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Response of Log amp to Different Signal Types

Log amps are generally specified for a sine wave input. Like diode detectors, log amps are not true rms responding. Differing signal waveforms in a log amp shift the effective value of the log amp's intercept upwards or downwards. Graphically, this looks like a vertical shift in the log amp's transfer function (see Fig. 19.), the logarithmic slope being unaffected. The figure shows the transfer function of the AD8307 when alternately fed by an un-modulated sine wave and by a forward-link CDMA channel of the same rms power (9 channels active.) The output voltage differs by the equivalent of 3.55dB (88.7 mV) over the complete dynamic range of the device.

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