# Effect of AC output impedance on a ratiometric measurement

I was planning to make a measurement with a ratiometric IC, so I will sense both the Vcc and the Vout from the chip as shown below. For cleaning up the noise and stability I also want to add a 5V voltage regulator right before the IC. The IC is basically measuring differential pressure.

I have heard from someone that I might need to verify whether the voltage regulator has a low AC output impedance. What could be reason for such inquiry for this application and why would it matter? What is meant by AC output impedance?

• If your sensors have much higher "impedance" than the voltage regulator' output "impedance", you shouldn't worry much about voltage divider effect. – Long Pham Sep 14 '18 at 9:35
• I have heard from someone that... Next time, ask WHY. There are plenty of people in the world willing to give their opinion. If you ask "why?" then you will learn to separate the well funded opinions from the less well funded ones. The rogue/unfunded opinion that the output impedance of a regulator matters sends on a quest to find a low AC output impedance regulator. So go back to the person who stated that you need the low AC impedance and let him/her prove to you why it matters. My guess is that this person cannot do so. – Bimpelrekkie Sep 14 '18 at 13:51
• For cleaning up the noise and stability That means you need decoupling capacitors. It is always good practice to add these at input and output of regulators. That will give you a low AC impedance. The AC output impedance of the regulator itself then does not matter anymore. See Andy's answer in your other question. – Bimpelrekkie Sep 14 '18 at 13:53
• @Bimpelrekkie It doesnt matter because I measure both Vcc and Vout? – user1234 Sep 14 '18 at 14:12
• No, as Andy explained: the output impedance of a regulator is largely irrelevant because the decoupling capacitors determine the impedance. Any noise coupling to the long lines will be sorted to ground by those capacitors. Even if you used a voltage regulator with a very high output impedance the capacitor would still provide a low AC impedance. So: the regulator's impedance is irrelevant. Regulators need decoupling capacitors. The decoupling capacitors also create a low AC impedance. – Bimpelrekkie Sep 14 '18 at 14:22