Fig. 2 is the basic operating schematic of the LTC5507. C1 couples the input to the chip and should be chosen for the frequency range desired. It should be a ceramic type for best results, and you must be careful of self-resonances, particularly at high frequencies. Remember, if you use a 0.1 uF, for example, it does not take much lead length to resonate above a few tens of MHz. As a result, it is not a good idea to use any sort of electrolytic capacitor, since the internal winding of the foil elements in the capacitor package is inductive and will certainly resonate at some frequency over the wide range of the device. C2 is the filter capacitor for the output of the peak detector and will affect the speed of response of the output. As a rule of thumb, both C1 and C2 should be selected according to the following guidelines suggested by Linear Technology: