5962-89805012A 查看數據表（PDF） - Analog Devices
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5962-89805012A Datasheet PDF : 17 Pages
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AD536A
THEORY OF OPERATION
The AD536A 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 AD536A follows the equation
V
rms
=
Avg
VIN
2
V rms
Figure 6 is a simplified schematic of the AD536A. Note that it is
subdivided into four major sections: absolute value circuit
(active rectifier), squarer/divider, current mirror, and buffer
amplifier. The input voltage (VIN), which can be ac or dc, is
converted to a unipolar current (I1) by the active rectifiers
(A1, A2). I1 drives one input of the squarer/divider, which has
the transfer function
I4 = II2/I3
The output current, I4, of the squarer/divider drives the current
mirror through a low-pass filter formed by R1 and the exter-
nally 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. Thus,
I4 = Avg[II2/I4] = II rms
CURRENT MIRROR
14
+VS
10
COM
ABSOLUTE VALUE;
0.2mA
FS
I3
R1 0.4mA
25kΩ FS
VOLTAGE-CURRENT
4
8
9
CONVERTER
A3
I2
IOUT
R2
25kΩ
RL
R4
VIN
50kΩ
I1
Q1
|VIN|R–1
Q3
BUF
5
IN
7
BUFFER
dB
OUT
1
A4
A1
Q2 Q4
Q5
6
12kΩ A2
25kΩ
BUF
R3
12kΩ
25kΩ
ONE-QUADRANT
SQUARER/DIVIDER
80kΩ OUT
3
NOTES
1. PINOUTS ARE FOR 14-LEAD DIP.
–VS
Figure 6. Simplified Schematic
Data Sheet
The current mirror also produces the output current, IOUT, which
equals 2I4. IOUT can be used directly or can be converted to a
voltage with R2 and buffered by A4 to provide a low impedance
voltage output. The transfer function of the AD536A results in
the following:
VOUT = 2R2 × 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. The 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.
CONNECTIONS FOR dB OPERATION
The logarithmic (or decibel) output of the AD536A is one of
its most powerful features. The internal circuit computing dB
works accurately over a 60 dB range. The connections for dB
measurements are shown in Figure 7.
Select the 0 dB level by adjusting R1 for the proper 0 dB reference
current (which is set to cancel the log output current from the
squarer/divider at the desired 0 dB point). The external op amp
provides a more convenient scale and allows compensation of
the +0.33%/°C scale factor drift of the dB output pin.
The temperature-compensating resistor, R2, is available online
in several styles from Precision Resistor Company, Inc., (Part
Number AT35 and Part Number ST35). The average temperature
coefficients of R2 and R3 result in the +3300 ppm required to
compensate for the dB output. The linear rms output is available
at Pin 8 on the DIP or Pin 10 on the header device with an output
impedance of 25 kΩ. Some applications require an additional
buffer amplifier if this output is desired.
For dB calibration,
1. Set VIN = 1.00 V dc or 1.00 V rms.
2. Adjust R1 for dB output = 0.00 V.
3. Set VIN = +0.1 V dc or 0.10 V rms.
4. Adjust R5 for dB output = −2.00 V.
Any other desired 0 dB reference level can be used by setting
VIN and adjusting R1 accordingly. Note that adjusting R5 for the
proper gain automatically provides the correct temperature
compensation.
Rev. E | Page 8 of 16
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