AD8313

The AD8313 is a complete multistage demodulating logarithmic amplifier, capable of accurately converting an RF signal at its differential input to an equivalent decibel-scaled value at its dc output. The AD8313 maintains a high degree of log conformance for signal frequencies from 0.1 GHz to 2.5 GHz and is useful over the range of 10 MHz to 3.5 GHz. The nominal input dynamic range is 65 dBm to 0 dBm (re: 50 W), and the sensitivity can be increased by 6 dB or more with a narrow band input impedance matching network or balun. Application is straightforward, requiring only a single supply of 2.7 V5.5 V and the addition of a suitable input and supply decoupling. Operating on a 3 V supply, its 13.7 mA consumption (for TA = +25) amounts to only 41 mW. A power-down feature is provided; the input is taken high to initiate a low current (20 mA) sleep mode, with a threshold at half the supply voltage.

The AD8313 uses a cascade of eight amplifier/limiter cells, each having a nominal gain of 8 dB and a 3 dB bandwidth of 3.5 GHz, for a total midband gain of 64 dB. At each amplifier output, a detector (rectifier) cell is used to convert the RF signal to baseband form; a ninth detector cell is placed directly at the input of the AD8313. The current-mode outputs of these cells are summed to generate a piecewise linear approximation to the logarithmic function, and converted to a low impedance voltagemode output by a transresistance stage, which also acts as a lowpass filter.

When used as a log amp, the scaling is determined by a separate feedback interface (a transconductance stage) that sets the slope to approximately 18 mV/dB; used as a controller, this stage accepts the setpoint input. The logarithmic intercept is positioned to nearly 100 dBm, and the output runs from about 0.45 V dc at 73 dBm input to 1.75 V dc at 0 dBm input. The scale and intercept are supply and temperature