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I'm designing a PCB using this component http://www.ti.com/lit/ds/symlink/afe4403.pdf

And from the datasheet page 90 layout guidelines I found point(2) which says

If the INP, INN lines are required to be routed over a long trace, TI recommends that VCM be used as a shield for the INP, INN lines.

And I got a reference circuit in which they had this

enter image description here

So normally when guard trace is required I put a GND trace with VIAs and double spacing on either side.

My question is, having bypass cap is always a good practice or only useful if the circuit demands?. How to determine a long trace,any rules or standards?. How to route such trace in PCB, do I've to put the cap close to source or destination end?.

Thanks

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Without knowing what the frequency content of the Vcm line or knowing what is driving the Vcm line, the reasons are speculatory as to why you would need the 10nF cap on the Vcm line.

Voltage references need bypass capacitors for stability and to limit the bandwidth of the feedback loop. When you extend the trace across the board you are adding inductance, which is going to change the load of the reference, to stabilize this you add a cap. Which is probably the function of that cap.

Since it is not known what the designers intended we can only infer as to why. If the datasheet suggests it its a good idea to follow the engineers suggestions because they design, build and test the devices.

To answer the other question: the length of a trace will add parasitics. You can use a PCB trace calculator to find the inductance, resistance, capacitance and transmission line effects. Normally these effects are in the range of nH's, 100's of micro ohms and pF's so they will matter only to higher frequencies (+50Mhz - kind of a nebulous number). But if your amplifier has bandwidth in this range then you need to worry about it.

schematic

simulate this circuit – Schematic created using CircuitLab

Run the guard trace all the way around the wires to the sensor. If you using a cable with a shield (and you should) then tie Vcm to the shield. The principle here is that Vcm provides the same voltage potential as the inputs and minimizes leakage and EMI.

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  • \$\begingroup\$ Thanks for the answer. I'll check with frequency content and find out the necessity. Coming to how you would do the PCB routing with resistor and cap, could you please explain little more. Consider the image in the link bertsimonovich.files.wordpress.com/2013/04/clip_image002.gif and let me know at which point you would connect resistor and capacitor. AFAIK the res and cap should be close to source pin (VCM). \$\endgroup\$ – Dee Feb 16 '17 at 19:35
  • \$\begingroup\$ Actually scratch that last thought on the resistor, I've included a diagram \$\endgroup\$ – Voltage Spike Feb 16 '17 at 19:45
  • \$\begingroup\$ Great! Thanks and this makes sense. But one last doubt, in your diagram you've used the guard trace covering both the INN and INP separately with a junction. Actually that can be done if there is enough space to route but in case there isn't then is it good to go with single guard trace in between like in the image I shared?. \$\endgroup\$ – Dee Feb 16 '17 at 19:53
  • \$\begingroup\$ Yes, you can put INN and INP together or just one trace between or put them in a plane of Vcm \$\endgroup\$ – Voltage Spike Feb 16 '17 at 20:04
  • \$\begingroup\$ Thanks. I've just tried in Altium to see how it works. Check this pic link and let me know if its acceptable. yadi.sk/i/mZr1lW3Z3EBG35 \$\endgroup\$ – Dee Feb 16 '17 at 21:05

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