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I have a PCB with two power rails, 5V and 12V, and two ground pours, GND-5V and GND-12V, respectively. The two ground pours are tied at one point (close to the connector). I'm powering a DAC off the 5V rail and buffering its output using an opamp that is also powered off the 5V rail. The output of the buffer is fed to the control pin of a DC-DC converter that is powered off the 12V rail. The datasheet for the DC-DC converter suggests a 0.1uF bypass capacitor for the control pin.

Should the return path of the bypass capacitor (shown as Cb in the image below) be GND-5V, as any spikes would be coming from the 5V rail, OR should it be GND-12V as that's the converter's return path? Note that the both the opamp and the DAC have bypass capacitors as well.enter image description here

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    \$\begingroup\$ Why does the data sheet suggest the capacitor - do they give a clue as to what the problem is - is it a sensitive input that needs a bit of protection or is it an input that can have superimposed on it some nasties by virtue of how it connects internally to the DC/DC thing? That dictates how the capacitor connects. \$\endgroup\$
    – Andy aka
    May 3, 2016 at 14:58
  • \$\begingroup\$ @Andyaka The datasheet simply says that is an AC bypass to insure smooth voltage control levels \$\endgroup\$
    – CuriousCat
    May 3, 2016 at 15:38
  • \$\begingroup\$ if it is to protect the converter then the cap should ref ground at the converter. \$\endgroup\$
    – Andy aka
    May 3, 2016 at 16:16
  • \$\begingroup\$ That was my assumption as well, hence why I have it drawn as such in the image. \$\endgroup\$
    – CuriousCat
    May 3, 2016 at 16:18
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    \$\begingroup\$ Just add a small resistor at the buffer amp and ground the capacitor to the DC-DC converter \$\endgroup\$
    – Sam
    May 4, 2016 at 3:30

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A capacitor placed on a line (to ground) to restrict "bad stuff coming out" is grounded locally but, on the other hand, protecting "bad stuff getting in" means it gets grounded local to what it protects. Sometimes both are needed.

Taking it one small step further means adding a series resistor to the control line to further enhance the protection. In either case you would have the resistor closest to the source of the signal.

Sometimes this just cannot be done (on those occasions when the control line is actively involved in a control loop). The main recourse here is to use a differential amplifier to transfer the good voltage at the noisy ground (the source) to be a good voltage at the clean end (where it needs to be inputted).

And, of course, there are those situations when a control voltage needs to pass from one completely different sub-system to another and there are several ways such as digitizing the signal and transmitting via an opto coupler or an isolation amplifier.

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