AD637
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
V
2
V rms
=
Avg
IN
V rms
Figure 1 is a simplified schematic of the AD637, it is subdivided
into four major sections; absolute value circuit (active rectifier),
square/divider, filter circuit and buffer amplifier. The input volt-
age V
IN
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
2
I
I
4
=
1
I
3
The output current of the squarer/divider, I4 drives A4 which
forms a low-pass filter with the external averaging capacitor. If
the RC time constant of the filter is much greater than the long-
est period of the input signal than A4s output will be propor-
tional to the average of I4. The output of this filter amplifier is
used by A3 to provide the denominator current I3 which equals
Avg. I4 and is returned to the squarer/divider to complete the
implicit rms computation.
I
2
I
4
=
Avg
1
=
I
1
rms
I
4
and
V
OUT
= V
IN
rms
If the averaging capacitor is omitted, the AD637 will compute the
absolute value of the input signal. A nominal 5 pF capacitor should
be used to insure stability. The circuit operates identically to that of
the rms configuration except that I3 is now equal to I4 giving
I
I
4
=
1
I
4
I
4
=
I
1
The denominator current can also be supplied externally by pro-
viding a reference voltage, V
REF
, to Pin 6. The circuit operates
identically to the rms case except that I3 is now proportional to
V
REF
. Thus:
2
I
1
I
4
=
Avg
I
3
and
2
V
IN
V
O
=
V
DEN
This is the mean square of the input signal.
STANDARD CONNECTION
2
the AD637 can be ac coupled through the addition of a non-
polar capacitor in series with the input as shown in Figure 2.
BUFFER
1
ABSOLUTE
VALUE
AD637
14 NC
13
V
IN
12
NC
+V
S
–V
S
V
O
=
C
AV
V
IN
3
OPTIONAL
AC COUPLING
CAPACITOR
2
3
BIAS
SECTION
4
5
25k
6
7
SQUARER/DIVIDER
25k
11
10
9
FILTER
8
Figure 2. Standard RMS Connection
The performance of the AD637 is tolerant of minor variations in
the power supply voltages, however, if the supplies being used
exhibit a considerable amount of high frequency ripple it is
advisable to bypass both supplies to ground through a 0.1
µF
ceramic disc capacitor placed as close to the device as possible.
The output signal range of the AD637 is a function of the sup-
ply voltages, as shown in Figure 3. The output signal can be
used buffered or nonbuffered depending on the characteristics
of the load. If no buffer is needed, tie buffer input (Pin 1) to
common. The output of the AD637 is capable of driving 5 mA
into a 2 kΩ load without degrading the accuracy of the device.
20
Load
MAX V
OUT
– Volts 2k
15
10
5
0
0
3
5
10
15
SUPPLY VOLTAGE – DUAL SUPPLY – Volts
18
Figure 3. AD637 Max V
OUT
vs. Supply Voltage
CHIP SELECT
The AD637 is simple to connect for a majority of rms measure-
ments. In the standard rms connection shown in Figure 2, only
a single external capacitor is required to set the averaging time
constant. In this configuration, the AD637 will compute the
true rms of any input signal. An averaging error, the magnitude
of which will be dependent on the value of the averaging capaci-
tor, will be present at low frequencies. For example, if the filter
capacitor C
AV
, is 4
µF
this error will be 0.1% at 10 Hz and in-
creases to 1% at 3 Hz. If it is desired to measure only ac signals,
The AD637 includes a chip select feature which allows the user
to decrease the quiescent current of the device from 2.2 mA to
350
µA.
This is done by driving the CS, Pin 5, to below 0.2 V
dc. Under these conditions, the output will go into a high im-
pedance state. In addition to lowering power consumption, this
feature permits bussing the outputs of a number of AD637s to
form a wide bandwidth rms multiplexer. If the chip select is not
being used, Pin 5 should be tied high.
–4–
REV. E
AD [ ANALOG DEVICES ]
