FAQ: Operating RMS Detectors at Low Frequencies

Document created by CarlosC Employee on Nov 2, 2010Last modified by AndyR on Jan 31, 2012
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Which RMS Power Detectors can be operated at low frequencies?




The term "low frequency" is relative.  To someone working with microwave signals, 50 MHz can mean low frequency. In that case, all of our rms detectors, except for our current Wafer Level Chip Scale Package (WLFCSP) product line (ADL5500, ADL5502, ADL5504, and ADL5505), can readily measure frequencies as low as 50 MHz. These include the AD8361, AD8362, AD8363, AD8364, ADL5501, and ADL5902.


If by low frequency you mean kHz (or lower!), then only devices that require external ac-coupling will work.  Devices such as the ADL5501 have on-chip ac-coupling which disallow for lower frequency performance.  The result of the internal ac coupling is a 3 dB high-pass corner frequency in the 10s of MHz which limits that power delivered to the detector.  This restricts operation to 50 MHz or more.


Any detector that has the ac coupling cap at the RF input externally has a controllable low frequency point.  These devices include the AD8361, AD8362, AD8363, and AD8364  More information on how to operate these devices at low frequencies, including tradeoffs, can be found in the application note AN-691.  http://www.analog.com/static/imported-files/application_notes/AN-691.pdf


Note that the AD8363 and AD8364 (not released at the time AN-691 was publish) follow similar trends as the AD8362.