output from Pin 6. Alternatively, R4 can be adjusted to
give the correct output with the lowest expected value of V

2. Connect the desired full scale input level to V

, either dc or

IN

.

IN

a calibrated ac signal (1 kHz is the optimum frequency);
then trim R1, to give the correct output from Pin 6, i.e.,
1000 V dc input should give 1.000 V dc output. Of course, a
±1.000 V peak-to-peak sine wave should give a 0.707 V dc
output. The remaining errors, as given in the specifications
are due to the nonlinearity.

The major advantage of external trimming is to optimize device
performance for a reduced signal range; the AD536A is inter­nally trimmed for a 7 V rms full-scale range.

by using a resistive divider between +V

and ground. The values

S

of the resistors can be increased in the interest of lowered power
consumption, since only 5 mA of current flows into Pin 10
(Pin 2 on the “H” package). AC input coupling requires only
capacitor C2 as shown; a dc return is not necessary as it is
provided internally. C2 is selected for the proper low frequency
break point with the input resistance of 16.7 kΩ; for a cutoff at
10 Hz, C2 should be 1 µF. The signal ranges in this connection
are slightly more restricted than in the dual supply connection.
The input and output signal ranges are shown in Figure 14. The
load resistor, R

CHOOSING THE AVERAGING TIME CONSTANT

, is necessary to provide output sink current.

L

C2

Figure 3. Single Supply Connection

The AD536A will compute the rms of both ac and dc signals.
If the input is a slowly-varying dc signal, the output of the
AD536A will track the input exactly. At higher frequencies, the
average output of the AD536A will approach the rms value of
the input signal. The actual output of the AD536A will differ
from the ideal output by a dc (or average) error and some
amount of ripple, as demonstrated in Figure 4.

Figure 2. Optional External Gain and Output Offset Trims

SINGLE SUPPLY CONNECTION

The applications in Figures l and 2 require the use of approxi­mately symmetrical dual supplies. The AD536A can also be
used with only a single positive supply down to +5 volts, as
shown in Figure 3. The major limitation of this connection is
that only ac signals can be measured since the differential input
stage must be biased off ground for proper operation. This
biasing is done at Pin 10; thus it is critical that no extraneous
signals be coupled into this point. Biasing can be accomplished

–4–

Figure 4. Typical Output Waveform for Sinusoidal Input

The dc error is dependent on the input signal frequency and the
value of C
value of C

. Figure 5 can be used to determine the minimum

AV

which will yield a given percent dc error above a

AV

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 C
C

, a tenfold increase in this capacitance will affect a tenfold

AV

. Since the ripple is inversely proportional to

AV

reduction in ripple. When measuring waveforms with high crest

REV. B

AD536A

C2

C3

C3

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).

The primary disadvantage in using a large C

to remove ripple

AV

is that the settling time for a step change in input level is in­creased proportionately. Figure 5 shows that the relationship
between C
microfarad of C

and 1% settling time is 115 milliseconds for each

AV

. The settling time is twice as great for de-

AV

creasing signals as for increasing signals (the values in Figure 5
are for decreasing signals). Settling time also increases for low
signal levels, as shown in Figure 6.

The two-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 one-pole filter. The values

, C2, and C3 can then be reduced to allow extremely fast

of C

AV

settling times for a constant amount of ripple. Caution should
be exercised in choosing the value of C

, since the dc error is

AV

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, available
from Analog Devices.

Figure 5. Error/Settling Time Graph for Use with the Stan­dard rms Connection in Figure 1

Figure 6. Settling Time vs. Input Level

A better method for reducing output ripple is the use of a
“post-filter.” Figure 7 shows a suggested circuit. If a single-pole
filter is used (C3 removed, R
twice the value of C

AV

8 and settling time is increased. For example, with C

shorted), and C2 is approximately

X

, the ripple is reduced as shown in Figure

= 1 µF

AV

and C2 = 2.2 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 C

and C2, can, therefore, be reduced to permit faster

AV

settling times while still providing substantial ripple reduction.

Figure 7. 2-Pole “Post” Filter

Figure 8. Performance Features of Various Filter Types

AD536A PRINCIPLE 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:

2





V

Vrms= Avg.

