AD536AKQ【PDF 10/17 页】IC TRUE RMS/DC CONV 14CDIP

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Sheet目录38

6页 7页 8页 9页 [10页]11页 12页 13页 14页

Data Sheet

AD536A

Rev. E | Page 9 of 16

AD536A

25k&

ABSOLUTE

VALUE

SQUARER/

DIVIDER

CURRENT

MIRROR

BUF

C1, C

COM

OUT

4.6V TO 18V

OUT

2.5V

OP77

AD580J

500k&

0dB

REF

ADJUST

dB SCALE

FACTOR ADJUST

TEMPERATURE

COMPENSATED

dB OUTPUT

+100mV/dB

dB OUT

3mV/dB

SPECIAL TC COMPENSATION RESISTOR, +3300ppm/癈,

PRECISION RESISTOR COMPANY PART NUMBER AT 35 OR PART NUMBER ST35.

LINEAR

rms

OUTPUT

24.9k&

60.4&

1k&

33.2k&

5k&

BUF OUT

BUF IN

0.1礔

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 ? 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 礦) up to only 5 kHz.

100k

10M

10k

0.1

0.01

10%

?dB

FREQUENCY Hz

7Vrms INPUT

1Vrms INPUT

100mVrms INPUT

10mVrms INPUT

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 = V

/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 = 1n).

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 祍) 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.

?= DUTY CYCLE =

CF = 1/?/DIV>

?/DIV>

(rms) = 1 V rms

100祍

?/DIV>

100祍

1 2 3 4 5 6 7 8 9 10 11

CREST FACTOR

Figure 9. Error vs. Crest Factor

1祍 10祍 100祍 1000祍

PULSEWIDTH(祍)

0.1

1VrmsCF=3

1VrmsCF=10

Figure 10. Error vs. Pulse Width Rectangular Pulse

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