AD636 Просмотр технического описания (PDF) - Analog Devices

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AD636

Low Level, True RMS-to-DC Converter

Analog Devices 

Data Sheet

slightly more restricted than in the dual supply connection. The

load resistor, RL, is necessary to provide current sinking capability.

CAV

–+

C2

3.3µF

VIN

NONPOLARIZED

VIN

1

NC 2

–VS

3

CAV

4

ABSOLUTE

VALUE

AD636

SQUARER

DIVIDER

+VS

14

0.1µF

13 NC

12 NC

11 NC

20kΩ

VOUT

RL

1kΩ TO 10kΩ

dB 5

BUF OUT

6

BUF IN

7

CURRENT

MIRROR

+

BUF

–

10kΩ

10kΩ

COM

10

RL

9

IOUT

8

0.1µF

39kΩ

NC = NO CONNECT

Figure 11. Single-Supply Connection (See Text)

CHOOSING THE AVERAGING TIME CONSTANT

The AD636 computes the rms of both ac and dc signals. If the

input is a slowly varying dc voltage, the output of the AD636

tracks the input exactly. At higher frequencies, the average

output of the AD636 approaches the rms value of the input

signal. The actual output of the AD636 differs from the ideal

output by a dc (or average) error and some amount of ripple, as

demonstrated in Figure 12.

EO

IDEAL

EO DC ERROR = EO – EO (IDEAL)

AVERAGE EO = EO

DOUBLE-FREQUENCY

RIPPLE

TIME

Figure 12. Typical Output Waveform for Sinusoidal Input

The dc error is dependent on the input signal frequency and the

value of CAV. Figure 13 can be used to determine the minimum

value of CAV, which yields a given % dc error above a 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 CAV. Because the ripple is inversely proportional

to CAV, a tenfold increase in this capacitance effects a tenfold

reduction in ripple. When measuring waveforms with high crest

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).

AD636

100

100

0.01%

10

0.1%

ERROR

1%

ERROR

1

10%

ERROR

VALUES FOR CAV AND ERROR

0.1

1% SETTLING TIME FOR

STATED % OF READING

AVERAGING ERROR*

ACCURACY ±20% DUE TO

COMPONENT TOLERANCE

*% dc ERROR + % RIPPLE (PEAK)

0.01

1

10

100

1k

10k

INPUT FREQUENCY (Hz)

10

1

0.1

0.01

100k

Figure 13. Error/Settling Time Graph for Use with the Standard RMS

Connection

The primary disadvantage in using a large CAV to remove ripple

is that the settling time for a step change in input level is

increased proportionately. Figure 13 shows the relationship

between CAV and 1% settling time is 115 ms for each microfarad

of CAV. The settling time is twice as great for decreasing signals

as for increasing signals (the values in Figure 13 are for decreasing

signals). Settling time also increases for low signal levels, as

shown in Figure 14.

10.0

7.5

5.0

2.5

1.0

0

1mV

10mV

100mV

1V

rms INPUT LEVEL

Figure 14. Settling Time vs. Input Level

A better method for reducing output ripple is the use of a post-

filter. Figure 15 shows a suggested circuit. If a single-pole filter

is used (C3 removed, RX shorted), and C2 is approximately

5 times the value of CAV, the ripple is reduced, as shown in

Figure 16, and the settling time is increased. For example, with

CAV = 1 µF and C2 = 4.7 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 CAV and C2 can therefore be reduced

to permit faster settling times while still providing substantial

ripple reduction.

Rev. E | Page 11 of 16

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