Energy detection (ED) has been of interest for various applications ranging from very low frequency biomedical signal read-out to high frequency wireless communication. The analog implementation of the ED requires circuits with a non-linear transfer characteristics which can be obtained through squaring or rectification. Comparison of these two methods is presented and implementation limitations are detailed. Interesting characteristics of rectifiers, derived from ideal transfer characteristics and from non-ideal circuit implementations, encourage rectifier's use in the ED rather than the squarer. A high speed and high precision current mode full-wave rectifier was implemented in GF 22FDX technology. Our circuit achieves a 60 dB dynamic range and 100 MHz bandwidth with an ultra-low power consumption of 4.3 mu W from 0.8V voltage supply.