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.
Best Answer
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.