AD608
supply as a reference, but also causes the RSSI output and the
ADC output to track over power supply variations, reducing
system errors and component costs.
RSSI OUTPUT
The logarithmic amplifier uses a successive-detection architecture.
Each of the five stages has a full-wave detector; two additional
high level detectors are driven by attenuators at the input to the
limiting amplifiers, for a total of seven detector stages. Because
each detector is a full-wave rectifier, the ripple component in
the resulting dc is at twice the IF. The AD608 low-pass filter has
a 2 MHz cutoff frequency, which is one decade below the 21.4 MHz
ripple that results from a 10.7 MHz IF.
POWER CONSUMPTION
The total power supply current of the AD608 is a nominal
7.3 mA. The power is signal dependent, partly because the RSSI
output increases (the current is increased by 200 μA at an RSSI
output of +1.8 V), but mostly due to the IF consumption of the
band-pass filter when driven to 891 mV, assuming a 4 dB loss
in this filter and a peak input of +5 dBm to the log-IF amp. In
addition, the power is temperature dependent because the
biasing system used in the AD608 is proportional to the
absolute temperature (PTAT).
For operation at lower IFs, such as 450 kHz or 455 kHz, the
AD608 requires an external low-pass filter with a single pole
located at 90 kHz, a decade below the 900 kHz ripple frequency
for these IFs. The RSSI range is from the noise level at approx-
imately −80 dBm to overload at +15 dBm and is specified for
1 dB accuracy from −75 dBm to +5 dBm. The +15 dBm
maximum IF input is provided to accommodate band-pass
filters of lower insertion loss than the nominal 4 dB for
10.7 MHz ceramic filters.
TROUBLESHOOTING
The most common causes of problems with the AD608 are
incorrect component values for the offset feedback loop, poor
board layout, and pickup of radio frequency interference (RFI),
which all cause the AD608 to lose the low end (typically below
−65 dBm) of its RSSI output and cause the limiter to swing
randomly. Both poor board layout and incorrect component
values in the offset feedback loop can cause low level oscillations.
Pickup of RFI can be caused by improper layout and shielding
of the circuit.
DIGITIZING THE RSSI
In typical cellular radio applications, the RSSI output of the
AD608 is digitized by an analog-to-digital converter (ADC).
The RSSI output of the AD608 is proportional to the power
supply voltage, which not only allows the ADC to use the
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ADI [ ADI ]
