Hi MUX Experts,
When looking at the TMUX12xx family, I see that the bandwidth differs from part to part and seems to be lower for higher channel parts. TMUX1208 = 65MHz, TMUX1247 = 250MHz.
Could you please explain the bandwidth differences of these devices? Is it number of switch dependent?
Thanks,
Reed
Hi Reed,
So the switch bandwidth is greatly dependent on Load and On Capacitance, and to a smaller extent Ron and R_Load, when we measured bandwidth for these parts we used the same capacitive load (50 Ohms and 5 pF in Parallel ) . This leaves the main difference in on Capacitance and On Resistance (shown Below)
For the TMUX1208 we have an On-Capacitance of 85pF - On Resistance ~ 5 Ohms
For the TMUX1247 we have an On-Capacitance of 27pF. - On Resistance ~ 3 Ohms
Load Capacitance is greatly impacted by how many switches are in one "Mux", we label this as configuration for our specs (2:1 Muxes will have the Lower Con while 8:1 Muxes will have higher Con typically). You can see this in the TMUX1208/9 Datasheet as shown below:
The TMUX1209 has about 1/2 of the ON Capacitance as the TMUX1208 because it is a 4:1 Configuration.
The amount of Muxes inside one package does not affect Con however, as the TMUX1209 has two 4:1 Muxes inside the Package and has a lower Con than the TMUX1208 which has 1 Mux inside the package but it is 8:1. We refer to the number of Muxes inside of an IC as the Channel Count.
The On Resistance and On Capacitance effectively create a low pass filter where the corner frequency is apprx. 1/(2 * Pi * Ron * Con) . The Larger Ron and Con are the lower the corner frequency is going to be.
We typically spec our bandwidth pretty conservatively though because the load capacitance and layout can degrade bandwidth performance pretty significantly.
For a more information on bandwidth in Muxes please see our Precision Lab Video on the Topic.
Best Regards,
Parker Dodson
