ADL5391 - when used as a square law detector (frequency 10 to 50MHz, input power -10dBm to 0dBm) according to figure 21 is the expected error close to 0dB? What does figure 20 say as to the signal to noise ratio at small amplitudes?
At moderate input levels, the error is very small as seen in figure 21. You will be running into the noise floor of the part at low signal levels eventually, but I believe what you are seeing at the lower input levels is the DC offsets that are becoming on par with the signal and introducing the error.
Hope this helps,
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