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REV. B

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AD536A

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700 World Wide Web Site: http://www.analog.com

Fax: 781/326-8703 © Analog Devices, Inc., 1999

Integrated Circuit

True RMS-to-DC Converter

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 +125ⴗ C 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 capabi-

lity 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 con-

veniently 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 re-

versal 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 lev-

els. 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 com-

mercial 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 her-

metically sealed ceramic chip carrier.

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 equiva-

lent 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 com-

plex signals with wide dynamic range.

3. The only external component required to perform measure-

ments 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 im-

proved bandwidth and temperature drift specifications.

PIN CONFIGURATIONS AND

FUNCTIONAL BLOCK DIAGRAMS

LCC (E-20A) Package

3 2 1 20 19

9 10 11 12 13

BUF

CURRENT

MIRROR

ABSOLUTE

VALUE

SQUARER

DIVIDER

25k⍀

AD536A

COM

OUT

NC = NO CONNECT

–V

BUF

OUT

BUF

NC NC

TO-116 (D-14) and

Q-14 Package

ABSOLUTE

VALUE

CURRENT

MIRROR

25k⍀

BUF

SQUARER

DIVIDER

AD536A

NC = NO CONNECT

–V

BUF

OUT

BUF

COM

OUT

TO-100 (H-10A)

Package

BUF

25k⍀

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

25k⍀

AD536A

COM

OUT

–V

BUF

OUT

BUF IN

Summary of content (8 pages)

PAGE 1
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.
PAGE 2
AD536A–SPECIFICATIONS (@ +25ⴗC, and ⴞ15 V dc unless otherwise noted) Model AD536AJ Typ Min TRANSFER FUNCTION CONVERSION ACCURACY Total Error, Internal Trim1 (Figure 1) vs. Temperature, TMIN to +70°C +70°C to +125°C vs. Supply Voltage dc Reversal Error Total Error, External Trim1 (Figure 2) ± 0.1 ± 0.01 ± 0.2 ± 3 ± 0.
PAGE 3
AD536A ABSOLUTE MAXIMUM RATINGS 1 STANDARD CONNECTION Supply Voltage Dual Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 18 V Single Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +36 V Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . 500 mW Maximum Input Voltage . . . . . . . . . . . . . . . . . . . . ± 25 V Peak Buffer Maximum Input Voltage . . . . . . . . . . . . . . . . . . . . . ± VS Maximum Input Voltage . . . . . . . . . . . . . . .
PAGE 4
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.
PAGE 5
AD536A 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 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, since the dc error is dependent upon this value and is independent of the post filter.
PAGE 6
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, 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: The current mirror also produces the output current, IOUT, which equals 2I4.
PAGE 7
AD536A 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.
PAGE 8
AD536A OUTLINE DIMENSIONS Dimensions shown in inches and (mm). C502e–0–6/99 D-14 Package TO-116 H-10A Package TO-100 –8– PRINTED IN U.S.A. E-20A Package LCC REV.