Datasheet

AD636 Data Sheet
Rev. E | Page 8 of 16
THEORY OF OPERATION
RMS MEASUREMENTS
The AD636 embodies an implicit solution of the rms equation
that overcomes the dynamic range as well as other limitations
inherent in a straightforward computation of rms. The actual
computation performed by the AD636 follows the equation:









rmsV
V
AvgrmsV
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, 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:
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,
rmsI1
I4
I2
AvgI4 







2
The current mirror also produces the output current, I
OUT
, 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 Q
3
, because the
voltage at this point is proportional to –log V
IN
. 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.
ABSOLUTE VALUE/
VOLTAGE–CURRENT
CONVERTER
A4 6
7
5
3
984
10
14
A1
A2
A3
1
COM
BUFFER
BUF
IN
10kΩ
Q5
Q4Q2
Q1
Q3
C
AV
I
OUT
8kΩ
8kΩ
+
|V
IN
|
R4
I1
I3
I4
I
REF
CURRENT MIRRO
R
V
IN
R4
20kΩ
R3
10kΩ
ONE-QUADRANT
SQUARER/
DIVIDER
–V
S
+V
S
R
L
dB
OUT
BUF
OUT
R2
10kΩ
20µA
FS
R1
25kΩ
10µA
FS
00787-013
+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
10kΩ
R
EXTERNA L
(OPTIONAL, SEE TEXT)
–V
S
+
V
S
BUFFER
INPUT
CURRENT
MIRROR
R
LOAD
R
E
40kΩ
5µA5µA
00787-014
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 kΩ 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 R
E
alters this
voltage divider such that increased negative swing is possible.