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I have an application, where I want to detect proximity of conductive materials (including humans). The idea is that I have a series of conductive sense strips and I measure capacitance between each pair consecutively.

I found an AD7147 CDC datasheet and tested it out in differential mode, seems to work just fine. What troubles me is distance between the sense strip and the IC, which can reach 2m. Obviously cabling should be shielded, IC even provides separate shielding ground pad. Since I'm not interested in absolute value but only dynamic change in measurement, any coaxial cable with fixed capacitance-per-length should do, right? As long as length remains fixed and cable itself is fixed to avoid triboelectric effects.

Are my assumptions correct or am I missing anything? Are there any known issues (possibly from analog accelerometer world) with such a setup?

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  • \$\begingroup\$ Page 33 suggests the chip can be 10cm away from the sensors, so 2m might be tricky. Are you talking about several strips, each with it's own coax? That might go over the 150pF to ground permitted on the ACshield pin. \$\endgroup\$ – Jack B Oct 3 '19 at 15:37
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Since I'm not interested in absolute value but only dynamic change in measurement, any coaxial cable with fixed capacitance-per-length should do, right?

Well, to a point. If the dynamic change becomes too small a fraction of the absolute capacitance, your signal will get lost in the noise. You really do care about all of the capacitance connected to the device pin, which can only measure the total capacitance that it sees.

There are ways to measure capacitance (or in general, impedance) at the end of a transmission line, but they require more sophisticated circuitry such as TDR or the equivalent frequency-domain algorithms.

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  • \$\begingroup\$ This chip has a driven shield, in which case the cable capacitance will not (directly) contribute to the measurement, will it? \$\endgroup\$ – Jack B Oct 3 '19 at 15:39
  • \$\begingroup\$ True, as long as you don't exceed the chip's ability to drive the shield -- i.e., the shield capacitance to ground. \$\endgroup\$ – Dave Tweed Oct 3 '19 at 16:41

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