Analog Devices AD604 Manuel D’Utilisation

 

Rev. E | Page 19 of 32 

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Figure 44 shows an implementation of an AGC amplifier with 
82 dB of gain range using a single AD604. The signal is applied 
to connector VIN and, because the signal source is 50 Ω, a 
terminating resistor (R1) of 49.9 Ω 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 Ω 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 μF 
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 gain range and gain error for 
the AD604 connected as shown in Figure 44. The gain range is 
−14 dB to +82 dB; the useful range is 0 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 14 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 μV rms (−79 dBm). The circuit as shown in 
Figure 44 has about 40 MHz of noise bandwidth; the 0.8 nV/√Hz 
of input referred voltage noise spectral density of the AD604 
results in an rms noise of 5.05 μV in the 40 MHz bandwidth.