Capacitor / filtering for PSU

[Rajinder]

Hi,
I am designing a 19' inch rack system. A Switch mode PSU offering 5V, +/-12V has been selected. I am using the 5V supply to generate a 3.3V supply for a micro and DAC. The 5V will also supply the +ve supply for a 0P1177 opamp. What filtering would I need for the 5V output from the PSU. Would a 100uF low ESR cap + 100nF ceramic do. I was also thinking of a PI filter too.
Thanks in advance.
Best regards,
Rajinder

 

[Arouse1973]

Hi Rajinder
Please supply a circuit diagram including all the parts you intend to use. This will help us in deciding what you need.
Thanks
Adam

 

[CapSteel]

I normally go with a 10uF electrolytic + 100nF ceramic. One 100nF as close as possible to every IC supply pin. The 10uF cap can often be shared among multiple ICs if the ICs are close enough (say <10cm). Not sure if there's anything special needed for switch mode power supplies though.

 

[AnalogKid]

It is common to see 10 to 100 ohm resistors in series with the two power supply connections, with the decoupling caps on the chip side. 100 ohms seems large to me, but something in the 10 to 47 ohm range should help kill the high frequency spike noise.

ak

 

[Rajinder]

Thanks for your replies. I thought that a PI filter may be used with a 100uF cap. The PSU is a switched mode offering +5V, +/-12V. It will be used to power a couple of opamp (OP1177), LP3985 LDO to generate 3V3. All these will have decoupling caps 100nF
I am not sure about what I need from the PSU +5V. An electrolytic, tantalum low ESR etc? I am not sure. Also the PSU has sense lines, I presume these go across the load? Please could you advise.

 

[Alec_t]

Also the PSU has sense lines, I presume these go across the load? Please could you advise.

Doesn't the datasheet show how they're used?

 

[Rajinder]

No, the PSU data sheet doesn't mention anything about bulk capacitor decoupling etc