# Why don't analog multiplexers specify PSRR?

I am designing a PCB that needs to switch an analog signal coming from multiple sensors with a multiplexer. I need to be able read single uVs of changes on the signal. I am looking to power the MUXs with a switching converter and am worried that the switching noise will couple through the MUX VDD into the analog signal.

The multiplexer I am looking at is the MUX36D08IDWR.

Why does this datasheet not specify a PSRR? Am I missing something?

I don't think you could give an accurate PSRR without defining external circuitry, because the impedances of the circuit on both sides of the switch determine the effective rate. Instead, you get the capacitance of each pin. Figure 39 from the datasheet gives the equivalent circuit for each switch position, and the important detail is that $$\C_{GDN} \approx C_{GDP} \$$ to minimize current injection from switching the MUX. The digital control signals are effectively connected to either VDD or VSS.

This capacitance is reported in the datasheet as CS and CD. These values could easily be converted from the datasheet as $$\C_G = C_{GDN} + C_{GDP} \$$.

An equivalent circuit shows that the amount of signal injection from the power supply depends on what your circuit impedances look like:

simulate this circuit – Schematic created using CircuitLab

If your supply is really bad (large magnitude of ripple), you may also get modulation of the switch resistance. If your signal path is sensitive to changes in switch resistance on the order of 10-100Ω, that's probably another source of concern.

This is a passive switch. In a transmission gate, the supply voltage does not have much of an effect on the signal.

• Thus it would be more like crosstalk with the supply voltage than PSRR. Aug 24, 2023 at 14:40

Why don't analog multiplexers specify PSRR?

Mainly because people buy them anyway :)

Analog switches have very much finite PSRR, but the value depends on the impedances on both sides of the switch. It'd be rather expensive to characterize this across frequency, input impedance, and output impedance. And yes, besides the obvious frequency, different input and output impedances affect PSRR differently depending on the switch implementation.

If you care about it, you have to characterize it yourself. And you'll quickly find that it takes some real effort - even in the particular application you have in mind, rather than generally.