Datasheet
AD637
Rev. K | Page 7 of 20
FUNCTIONAL DESCRIPTION
FILTER/AMPLIFIER
24k
24k
ONE QUADRANT
SQUARER/DIVIDER
BUFFER
AMPLIFIER
Q1
Q2
Q3
Q4
125
6k
6k
12k
24k
A5
A1
A2
ABSOLUTE VALUE VOLTAGE TO
CURRENT CONVERTER
I
1
I
3
I
4
A4
A3
BIAS
Q5
C
AV
+V
S
RMS
OUT
COMMON
CS
DEN
INPUT
OUTPUT
OFFSET
dB
OUTPUT
AD637
BUFF OUT
BUFF IN
–V
S
00788-004
14
1
13
10
4
6
5
3
7
9
11
8
V
IN
Figure 4. Simplified Schematic
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
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
=
rmsV
V
AvgrmsV
IN
2
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 (V
IN
), which can be ac or dc, is converted to a unipolar
current I
1
by the active rectifiers A1 and A2. I
1
drives one input
of the squarer/divider, which has the transfer function
3
1
4
I
I
I
2
=
The output current of the squarer/divider I
4
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 I
4
. The output of this filter amplifier is used by A3
to provide the denominator current I
3
, which equals Avg I
4
and
is returned to the squarer/divider to complete the implicit rms
computation
rmsI
I
I
AvgI
1
4
1
4
=
⎥
⎦
⎤
⎢
⎣
⎡
=
2
and
V
OUT
= V
IN
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 I
3
is now equal to
I
4
, giving
4
1
I
I
2
4
I =
I
4
= |I
1
|
The denominator current can also be supplied externally by
providing a reference voltage (V
REF
) to Pin 6. The circuit operates
identically to the rms case, except that I
3
is now proportional to
V
REF
. Therefore,
3
1
I
I
Avg
2
4
I =
and
DEN
IN
OUT
V
V
V
2
=
This is the mean square of the input signal.