FAQ: AD8362 RMS Detector: Equations for RMS Averaging Capacitor and Corner Frequency of Offset Control Loop

What is the correct equation for the determining the corner frequencies associated with the rms averaging and the offset control loop in the AD8362 RMS Detector?

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There are some errors in the Rev. D datasheet. Under the heading “TIME-DOMAIN RESPONSE OF THE CLOSED LOOP” there is an equation

f_LP = 1/(CLPF x 1.1 KΩ)

This is incorrect. The correct equation is:

F_LP = 1/(CLPF x 1.1 KΩ x 2π)

In the section “CHOOSING A VALUE FOR CLPF”, the equation is written as:

CLPF = 900 μF/(2π x F_LP).

This expresses the correct equation in a slightly different format

Equation 13 on page 21 of the Rev. D datasheet is also incorrect. The correct equation for determining CHPF, which sets the high-pass corner of the offset control loop is:

CHPF = 200uF/Fhp