AD8306

The AD8306 is a versatile and easily applied log-limiting amplifier. Being complete, it can be used with very few external components, and most applications can be accommodated using the simple connections shown in the preceding section. A few examples of more specialized applications are provided here.

The AD8306 is a complete IF limiting amplifier, providing bothan accurate logarithmic (decibel) measure of the input signal(the RSSI function) over a dynamic range of 100 dB, and a programmable limiter output, useful from 5 MHz to 400 MHz. It is easy to use, requiring few external components. A single supply voltage of +2.7 V to +6.5 V at 16 mA is needed, corre- sponding to a power consumption of under 50 mW at 3 V, plus
the limiter bias current, determined by the application and typically 2 mA, providing a limiter gain of 90 dB when using 200Ω loads. A CMOS-compatible control interface can enable the AD8306 within about 500 ns and disable it to a standby current of under 1 µA.

The six cascaded amplifier/limiter cells in the main path have a small signal gain of 12.04 dB (*4), with a 3 dB bandwidth of 850 MHz, providing a total gain of 72 dB. The programmable output stage provides a further 18 dB of gain. The input is fully differential and presents a moderately high impedance (1 kΩinparallel with 2.5 pF). The input-referred noise-spectral-density,when driven from a terminated 50 Ω, source is 1.28 nV/Hz,
equivalent to a noise figure of 3 dB. The sensitivity of the AD8306 can be raised by using an input matching network.Each of the main gain cells includes a full-wave detector. An additional four detectors, driven by a broadband attenuator, are used to extend the top end of the dynamic range by over 48 dB.

The overall dynamic range for this combination extends from91 dBV (78 dBm at the 50 Ωlevel) to a maximum permissible value of +9 dBV, using a balanced drive of antiphase inputs each of 2 V in amplitude, which would correspond to a sine wave power of +22 dBm if the differential input were terminated in 50 Ω.Through laser trimming, the slope of the RSSI output is closely controlled to 20 mV/dB, while the intercept is set to 108 dBV(95 dBm re 50 Ω). These scaling parameters are determined by a band-gap voltage reference and are substantially indepen-dentoftemperature and supply. The logarithmic law conformance is typically within ±0.4 dB over the central 80 dB of this range at any frequency between 10MHz and 200 MHz, and is degraded only slightly at 400 MHz.

The RSSI response time is nominally 73 ns (10%90%). The averaging time may be increased without limit by the addition of an external capacitor. The full output of 2.34 V at the maximum input of +9 dBV can drive any resistive load down to 50 Ωand this interface remains stable with any value of capacitance on the output.

The AD8306 is fabricated on an advanced complementary bipolar process using silicon-on-insulator isolation techniques and is available in the industrial temperature range of 40°C to+85°C, in a 16-lead narrow body SO package. The AD8306 is also available for the full military temperature range of 55°C to+125°C, in a 16-lead side-brazed ceramic DIP.