AD637JD/+ Analog Devices Inc, AD637JD/+ Datasheet - Page 7

• Manufacturer Part Number: AD637JD/+
• Description: IC RMS/DC CONV PRECISION 14-CDIP
• Manufacturer: Analog Devices Inc

Specifications of AD637JD/+

• Rohs Status: RoHS non-compliant
• Current - Supply: 2.2mA
• Voltage - Supply: ±3.0V ~ 18V
• Mounting Type: Through Hole
• Package / Case: 14-CDIP (0.300", 7.62mm)

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