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I'm building an EEG circuit and as such there are approximately 128 channel leads. At the moment I'm just running standard wires but to improve space and neatness I'm going to print these on a flex PCB which will terminate in a board-to-board connector. When looking at the pitch sizes I'm tempted to use the 0.35 mm, however I am concerned due to the lower space between traces this will lead to a decreased signal-to-noise ratio and convolution.

Would it be less risk to go with a higher pitch i.e. a 0.5 mm or even a 1 mm or should a 0.35 mm be fine?

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  • \$\begingroup\$ What is the foundation of your concern? \$\endgroup\$ – Andy aka Oct 6 '16 at 12:19
  • \$\begingroup\$ Thanks for replying Andy, I'm not quite sure how to answer your question but primarily I'm concerned that the individual signals may become corrupt and unusable if the pitch size ie .35mm is too close together. That being said, smaller compact size for this particular circuit design is important so I want to go as small as possible without decreasing the SNR. \$\endgroup\$ – gerardshield Oct 6 '16 at 20:52
  • \$\begingroup\$ Isn't cross talk a concern to you? \$\endgroup\$ – Andy aka Oct 6 '16 at 21:08
  • \$\begingroup\$ Yes this is what I'm concerned about by choosing a .35mm pitch - an increase in crosstalk would lead to the decrease in SNR. I am hoping this won't be an issue though because I'd like to use as small a connector as possible to save space. \$\endgroup\$ – gerardshield Oct 6 '16 at 23:24
  • \$\begingroup\$ There are web sites with capacitance calculators. This value is based on dimensions. So, you get a figure and cross talk will rise with frequency due to the capacitance but the other half if the story is amplifier input impedance. Basically the lower the input impedance the smaller the effects due to capacitance so maybe do some checks on likely values and report back. \$\endgroup\$ – Andy aka Oct 7 '16 at 0:07
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From comment:

There are web sites with capacitance calculators. This value is based on dimensions. So, you get a figure and cross talk will rise with frequency due to the capacitance but the other half if the story is amplifier input impedance. Basically the lower the input impedance the smaller the effects due to capacitance so maybe do some checks on likely values and report back. – Andy aka Oct 7 at 0:07

Paraphrased:

  • Use online calculator to calculate the inter-lead capacitance
  • Calculate inter-lead impedance at a frquency of interest
  • Find out the input impedance of your amplifier
  • You can now determine amount of capacitive crosstalk using simple resistor divider formula.

My note: the standard way to avoid crosstalk is to ground every second pin ground. This is what's done in high-speed single-ended parallel interfaces such as PATA 80.

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