AD8307

The AD8307 is the first logarithmic amplifier in an 8-lead (SO-8) package. It is a complete 500 MHz monolithic demodulating logarithmic amplifier based on the progressive compression (successive detection) technique, providing a dynamic range of 92 dB to ±3 dB law-conformance and 88 dB to a tight ±1 dB error bound at all frequencies up to 100 MHz. It is extremely stable and easy to use, requiring no significant external compo- nents. A single supply voltage of 2.7 V to 5.5 V at 7.5 mA is needed, corresponding to an un precedented power consumption of only 22.5 mW at 3 V. A fast-acting CMOS-compatible con-trol pin can disable the AD8307 to a standby current of under 150µA.

Each of the cascaded amplifier/limiter cells has a small-signal gain of 14.3 dB, with a 3 dB bandwidth of 900 MHz. The input is fully differential and at a moderately high impedance (1.1 kΩ  in parallel with about 1.4 pF). The AD8307 provides a basic dynamic range extending from approximately 75 dBm (where dBm refers to a 50Ω source, that is, a sine amplitude of about ±56 µV) up to +17 dBm (a sine amplitude of ±2.2 V).A simple input-matching network can lower this range to 88dBm to +3 dBm. The logarithmic linearity is  typically within ±0.3dB up to 100 MHz over the central portion of this range, and is degraded only slightly at 500 MHz. There is no minimum frequency limit; the AD8307 may be used at audio frequencies (20 Hz) or even lower.

The output is a voltage scaled 25 mV/dB, generated by a current of nominally 2 µA/dB through an internal 12.5 kΩ resistor. Thisvoltage varies from 0.25 V at an input of 74 dBm (that is, the ac intercept is at 84 dBm, a 20 µV rms sine input), up to 2.5 V for an input of +16 dBm. This slope and intercept can be trimmed using external adjustments. Using a 2.7V supply, the output scaling may be lowered, for example to 15 mV/dB, to permit utilization of the full dynamic range.

The AD8307 exhibits excellent supply insensitivity and tem-perature stability of the scaling parameters. The unique combi-nation of low cost, small size, low power consumption, high accuracy and stability, very high dynamic range, and a frequency range encompassing audio through IF to UHF, make this prod-uct useful in numerous applications requiring the reduction of a signal to its decibel equivalent.

The AD8307 is available in the industrial temperature range of40°C to +85°C, and in 8-lead SOIC and PDIP packages.