AD536A Analog Devices, AD536A Datasheet - Page 6

• Manufacturer Part Number: AD536A
• Description: Integrated Circuit True RMS-to-DC Converter
• Manufacturer: Analog Devices

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, V
unipolar current I
input of the squarer/divider, which has the transfer function:
The output current, I
mirror through a low-pass filter formed by R1 and the externally
connected capacitor, C
greater than the longest period of the input signal, then I
effectively averaged. The current mirror returns a current I
which equals Avg. [I
the implicit rms computation. Thus:
Figure 9. Simplified Schematic
1
I
, by the active rectifier A
4
4
IN
= Avg. I
], back to the squarer/divider to complete
4
, of the squarer/divider drives the current
, which can be ac or dc, is converted to a
AV
. If the R1, C
I
4
[
= I
1
2
1
/ I
2
/I
4
3
]
= I
AV
1
time constant is much
rms
1
, A
2
. I
1
drives one
Figure 10. dB Connection
4
is
3
,
–6–
The current mirror also produces the output current, I
which equals 2I
voltage with R2 and buffered by A4 to provide a low impedance
voltage output. The transfer function of the AD536A thus
results:
The dB output is derived from the emitter of Q3, since the
voltage at this point is proportional to –log V
lower, Q5, buffers and level shifts this voltage, so that the dB
output voltage is zero when the externally supplied emitter
current (I
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.
dB Calibration:
1. Set V
2. Adjust R1 for dB out = 0.00 V
3. Set V
4. Adjust R5 for dB out = –2.00 V
Any other desired 0 dB reference level can be used by setting
V
the proper gain automatically gives the correct temperature
compensation.
IN
and adjusting R1, accordingly. Note that adjusting R5 for
IN
IN
REF
= 1.00 V dc or 1.00 V rms
= +0.1 V dc or 0.10 V rms
) to Q5 approximates I
4
. I
V
OUT
OUT
can be used directly or converted to a
= 2R2 I rms = V
3
.
IN
rms
IN
. Emitter fol-
OUT,
REV. B