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I'm fairly new to PCB design, so maybe I start with a basic 0 ohm resistor like this:

enter image description here

I have both output pins (SLEEP & RESET) on the DAC chip. I want to have two options on my PCB, one with 3-pin header connector (THT from SAMTEC here the TSW) to tie 1&2 or 2&3 together, tying the pin to 1.8V or GND, respectively. Is this the right way to do it? Now to routing: I'm thinking of having it something like both resistors sharing a pad, so people only solder one of the resistors (no short on FPGA I/Os):

enter image description here

Is this correct?

I think that this isn't optimal, since I want to have the whole SPI connection from external source or FPGA and don't want to solder & desolder four 0 ohm resistors each time I want to change between both options. I also want to feed 10 LVDS pairs (20 wires) to 10 DACs from the FPGA, but additionally I want to use the LMK00308 as a clock distribution and try both options on the PCB. How can I best have something like the mentioned TSW 2-pin header to switch between both those options? Is a multiplexer a good idea? I definitely can't afford any clock phase delays to my reference clock to the DACs.

A solder bridge might be easier than soldering. I also don't want to put the 0 ohm resistors on the LVDS wires, since these have matched length and 50 ohm impedance:

enter image description here

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    \$\begingroup\$ Solder bridges are actually kind of finicky; it can be hard to get the solder to actually bridge when you want it to. I prefer to use zero-ohm links personally. \$\endgroup\$
    – Hearth
    Nov 26, 2023 at 19:25
  • \$\begingroup\$ For volume manufacturing, I would avoid overlapping the resistor footprints. The additional unmasked area, will cause non-symmetrical forces on the resistor from surface tension in the liquid solder. This can cause misalignment of the resistor or tombstoning. If you intend to only ever hand solder, then you should be OK. Your contract manufacturer can advise further. \$\endgroup\$
    – elchambro
    Nov 26, 2023 at 23:03

2 Answers 2

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Those of us old enough to remember configuring motherboards know that if there are a lot of these to do, there are really two options: switches (available in SPST and SPDT, at the very least) and 0.1" jumpers.

The switches were available in DIP packages with different numbers of switches on them -- 4 and 8's were common. Finding them in SMT packages is a bit rougher, but they exist (oddly, DIP packages seem to still be a dime a dozen, but that means your board would need an extra assembly step). For example, https://www.digikey.com/en/products/detail/cts-electrocomponents/218-8LPSTR/5223876

SMT 8 position switch from Digikey

0.1" jumpers are a dime a dozen (Example: https://www.digikey.com/en/products/detail/sullins-connector-solutions/QPC02SXGN-RC/2618262)

Digikey jmper

The jumpers were used with single or double row 0.1" male headers. You use them to complete circuits. It was a breeze w/ thruhole parts, but getting the geometry correct if you want only SMT might be trickier.

Both options are obviously more expensive than a pick and place only solution involving 0 ohm resistors and/or solder jumpers, but both are probably more idiot proof and less susceptible to board damage from repeated rework every time you need to change something. Also, probably easier to understand what's going on by looking at the board than the other solutions, as all the labels would be in one place.

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  • \$\begingroup\$ Thank you for the suggestion. I might be missing something, but how is this suitable for my application? It's something like a three line connection that I have (1. DAC, 2. FPGA, 3. Clock module) and the first mechanical connector you mentioned would only cut the connection one way. Do you mean I should use two of them once between DAC-FPGA and once DAC-Clock? But then How do I connect both wires to the DAC? This might also add some phase delay, so not favourable. The 0.1 jumper is the one I'm using with the TSW I mentioned, thank you. P.S. It's only a prototype and price isn't a big issue \$\endgroup\$ Nov 27, 2023 at 16:41
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    \$\begingroup\$ @johnny_1010 you're asking about using zero ohm resistors (aka wires) to dynamically reroute signals. Switches do the same thing, are more convenient for field reconfiguration, and don't require rework skills. \$\endgroup\$ Nov 27, 2023 at 17:04
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It may not be appropriate in your particular case, but you can get x4 (and other multiples, but x4 is more common) zero-ohm networks. Image from lcsc shows a part measuring 1 x 2mm overall.

enter image description here

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