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AD608AR-REEL bảng dữ liệu(PDF) 11 Page - Analog Devices |
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AD608AR-REEL bảng dữ liệu(HTML) 11 Page - Analog Devices |
11 / 16 page AD608 Rev. C | Page 11 of 16 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. 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. 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 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. 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). 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. |
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