AD636JD【PDF 9/17 页】IC TRUE RMS/DC CONV MONO 14-CDIP

=

rms

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

input voltage, VIN, which can be ac or dc, is converted to a

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

connected capacitor, C

AV

. If the R1, C

AV

time constant is much

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

=

?/DIV>

=

2

The current mirror also produces the output current, IOUT, which

equals 2I4. I

OUT

can be used directly or converted to a voltage

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

output. The transfer function of the AD636 thus results

V

OUT

= 2 R2 I rms = V

IN

rms

The dB output is derived from the emitter of Q3, because the

voltage at this point is proportional to log VIN. Emitter follower,

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

voltage is zero when the externally supplied emitter current

(I

REF

) to Q5 approximates I3.

ABSOLUTEVALUE/

VOLTAGECURRENT

CONVERTER

A4

6

7

5

3

9

8

4

10

14

A1

A2

A3

1

COM

BUFFER

BUF

IN

10k&

Q5

Q4

Q2

Q1

Q3

C

AV

I

OUT

8k&

+

|V

IN

|

R4

I1

I3

I4

I

REF

CURRENT MIRRO

V

IN

R4

20k&

R3

10k&

ONE-QUADRANT

SQUARER/

DIVIDER

V

S

+V

S

R

L

dB

OUT

BUF

OUT

R2

10k&

20礎

FS

R1

25k&

10礎

FS

+V

S

C

AV

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

10k&

R

EXTERNAL

(OPTIONAL, SEE TEXT)

V

S

+

S

BUFFER

INPUT

CURRENT

MIRROR

R

LOAD

R

E

40k&

5礎

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 k?emitter resistor, setting up a voltage

divider between V

S

and ground. This reduced effective V

S

,

limits the available negative output swing of the buffer. The

addition of an external resistor in parallel with RE alters this

voltage divider such that increased negative swing is possible.

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