# What does “50 ohm system” mean?

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 ?

• It's referring to trace impedance. SPI communication lines would only be 50Ω impedance if you designed them to be 50Ω impedance. – Samuel Jun 6 '14 at 0:52
• @Samuel Is it good practice to make SPI communication lines 50ohm ? I guess this leads to a follow up question – efox29 Jun 6 '14 at 0:55
• 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. – Samuel Jun 6 '14 at 1:01
• 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. – Connor Wolf Jun 6 '14 at 1:46
• @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 ? – efox29 Jun 6 '14 at 2:42