SSM2110_15 Datasheet, PDF (8/11 Pages) Analog Devices – TRUE RMS TO DC CONVERTER

ANALOG DEVICES fAX-ON-DEMAND HOTLINE
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range of the device will be roughly symmetrical about the inter-
nal negative voltage reference. Also. the output impedance will
be low enough to drive the log amplifier's input(s) without intro-
ducing significant errors. The bias current into the pins of the log
amplifier is typically less than 1J.lAbut can be as high as 21lA. For
this reason the current through the log recovery transistor A"
should always be set higher than 51-1AT. he current 'REFI should
not be set too high (above SellA) because the base current of
The transfer characteristic of the log recovery amplifier is given
by the following equation:
lOUT = 64mV.1 V1N
RSCALE VT
Combining the two equations yields the overall transfer charac-
teristic for the ouput voltage VlOGIRMSj:
0'0 induces errors in the RMS computing loop. If higher reference
currents are required then they should be taken care of by
changing the current through Olj' This can be done by changing
V_{LOG (AMS) =} ^O.148xRRMS _x Iog ^{(VINlR,N) 2}
RSCALE
( IREFI x I REF2
RREF2'The Log Recovery Amplifier Section explains this in more
detail.
The output sensitivity at EMITTER (pin 8) is about +60mV for
( ) where, IREF' '" 8.1 V
~EFI
every 1Ode of signal level increase at 25°C. This sensitivity has
a temperature coefficient of +3300ppm/oC.
( ) ^7.5V
'REF2 '"
+ 1IJA
RREF2
OBSOLETE The fog of absolute value output can be converted to a log of
mean value by connecting a capacitor between LOGABSVAL(pin
2) and V- (see Figure 7). Since this is an emitter follower output.
the response to a large-signal level increase will be fast while
the time constant of the output following a large-signal level de-
crease will be determined by the product of capacitor CAVGand
resistor RREFI'
One might think that connecting a capacitor to the log output
would produce the average of the log of the absolute value.
However, since the capacitor enforces an AC ground at the
emitter of the output transistor, the capacitor charging currents
are proportionalto the antilog ofthe signal atthe base. Since the
base voltage is the log of the absolute value, the log and the
antilog terms cancel, and the capacitor is charged as a linear
integrator with a current directly proportional to the absolute value
of the input current. This effectively inverts the order of the av-
eraging and logging operations. The signal at the output. there-
fore, is the log of the average of the absolute value of the input
Ideally this voltage is completely independent of temperature.
However, due to the temperature coefficientof several transis-
tors internal to the SSM-211 0, this is not entirely true. The tem-
perature coefficient is approximately :t75ppm/aC.
With the values shown in Figures 7 and S, this transfer function
corresponds to an output change of 50mV/dB, The reference
current is set to 10IJ.Ato provide the widest possible dynamic
range. The following results can be expected for the circuit in
Figure 8:
V,N(RMSI
100)l.V
I'NIRMS) ^'OnA
v, "" OUT ^-3V
tmV
100nA
-2V
10mA
')l.A
-'V
100mV ,,,
'V Ii
1OI'A 100)l.A
i
ov
1V _!
,av
1mA
2V
An RSCAlEvalue of approximately 4.7kQ gives the best overall
linearity and temperature compensation performance. This IS an
signal.
improvement of about a factor of 40 over the uncompensated
LOG RECOVERY AMPLIFIER (PINS 9,10,11 AND 12)
The log recovery amplifier is a linearized voltage-to-current
transconductor whose gain can be made proportional to abso-
lute temperature. It is used to reference the log output( s) to ground
and also to temperature compensate the VT (KT/q) terms in the
log output recovery transistors (0/0'0 anda,,).
One input of the log recovery amplifier is usually connected to
EMITTER (the emitter of the log recovery transistor-pin 8) while
the other is connected to VREF(pin 3).
Figure 6 shows the internal and external connections used to
drift. A 2k.a resistor in series with a silicon diode can be connected
from LOG SCALE (pin 12) to the negative supply to defeat the
temperature compensation for certain applications such as
compressorllimiters where the log drift will cancel the thermal
gain drift of a VCA's dB/volt control port.
The maximum output current for both the compensated and
uncompensated examples above is :!:250~A. This output current
is converted to a voltage with the circuits in Figures 7 and S. For
these circuits this corresponds to a maximum output voltage of
:!:3.975V.If RSCAlE is changed from the nominal value of 4.7kn
the maximum output current will also change by the following
obtain an output voltage equal to VlOG(RMSr The transfer char-
acteristic of the log recovery transistors is given by the following
equation:
equation:
ILOGOUT(MAX'")' 1.1SV
RSCALE
.1VIN ,. VT x In ^{VIN(RMS)/RIN) 2}
( IREF' x IAEF2 )
( ) where, IREF' = 8.W
RREF'
( ) ^{IREF2 '" 7.5V + 1J.lA}
~EF2
If the log recovery amplifier is not used, +LOGIN (pin 10) and -
LOG IN (pin 11) must be connected to V REF(pin 3) for proper
operation of the rest of the circuit.
It is possible to use one of the log configurations in Figure 7 or 8
in conjunction with the linear output circuits (Figures 1.3.4 and
5) but care must be taken in choosing the appropriate resistor
values. This provides the user with substantial flexibility from a
single device.