IN




Vrms








REV. B

–5–

AD536A

Figure 9 is a simplified schematic of the AD536A; it is subdi­vided into four major sections: absolute value circuit (active
rectifier), squarer/divider, current mirror, and buffer amplifier.
The input voltage, V
unipolar current I

, which can be ac or dc, is converted to a

IN

, by the active rectifier A1, A2. I1 drives one

1

input of the squarer/divider, which has the transfer function:

2

= I

/I

I

4

1

3

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, C
greater than the longest period of the input signal, then I
effectively averaged. The current mirror returns a current I
which equals Avg. [I

. If the R1, CAV time constant is much

AV

], back to the squarer/divider to complete

4

is

4

,

3

the implicit rms computation. Thus:

I4= Avg. I

2

/ I

[]

= I1rms

1

4

Figure 9. Simplified Schematic

The current mirror also produces the output current, I
which equals 2I4. I

can be used directly or converted to a

OUT

OUT,

voltage with R2 and buffered by A4 to provide a low impedance
voltage output. The transfer function of the AD536A thus
results:

= 2R 2 I rms =VINrms

V

OUT

The dB output is derived from the emitter of Q3, since the
voltage at this point is proportional to –log V

. Emitter fol-

IN

lower, Q5, buffers and level shifts this voltage, so that the dB
output voltage is zero when the externally supplied emitter
current (I

CONNECTIONS FOR dB OPERATION

) to Q5 approximates I3.

REF

A powerful feature added to the AD536A is the logarithmic or
decibel output. The internal circuit computing dB works accu­rately over a 60 dB range. The connections for dB measure­ments are shown in Figure 10. The user selects the 0 dB level by
adjusting R1, for the proper 0 dB reference current (which is set
to exactly cancel the log output current from the squarer-divider
at the desired 0 dB point). The external op amp is used to pro­vide a more convenient scale and to allow compensation of the
+0.33%/°C scale factor drift of the dB output pin. The special
T.C. resistor, R2, is available from Tel Labs in Londonderry,
N.H. (model Q-81) or from Precision Resistor Inc., Hillside,
N.J. (model PT146). The averaged temperature coefficients of
resistors R2 and R3 develop the +3300 ppm needed to reverse
compensate the dB output. The linear rms output is available at
Pin 8 on DIP or Pin 10 on header device with an output imped­ance of 25 kΩ; thus some applications may require an additional
buffer amplifier if this output is desired.

dB Calibration:

1. Set V

= 1.00 V dc or 1.00 V rms

IN

2. Adjust R1 for dB out = 0.00 V

3. Set V

= +0.1 V dc or 0.10 V rms

IN

4. Adjust R5 for dB out = –2.00 V

Any other desired 0 dB reference level can be used by setting
V

and adjusting R1, accordingly. Note that adjusting R5 for

IN

the proper gain automatically gives the correct temperature
compensation.

Figure 10. dB Connection

–6–

REV. B

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 below
represent the frequency response of the AD536A at input levels
from 10 millivolts to 7 volts 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 volt rms signal will pro­duce less than 1% of reading additional error up to 120 kHz. A
10 millivolt signal can be measured with 1% of reading addi­tional error (100 µV) up to only 5 kHz.

Figure 11. High Frequency Response

AD536A

Figure 12. Error vs. Crest Factor

AC MEASUREMENT ACCURACY AND CREST FACTOR

Crest factor is often overlooked in determining the accuracy of
an ac measurement. Crest factor is defined as the ratio of the
peak signal amplitude to the rms value of the signal (CF = V
V rms). Most common waveforms, such as sine and triangle
waves, have relatively low crest factors (<2). Waveforms which
resemble low duty cycle pulse trains, such as those occurring in
switching power supplies and SCR circuits, have high crest
factors. For example, a rectangular pulse train with a 1% duty

cycle has a crest factor of 10 (CF = 1

Figure 12 is a curve of reading error for the AD536A for a 1 volt
rms input signal with crest factors from 1 to 11. A rectangular
pulse train (pulsewidth 100 µs) was used for this test since it is
the worst-case waveform for rms measurement (all 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 volt rms input amplitude.

).

η

/

P

Figure 13. AD536A Error vs. Pulsewidth Rectangular
Pulse

REV. B

Figure 14. AD536A Input and Output Voltage Ranges
vs. Supply

–7–

AD536A

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

D-14 Package

TO-116

C502e–0–6/99

H-10A Package

TO-100

E-20A Package

LCC

PRINTED IN U.S.A.

–8–

REV. B