I need to detect power from 2.5 - 6.0 GHz. The linearity of this part vs frequency can be 15 dB? (Detected voltage at 2.5 GHz vs 5.8 GHz) Am I reading the data sheet wrong? Is there a more linear detector across the 2.5 to 6 GHz band? Is there anything I can do on the input to improve the response?
Take a look at this plot of the ADL5511's rms output vs frequency. As RF detectors go, this is probably the flattest performance vs frequency that you can get. It shoudl be possible to build some kind of frequency-dependent attenuator in front of the device to try and flatten out the roll-off from 3 to 6 GHz.