AD636K Просмотр технического описания (PDF) - Analog Devices
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AD636K Datasheet PDF : 16 Pages
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AD636
Data Sheet
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:
V
rms
Avg
VIN 2
V rms
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, VIN, 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:
I4 I12
I3
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, CAV. If the R1, CAV 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,
I4
Avg
I22
I4
I1
rms
The current mirror also produces the output current, IOUT, which
equals 2I4. IOUT 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
VOUT = 2 R2 I rms = VIN rms
The dB output is derived from the emitter of Q3, because the
voltage at this point is proportional to –log VIN. Emitter follower,
Q5, buffers and level shifts this voltage, so that the dB output
voltage is zero when the externally supplied emitter current
(IREF) to Q5 approximates I3.
CURRENT MIRROR
14 +VS
VIN 1
10 COM
ABSOLUTE VALUE/
VOLTAGE–CURRENT
CONVERTER
R4
20kΩ
A1
R3 8kΩ
10kΩ
|VIN|
+
R4
8kΩ
A2
R1
25kΩ
20µA
FS
10µA
FS
I3 4
8
CAV IOUT
R2
10kΩ
I4
CAV
9 RL
+VS
I1
Q1
A3
Q3
Q2 Q4
IREF
BUF
IN BUFFER
7
A4
Q5
10kΩ
5
dB
OUT
6
BUF
OUT
ONE-QUADRANT
SQUARER/
DIVIDER
3 –VS
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.
+VS
BUFFER
INPUT
CURRENT
MIRROR
5µA 5µA
BUFFER
10kΩ OUTPUT
RE
40kΩ
RLOAD
–VS
REXTERNA L
(OPTIONAL, SEE TEXT)
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 −VS and ground. This reduced effective −VS,
limits the available negative output swing of the buffer. The
addition of an external resistor in parallel with RE alters this
voltage divider such that increased negative swing is possible.
Rev. E | Page 8 of 16
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