5962-8963701CA(2015) Просмотр технического описания (PDF) - Analog Devices

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5962-8963701CA
(Rev.:2015)

High Precision, Wideband RMS-to-DC Converter

Analog Devices 

AD637

Data Sheet

THEORY OF OPERATION

BUFF OUT 14

BUFF IN 1

A5

BUFFER

AMPLIFIER

ONE QUADRANT

SQUARER/DIVIDER

I4

FILTER/AMPLIFIER

24kΩ

A4

8 CAV

11 +VS

9

RMS

OUT

24kΩ

I1

ABSOLUTE VALUE VOLTAGE TO

CURRENT CONVERTER

6kΩ

VIN 13

12kΩ

A1

6kΩ

A2

Q4

Q1

Q2 Q3

A3

Q5

BIAS

I3 24kΩ

125Ω

AD637

Figure 4. Simplified Schematic

7

dB

OUTPUT

3 COMMON

5 CS

6

DEN

INPUT

4

OUTPUT

OFFSET

10 –VS

The AD637 embodies an implicit solution of the rms equation

that overcomes the inherent limitations of straightforward rms

computation. The actual computation performed by the AD637

follows the equation:

V

rms



Avg





VIN 2





V rms 

Figure 4 is a simplified schematic of the AD637, subdivided

into four major sections: absolute value circuit (active rectifier),

squarer/divider, filter circuit, and buffer amplifier. The input

voltage (VIN), which can be ac or dc, is converted to a unipolar

current (I1) by the A1 and A2 absolute value circuit. I1 drives

one input of the squarer/divider, which has the transfer function:

I4



I12

I3

The output current of the squarer/divider, I4, drives A4, forming

a low-pass filter with the external averaging capacitor. If the RC

time constant of the filter is much greater than the longest period

of the input signal, then the A4 output is proportional to the

average of I4. The output of this filter amplifier is used by A3

to provide the denominator current I3, which equals Avg I4 and

is returned to the squarer/divider to complete the implicit rms

computation

I4



Avg







I12

I4









I1 rms

and

VOUT = VIN rms

To compute the absolute value of the input signal, the averaging

capacitor is omitted. However, a small capacitance value at the

averaging capacitor pin is recommended to maintain stability;

5 pF is sufficient for this purpose. The circuit operates identically

to that of the rms configuration, except that I3 is now equal to

I4, giving

I4



I12

I4

I4 = |I1|

The denominator current can also be supplied externally by

providing a reference voltage (VREF) to Pin 6. The circuit operates

identically to the rms case, except that I3 is now proportional to

VREF. Therefore,

I4



Avg

I12

I3

and

VOUT



VIN 2

VDEN

This is the mean square of the input signal.

Rev. L | Page 12 of 25

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