AD8309ARU Analog Devices Inc, AD8309ARU Datasheet - Page 10

• Manufacturer Part Number: AD8309ARU
• Description: IC LOGARITHM AMP 100DB 16-TSSOP
• Manufacturer: Analog Devices Inc
• Type: Logarithmic Amplifierr

Specifications of AD8309ARU

• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP
• Rohs Status: RoHS non-compliant
• Applications: Receiver Signal Strength Indication (RSSI)
• No. Of Amplifiers: 1
• No. Of Pins: 16
• Peak Reflow Compatible (260 C): No
• Bandwidth: 500MHz
• Leaded Process Compatible: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD8309
sensitivity to disturbances on the supply lines. With careful
design, the sensitivities to many other parametric variations, and
the effects of temperature and supply voltage, can be reduced to
negligible proportions.
Figure 24. Basic Log Amp Structure Using A/0 Stages and
Transconductance (g
The output of each gain cell has an associated transconductance
(g
cell to a pair of differential currents; these are summed by sim-
ply connecting the outputs of all the g
parallel. The total current is then converted back to a voltage by
a transresistance stage, which determines the slope of the loga-
rithmic output. This general scheme is depicted, in a simplified
single-sided form, in Figure 24. Additional detectors, driven by
a passive attenuator, may be added to extend the top end of the
dynamic range.
The slope voltage may now be decoupled from the knee-voltage
E
biased with currents (not shown in the Figure) which can be
derived from a band-gap reference and thus be stable with tem-
perature. This is the architecture used in the AD8309. It affords
complete control over the magnitude and temperature behavior
of the logarithmic slope.
A further step is yet needed to achieve the demodulation response,
required in a log-limiter amp is to convert an alternating input
into a quasi- dc baseband output. This is achieved by modifying
the g
rectification function. Early log amps based on the progressive
compression technique used half-wave rectifiers, which made
post-detection filtering difficult. The AD640 was the first com-
mercial monolithic log amp to use a full-wave rectifier; this
proprietary practice has been used in all subsequent Analog
Devices types.
We can model these detectors as being essentially linear g
but producing an output current that is independent of the sign
of the voltage applied to the input. That is, they implement the
absolute-value function. Since the output from the later A/0 stages
closely approximates an amplitude symmetric square wave for
even moderate input levels, the current output from each detec-
tor is almost constant over each period of the input. Somewhat
earlier detectors stages in the chain produce a waveform having
only very brief “dropouts” at twice the input frequency. Only
those detectors nearest the log amp’s input produce a low level
waveform that is approximately sinusoidal. When all these (cur-
rent mode) outputs are summed, the resulting signal has a wave-
form which is readily filtered, to provide a low residual ripple on
the output.
K
m
DETECTORS
V
+TOP-END
) cell, which converts the differential output voltage of the
= 2kT/q, which is inherently PTAT. The detector stages are
IN
m
cells used for summation purposes to implement the
g
m
STAGE 1
A/0
CURRENT-SUMMING LINE
g
m
m
STAGE 2
A/0
) Cells for Summing
g
m
STAGE N
A/0
m
(detector) stages in
g
m
R
–
SLOPE
m
V
V
LOG
LIM
cells,
–10–
Intercept Calibration
Monolithic log amps from Analog Devices incorporate accurate
means to position the intercept voltage V
wave power for a demodulating log amp, when driven at a spe-
cific impedance level). Using the scheme shown in Figure 24,
the value of the intercept level departs considerably from that
predicted by the simple theory. Nevertheless, the intrinsic inter-
cept voltage is still proportional to E
tional to absolute temperature).
Recalling that the addition of an offset to the output produces
an effect which is indistinguishable from a change in the posi-
tion of the intercept, it will be apparent that we can cancel the
“left-right” motion of V
tion of E
having the required temperature behavior.
The precise temperature-shaping of the intercept-positioning
offset can result in a log amp having stable scaling parameters,
making it a true measurement device, for example, as a calibrated
Received Signal Strength Indicator (RSSI). In this application,
one is more interested in the value of the output for an input
waveform which is often sinusoidal (CW). The input level be
stated as an equivalent power, in dBm, but it is essential to
know the impedance level at which this “power” is presumed to
be measured. In an impedance of 50 , 0 dBm (1 mW) corre-
sponds to a sinusoidal amplitude of 316.2 mV (223.6 mV rms).
For the AD8309, the intercept may be specified in dBm when
the input impedance is lowered to 50 , by the addition of a
shunt resistor of 52.3 , in which case it occurs at –95 dBm.
However, the response is actually to the voltage at the input, not
the power in the termination resistor, and should be specified in
dBV. A –95 dBm sine input across a 50
sponds to an amplitude of 5.6 V, or –108 dBV, where 0 dBV is
specified as a sine waveform of 1 V rms, that is, 2.8 V p-p.
Note that a log amp’s intercept is a function of waveform. For
example, a square-wave input will read 6 dB higher than a sine-
wave of the same amplitude, and a Gaussian noise input 0.5 dB
higher than a sine wave of the same rms value. Further, a log
amp driven by the sum of two sinusoidal voltages of equal am-
plitude will show an output that is only 2.1 dB higher than the
response for a single sine wave drive, rather than the 3 dB that
might be expected if the device truly responded to input power.
These are characteristics exhibited by all demodulating log amps.
Dynamic Range
The lower end of the dynamic range is determined largely by the
thermal noise floor, measured at the input of the amplifier chain.
For the AD8309, the short-circuit input-referred noise-spectral
density is 1.1 nV/ Hz, and 1.275 nV/ Hz when driven from a
net source impedance of 25
sponds to a noise power of –78 dBm in a 500 MHz bandwidth.
The upper end of the dynamic range is extended upward by the
addition of top-end detectors driven by a tapped attenuator. These
smaller signals are applied to additional full-wave detectors
whose outputs are summed with those of the main detectors.
With care in design, this extension in the dynamic range can be
‘seamless’ over the full frequency range. For the AD8309 it
amounts to a further 48 dB. When using a supply of 4.5 V or
greater, an input amplitude of 4 V can be accommodated, corre-
sponding to a power level of +22 dBm in 50 . (A larger input
voltage may cause damage.)
K
by simply adding an offset at its demodulated output
X
resulting from the temperature varia-
(a terminated 50 ). This corre-
K
, which is PTAT (propor-
X
resistance corre-
(or equivalent sine-
REV. B