AD536A
Figure 9 is a simplified schematic of the AD536A; it is subdi-
vided 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 rectifier A1, A2. I1 drives one
input of the squarer/divider, which has the transfer function:
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 AD536A thus
results:
VOUT = 2R2 I rms = VIN rms
I4 = I12/I3
The dB output is derived from the emitter of Q3, since the
voltage at this point is proportional to –log VIN. Emitter fol-
lower, 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.
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. Thus:
CONNECTIONS FOR dB OPERATION
A powerful feature added to the AD536A is the logarithmic or
decibel output. The internal circuit computing dB works accu-
rately over a 60 dB range. The connections for dB measure-
ments are shown in Figure 10. The user selects the 0 dB level by
adjusting R1, for the proper 0 dB reference current (which is set
to exactly cancel the log output current from the squarer-divider
at the desired 0 dB point). The external op amp is used to pro-
vide a more convenient scale and to allow compensation of the
+0.33%/°C scale factor drift of the dB output pin. The special
T.C. resistor, R2, is available from Tel Labs in Londonderry,
N.H. (model Q-81) or from Precision Resistor Inc., Hillside,
N.J. (model PT146). The averaged temperature coefficients of
resistors R2 and R3 develop the +3300 ppm needed to reverse
compensate the dB output. The linear rms output is available at
Pin 8 on DIP or Pin 10 on header device with an output imped-
ance of 25 kΩ; thus some applications may require an additional
buffer amplifier if this output is desired.
2
I4 = Avg. I1 /I4 = I1 rms
[
]
dB Calibration:
1. Set VIN = 1.00 V dc or 1.00 V rms
2. Adjust R1 for dB out = 0.00 V
3. Set VIN = +0.1 V dc or 0.10 V rms
4. Adjust R5 for dB out = –2.00 V
Any other desired 0 dB reference level can be used by setting
Figure 9. Simplified Schematic
VIN and adjusting R1, accordingly. Note that adjusting R5 for
the proper gain automatically gives the correct temperature
compensation.
Figure 10. dB Connection
–6–
REV. B
ADI [ ADI ]
