If an analog switch IC is used for on-off switching of a signal path, how much isolation can be achieved in the signal-off state may be of critical importance. Several analog switches from different vendors were evaluated for their on-off ratios using the following circuit board as a test fixture.

 
F1 analog switch (cr) Figure 1: Analog switch test fixture with BNC connectors and a copper shield.

 
The test fixture was fitted with BNC connectors and a copper shield was inserted between those connectors to make sure, as much as possible, that the measured isolation values were really due to the ICs under test and not from preventable stray coupling.

 
F2 result analog switch (cr) Figure 2: Results of the test fixture with given frequency of 6kHz as one octave above an assumed 3kHz speech content frequency limit.  

These off-state isolation values are believed to be dominated by IC capacitances. By that reasoning (which has not been verified), off-state isolations would be expected to worsen by 6dB per octave for rising frequencies versus these 6kHz test results.

First seen on EDN.