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I'm designing a part of a system that will receive 4 analog signals carried on 4 twisted pair cables which are input to an ADC. These signals are low speed (probably in KHz) bipolar ranging from -10V to +10V that should be sampled every 30us. The maximum allowed error is 15mV.

What are the routing consideration of these signals on my PCB? Should they be treated as differential pairs?

Any links with detailed technical info are much appreciated.

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    \$\begingroup\$ If there was a good reason to use twisted pair for the cable, its probably a good idea to route the tracks as close as possible to each other and reasonably matched length. You probably don't need to route with actual coupled transmission lines. But you haven't said what frequencies are involved, what signal levels, acceptable measurement errors, and what interference sources you're worried about, so there's nothing we can say with certainty. \$\endgroup\$ – The Photon Jul 27 '12 at 17:50
  • \$\begingroup\$ @ThePhoton , these signals are low speed (possibly in KHz) bipolar ranging from -10V to +10V that should be sampled every 30us. The maximum allowed error is 15mV. These are the specs I know. \$\endgroup\$ – Abdella Jul 27 '12 at 17:56
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    \$\begingroup\$ Those are fairly relaxed specs in terms of speed and susceptability to interference. Even so, it won't hurt and shouldn't be difficult to route the pairs close together. Ideally also maintain a consistent ground reference for the length of the tracks. If you can't maintain ground reference or have to cross a ground cut, it will be more important to route the pairs together. \$\endgroup\$ – The Photon Jul 27 '12 at 18:05
  • \$\begingroup\$ @ThePhoton , should I make guard tracks between and around them, or it's better that they be close to each other with no guards? Also how close should they be? \$\endgroup\$ – Abdella Jul 27 '12 at 18:08
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    \$\begingroup\$ Definitely not guard tracks between the signals --- keep them as close together as you can (but don't agonize about it -- your specs are fairly relaxed). Guard tracks to either side will probably not make much difference either way. Do avoid placing any strong fast signals (say a digital clock, or a switching regulator high-current trace) near your analog signals. \$\endgroup\$ – The Photon Jul 27 '12 at 20:36
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At this low frequency it doesn't really matter whether they are routed as a transmission line on the PCB or not. The more important issue is capacitive coupling from other traces that could inject noise. You are only looking for a little over 10 bit accuracy, so nothing out of the ordinary needs to be done.

Some low pass filtering would be a good idea. You are sampling at 33 kHz, so clearly you can squash anything past 15 kHz, in fact that is a good idea. Hopefully you are super-sampling in the A/D and will then decimate and low pass filter later in firmware. This allows your anti-aliasing filter to be more easily realizable in analog. Let's say you've done that part right and really only care about maybe 1 or 2 kHz upper frequency. In that case, put two poles of passive low pass filtering at 5 kHz on each signal in differential mode. You need to put a little low pass filtering on each part of the signal individually because high enough common mode noise can fool active front ends and look like differential mode signal. The downside is that any imballance in the two filters on each line will convert some common mode noise into differential signal, so make these filters high enough, like a few 10s of kHz.

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  • \$\begingroup\$ Thanks Olin. Should I make guard tracks between and around each pair?, or it's better that they be close to each other with no guards? Also how close should they be? Also do you mean that I need two low pass filters for each pair? \$\endgroup\$ – Abdella Jul 27 '12 at 18:58
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Double sided PTH PCB having proper ground layer below the input tracks and having ground shield tracks- guard tracks & small caps at the input side will be helpful for proper operation.

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