Analog Devices AD604 - Manual (Page 19)

Figure 44. AGC Amplifier with 82 dB of Gain Range
ULTRALOW NOISE AGC AMPLl
96
g
A INR
a
ING' e W
Figure 44 shows animplementálion ofaw^GcampHTter '
82 dB of gain range using a single AD604. The signal is applied
to connector VIN and, because the signal source is 50 O, a
terminating resistor (R1) of 49.9 O is added. The signal is then
amplified by 14 dB (Pin FBK1 shorted to PAO1) through the
Channel 1 preamplifier and is further processed by the Channel 1
DSX. Next, the signal is applied directly to the Channel 2 DSX. The
second preamplifier is powered down by connecting its COM2 pin
to the positive supply as explained in the Preamplifier section.
C1 and C2 level shift the signal from the preamplifier into the
first DSX and, at the same time, eliminate any offset contribution
of the preamplifier. C3 and C4 have the same offset cancellation
purpose for the second DSX. Each set of capacitors, combined
with the 175 O input resistance of the corresponding DSX,
provides a high-pass filter with a -3 dB corner frequency of
about 9.1 kHz. VOCM is decoupled to ground by a 0.1 pF
capacitor, while VREF can be externally provided; in this
application, the gain scale is set to 20 dB/V by applying 2.500 V.
Because each DSX amplifier operates from a single 5 V supply,
the output is ac-coupled via C6 and C7. The output signal can
be monitored at the connector labeled RF OUT.
Figure 45 and Figure 46 shoW the gai«-range-apd gain error for
the AD604-CO«n|Cted as shown i\ Figure-44. The gain range is
-l4toB^o^82-dl; JhJus^ful r/ng^ is^ dB to +82 dB if the RF
output amplitude is controlled to ±400 mV (+2 dBm). The main
limitation on the lower end of the signal range is the input
capability of
the preamplifier. This limitation can be overcome by adding an
attenuator in front of the preamplifier, but that would defeat the
advantage of the ultralow noise preamplifier. It should be noted
that the second preamplifier is not used because its ultralow
noise and the associated high-power consumption are overkill
after the first DSX stage. It is disabled in this application by
connecting the COM2 pin to the positive supply. Nevertheless,
the second preamplifier can be used, if so desired, and the
useful gain range increases by l4 dB to encompass 0 dB to
96 dB of gain. For the same +2 dBm output, this allows signals
as small as -94 dBm to be measured.
To achieve the highest gains, the input signal must be band-
limited to reduce the noise; this is especially true if the second
preamplifier is used. If the maximum signal at OUT2 of the AD604
is limited to ±400 mV (+2 dBm), the input signal level at the
AGC threshold is +25 pV rms (-79 dBm). The circuit as shown in
Figure 44 has about 40 MHz of noise bandwidth; the 0.8 nV/VHz
of input referred voltage noise spectral density of the AD604
results in an rms noise of 5.05 pV in the 40 MHz bandwidth.
Rev. E | Page 19 of 32