Data Sheet

AD536A

Rev. E | Page 9 of 16

14

13

12

11

10

9

8

1

2

3

4

5

6

7

AD536A

25kΩ

ABSOLUTE

VALUE

SQUARER/

DIVIDER

CURRENT

MIRROR

BUF

NC

C2

NC

NC

NC

dB

COM

4.6V TO 18V

–E

+E

2.5V

OP77

AD580J

R1

500kΩ

0dB

REF

ADJUST

dB SCALE

FACTOR ADJUST

TEMPERATURE

COMPENSATED

dB OUTPUT

+100mV/dB

dB OUT

3mV/dB

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

LINEAR

rms

OUTPUT

R6

24.9kΩ

R3

60.4Ω

1kΩ

R4

33.2kΩ

R5

5kΩ

BUF OUT

BUF IN

0.1µF

7

4

3

6

2

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.

100k

1M

10M

1k

10k

10

1

0.1

0.01

1%

10%

±3dB

FREQUENCY (Hz)

7V rms INPUT

1V rms INPUT

100mV rms INPUT

10mV rms 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

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).

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.

η = DUTY CYCLE =

CF = 1/√η

100µs

T

0

100µs

T

1

0

–1

–2

–3

–4

1

2

3

4

5

6

7

8

9

10

11

CREST FACTOR

Figure 9. Error vs. Crest Factor

1µs

10µs

100µs

1000µs

PULSE WIDTH (µs)

10

1

0.1

1V rms CF = 3

1V rms CF = 10

Figure 10. Error vs. Pulse Width Rectangular Pulse