Altium and Decoupling Capacitors

[mcckevin]

Hi --

Relative Altium newbie here. I'm designing a PCB with an IC that is finicky about its decoupling capacitors (the lmx2594 frequency synthesizer IC). One pin wants 1uF, another pin wants 10uF in parallel with a 0.1uF, etc. Of course, these caps have to be physically close to the IC to do any good.

When I transfer the schematic to the PCB, all the components are placed off to the side of the PCB with no indication of which decoupling cap belongs to which IC or to which pin. The decoupling capacitors just show up between Vcc and GND, which is topologically true but not what I want.

In the schematic, is there a recommended way of specifying which decoupling caps should be close to which pin, such that the data will be communicated to the PCB layout? Currently, I put copious notes on the schematic but was looking for a better way. Couldn't find anything on this topic on the web. Thanks.

Kevin

[ajb]

You could use net ties.  These are special components that allow two nets to be electrically connected at a single point.  So in the schematic you'd connect, say, pin Vdd1 and it's associated caps to an isolated net, and then use a net tie component to connect that net to your main Vdd net.  The downside is that net tie components have to be explicitly placed on the PCB.  Alternatively you could just use 0-ohm resistors, which have the benefit of being removable if you want to bodge in a filter or measure current or something on the test board.

Personally, though, for situations like this I explicitly show which specific caps are connected to which pins on the schematic, then use Cross Select mode to grab the parts associated with each pin on the PCB by selecting them in the schematic, and simply move them to the appropriate location next to the IC.  (And if you aren't already using it, Cross Select plus the Tools->Component Placement ->Arrange commands are really nice for quickly organizing parts on a PCB to reflect their logical grouping on the schematic sheet.  Rooms can also be useful for organizing parts based on multisheet topologies, but they tend to be more annoying than helpful on small projects.)

[mengfei]

I just use the Cross Select Mode option in the SCH to group components that belong to each other which works vice versa.

the only problem would be is that if in the SCH those coupling cap's are drawn separate from the part it's suppose to go & that there are a couple of them in which you could just choose manually OR place them directly to the IC pin for Shift+Ctrl+X to work.