I'm looking to use the following switch to switch an SPI bus ( CLK <= 2Mhz) between two different devices.

I'm reading the datasheet to see if the the frequency of the signals are going to be ok, and I'm not sure I quite understand the High Frequency Performance section (Pg 9 of the datasheet)

In 50Ω systems, signal response is reasonably flat up to 50MHz (see Typical Operating Characteristics). Above 20MHz, the on-response has several minor peaks that are highly layout dependent. The problem is not turning the switch on, but turning it off. The off-state switch acts like a capacitor and passes higher frequencies with less attenuation. At 10MHz, off-isolation is about -50dB in 50Ω systems, becoming worse (approximately 20dB per decade) as frequency increases. Higher circuit impedances also degrade off-isolation. Adjacent channel attenuation is about 3dB above that of a bare IC socket and is entirely due to capacitive coupling.

What would be considered to be a 50ohm system ? Is this referencing to the trace impedance ? Would SPI communication lines be considered 50ohm ?

  • 2
    \$\begingroup\$ It's referring to trace impedance. SPI communication lines would only be 50Ω impedance if you designed them to be 50Ω impedance. \$\endgroup\$ – Samuel Jun 6 '14 at 0:52
  • \$\begingroup\$ @Samuel Is it good practice to make SPI communication lines 50ohm ? I guess this leads to a follow up question \$\endgroup\$ – efox29 Jun 6 '14 at 0:55
  • \$\begingroup\$ It entirely depends on the frequency you want to operate your SPI bus at. For 2MHz I would not bother. If you want to figure it out yourself in the future, read up on transmission lines and electrical length. \$\endgroup\$ – Samuel Jun 6 '14 at 1:01
  • \$\begingroup\$ Well, it somewhat depends. How long is your 2 Mhz bus? If it's longer then 6-12 inches, you should start at least considering signal integrity issues. \$\endgroup\$ – Connor Wolf Jun 6 '14 at 1:46
  • \$\begingroup\$ @ConnorWolf it will less than that. I suspect at most 2 inches. Given that the datasheet talks about 50Ω systems, would I suffer signal integrity issues without it ? \$\endgroup\$ – efox29 Jun 6 '14 at 2:42

What this language is telling you is that these devices can be used in distributed impedance systems. There are two rough areas of analysis, lumped and distributed. Distributed system use the wave equation to describe them, are typically built using waveguides, coax and strip lines etc. and operate with characteristic impedances. Lumped systems are modelled as collections of R,C and L's. There is also a division between high frequency and low frequency ( and those cut offs have different meanings to different people) but around 10's of MHz is normal for the lumped vs. distributed model. Low speed being the domain of lumped ...

Basically those parts can be used to switch high speed signals using coax etc. that means you're safe to use them at your slower speeds. You might be paying for performance you don't use but it will work nevertheless. At the slower speeds you won't need to worry about 50 Ohm termination and reflections etc.


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