AD636

Data Sheet

Rev. E | Page 8 of 16

THEORY OF OPERATION

RMS MEASUREMENTS

The AD636 embodies an implicit solution of the rms equation

that overcomes the dynamic range as well as other limitations

inherent in a straightforward computation of rms. The actual

computation performed by the AD636 follows the equation:





















rms

V

V

Avg

rms

V

IN

2

The AD636 is comprised of four major sections: absolute value

circuit (active rectifier), squarer/divider, current mirror, and

buffer amplifier (see Figure 7, for a simplified schematic). The

unipolar current I1, by the active rectifier A1, A2. I1 drives one

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

I3

I1

I4

2



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

mirror through a low-pass filter formed by R1 and the externally

greater than the longest period of the input signal, then I4 is

effectively averaged. The current mirror returns a current I3,

which equals Avg. [I4], back to the squarer/divider to complete

the implicit rms computation. Therefore,

rms

I1

I4

I2

Avg

I4



















2

with R2 and buffered by A4 to provide a low impedance voltage

output. The transfer function of the AD636 thus results

Q5, buffers and level shifts this voltage, so that the dB output

voltage is zero when the externally supplied emitter current

ABSOLUTE VALUE/

VOLTAGE–CURRENT

CONVERTER

A4

6

7

5

3

9

8

4

10

14

A1

A2

A3

1

COM

BUFFER

BUF

IN

10kΩ

Q5

Q4

Q2

Q1

Q3

8kΩ

8kΩ

+

R4

I1

I3

I4

CURRENT MIRROR

R4

20kΩ

R3

10kΩ

ONE-QUADRANT

SQUARER/

DIVIDER

dB

OUT

BUF

OUT

R2

10kΩ

20µA

FS

R1

25kΩ

10µA

FS

Figure 7. Simplified Schematic

THE AD636 BUFFER AMPLIFIER

The buffer amplifier included in the AD636 offers the user

additional application flexibility. It is important to understand

some of the characteristics of this amplifier to obtain optimum

performance. Figure 8 shows a simplified schematic of the buffer.

Because the output of an rms-to-dc converter is always positive,

it is not necessary to use a traditional complementary Class AB

output stage. In the AD636 buffer, a Class A emitter follower is

used instead. In addition to excellent positive output voltage

swing, this configuration allows the output to swing fully down

to ground in single-supply applications without the problems

associated with most IC operational amplifiers.

BUFFER

OUTPUT

10kΩ

(OPTIONAL, SEE TEXT)

BUFFER

INPUT

CURRENT

MIRROR

40kΩ

5µA

5µA

Figure 8. Buffer Amplifier Simplified Schematic

When this amplifier is used in dual-supply applications as an

input buffer amplifier driving a load resistance referred to

ground, steps must be taken to ensure an adequate negative

voltage swing. For negative outputs, current flows from the load

resistor through the 40 kΩ emitter resistor, setting up a voltage

limits the available negative output swing of the buffer. The

voltage divider such that increased negative swing is possible.