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I have an array of 16 electrodes that I want to be disconnected from everything until selected and then connected to ground, one at a time. Would a multiplexer work for this? My application is sensing the capacitance between pairs of electrodes in sequence using an TI FDC1004 which senses capacitance between a 25kHz signal and ground. The capacitance between electrodes 1-16 would be measured, 2-16, 3-16, 4-16, etc. for a total 128 measurements. enter image description here

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  • \$\begingroup\$ What potential difference are we talking about between Cin and ground? \$\endgroup\$
    – winny
    Feb 12 at 14:36
  • \$\begingroup\$ @winny The spec sheet states that the average DC voltage applied to the sensing electrode is 1.2V \$\endgroup\$ Feb 12 at 18:51
  • \$\begingroup\$ Good. How much stray capacitance is acceptable in the MUX/switch? \$\endgroup\$
    – winny
    Feb 12 at 20:58

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Depends on how sensitive your capacitance sensing is. In general, no, in capacitive button applications, which do pretty much what you describe, that often goes wrong because even in its off mode, the the contacts in a multiplexer circuit have enough capacitance to cause spurious changes, between adjacent pins and between the disconnected pin and the common ground connection. You typically counter that with plenty of post processing in software - based on the fact that for capacitive buttons, the actual capacitance doesn't matter, you just want to reliably sense a finger-induced change of capacitance.

Separate relays (with intentionally low capacitance) might work. But then you'd introduce something large, unpredictable into your measurement, which you would have to calibrate out.

Then again, things might be benign: the sensing frequency seems to be chosen intentionally low, so that the internal muxes work well. You'll honestly probably have to try

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  • \$\begingroup\$ When you say, "have to try", do you mean trying to connect ground to the drain pin on an analog semiconductor-based multiplexer (ADG1206, for example)? Is there any chance of damaging the device? \$\endgroup\$ Feb 12 at 18:50

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