5962-89805012A データシートの表示（PDF） - Analog Devices

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5962-89805012A

Integrated Circuit True RMS-to-DC Converter

Analog Devices 

Data Sheet

AD536A

VIN

1

NC 2

–VS

–VS 3

+VS C1, CAV CAV 4

C2

dB 5

0.1µF

BUF OUT 6

dB OUT

3mV/dB

BUF IN

7

ABSOLUTE

VALUE

AD536A

SQUARER/

DIVIDER

CURRENT

MIRROR

BUF

25kΩ

+VS

14

+VS

+E 4.6V TO 18V

13 NC

12 NC

EOUT

AD580J

11 NC

2.5V

dB SCALE

R4 FACTOR ADJUST

COM

10

R1

500kΩ

–E

33.2kΩ

+VS R5

9 RL

8 IOUT

0dB

REF

ADJUST

LINEAR

rms

OUTPUT

R6

24.9kΩ

R21

R3

60.4Ω

5kΩ

7

2

OP77

3

4

6

TEMPERATURE

COMPENSATED

dB OUTPUT

1kΩ

+100mV/dB

–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 rectan-

gular 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.

VP

0

T

өO

100µs

η = DUTY CYCLE =

CF = 1/√η

T

өIN (rms) = 1 V rms

100µs

1

10

7V rms INPUT

1%

10%

0

1

±3dB

1V rms INPUT

–1

0.1

100mV rms INPUT

–2

0.01

10mV rms INPUT

–3

1k

10k

100k

1M

10M

FREQUENCY (Hz)

Figure 8. High Frequency Response

AC MEASUREMENT ACCURACY AND 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 (<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√n).

–4

1 2 3 4 5 6 7 8 9 10 11

CREST FACTOR

Figure 9. Error vs. Crest Factor

10

1V rms CF = 10

1

1V rms CF = 3

0.1

1µs

10µs 100µs 1000µs

PULSE WIDTH (µs)

Figure 10. Error vs. Pulse Width Rectangular Pulse

Rev. E | Page 9 of 16

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