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AD637
FUNCTIONAL DESCRIPTION
FILTER/AMPLIFIER
BUFF IN
BUFF OUT 14
1
A5
BUFFER
AMPLIFIER
ONE QUADRANT
SQUARER/DIVIDER
I 4
24k ?
A4
8
11
9
C AV
+V S
RMS
OUT
24k ?
I 1
Q4
7
3
dB
OUTPUT
COMMON
ABSOLUTE VALUE VOLTAGE TO
CURRENT CONVERTER
6k ? 6k ?
A2
Q1
Q2
Q3
A3
Q5
I 3
BIAS
24k ?
5
6
4
CS
DEN
INPUT
OUTPUT
OFFSET
V IN 13
12k ?
A1
125 ?
AD637
Figure 4. Simplified Schematic
10
–V S
? V IN 2 ?
I 4 =
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 ? ?
? ? V rms ? ?
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
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
I 1 2
I 4
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
I 1
I 4 = Avg
I 4 =
2
I 3
V REF . Therefore,
I 1 2
I 3
The output current of the squarer/divider I 4 drives A4, forming
a low-pass filter with the external averaging capacitor. If the RC
and
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
V OUT =
V IN 2
V DEN
to provide the denominator current I 3 , which equals Avg I 4 and
This is the mean square of the input signal.
I 4 = Avg ? 1 ? = I 1 rms
is returned to the squarer/divider to complete the implicit rms
computation
? I 2 ?
? I 4 ?
and
V OUT = V IN rms
Rev. K | Page 7 of 20
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