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let's consider AD8318, a chip which contains a logarithmic amplifier, often used for power measurements.
I have read its datasheet and there is a thing that I do not understand: no maximum input RF voltage is declared. We may see from page 6 that only maximum input power is declared (12 dBm). Why? How can I know which is the maximum input RF voltage?
Becasue there isn't a max voltage, only a max power. You can put in whatever voltage you want, as long as the power referenced to 50Ω is kept below 12dBm. The requirement forces you to look at the frequency content of your signal and ensure that the total power of that signal does not exceed the rating. So measure or calculate the total power of whatever is upstream and keep it below that requirement.
At DC you should limit the voltage to this requirement.
Using P = V^2/R, at Power = 0.001 watt, and R = 50 ohm, you can compute Vrms = 0.223 volts.
Then scale up by 2 * 1.414, to compute Vpeakpeak = 0.632 volts.
The +12dBm input is 4 factors of 2X power (3dB) stronger than 0dBm.
The +12dBm input is 2 factors of 2X voltage (6.01dB) stronger than 0dBm.
Thus we know the +12dBm is 2 * 2 * 0.223 voltRMS.
