The AD8368 is a variable gain amplifier (VGA) with analog linear-in-dB gain control that can be used from low frequencies to 800 MHz. Its excellent gain range, conformance, and flatness are attributed to the Analog Devices, Inc. X-AMP® architecture, an innovative technique for implementing high performance variable gain control.
The gain range of −12 dB to +22 dB is scaled accurately to 37.5 dB/V with excellent conformance error. The AD8368 has a 3 dB bandwidth of 800 MHz that is nominally independent of gain setting. At 140 MHz, the OIP3 is 33 dBm at maximum gain. The output noise floor is −143 dBm/Hz, which corresponds to a 9.5 dB noise figure at maximum gain. The single-ended input and output impedances are nominally 50 Ω.
The gain of the AD8368 can be configured to be an increasing or decreasing function of the gain control voltage depending on whether the MODE pin is pulled to the positive supply or to ground, respectively. When MODE is pulled high, the AD8368 operates as a typical VGA with increasing gain.
By connecting MODE to ground and using the on-board rms detector, the AD8368 can be configured as a complete automatic gain control (AGC) system with RSSI. The output power is accurately leveled to the internal default setpoint of 63 mV rms (−11 dBm referenced to 50 Ω), independent of the waveform crest factor. Because the uncommitted detector input is available at DETI, the AGC loop can level the signal at the AD8368 output or at any other point in the signal chain over a maximum input power range of 34 dB. Furthermore, the setpoint level can be raised by dividing down the output signal before applying it to the detector.
The AD8368 operates from a supply voltage of 4.5 V to 5.5 V and consumes 60 mA of current. It can be fully powered down to <3 mA by grounding the ENBL pin. The AD8368 is fabricated using the Analog Devices proprietary SiGe SOI complementary bipolar IC process. It is available in a 24-lead LFCSP and operates over the industrial temperature range of −40°C to +85°C. Application boards are available upon request.
