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Old 25th November 2014, 04:43 PM   #1
Hengy is offline Hengy  Canada
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Question Preamp: opamp low pass filter for PS noise

I am in the planning stages of building a completely custom preamp. I was running some simulations in ltspice, using the lt1115 opamp. It oscillated quite badly, so I tried putting a 68pf cap baypassing the feedback resistor. (The opamp was in a negative feedback topology) The cap fixes the oscillation, and created low pass filter, with a -3dB frequency around 30kHz.

This got me thinking: I'm planning on using a switching power supply. Its much cheaper, and I've seen them successfully used (no noise) in many projects on the internet.

So, does the low pass filter that fixed the oscillation also filter out PS noise? Say the switching power supply operated at a frequency of 100kHz, with some ugly 100mv spikes. If this is injected into the signal, will the low pass filter it out?

Im very interested to hear some explaination on this, as I do not have the knowledge to answer it.

Hengy

Last edited by Hengy; 25th November 2014 at 04:46 PM.
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Old 26th November 2014, 04:15 AM   #2
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that cap will do nothing to the supply noise. you will need decoupling caps for that.
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Old 26th November 2014, 05:45 AM   #3
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With an SMPSU, decoupling caps and series inductors. But inductors are only worth the effort if your source material is up to snuff.
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Old 26th November 2014, 10:42 AM   #4
DF96 is offline DF96  England
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Correctly applied negative feedback reduces anything in the output which is not in the input. This includes stuff from the PSU. However, feedback does not work too well at higher frequencies as there is too little loop gain. You therefore need to reduce the PSU noise reaching the circuit, and use NFB as the 'final polish'.
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Old 26th November 2014, 01:43 PM   #5
Hengy is offline Hengy  Canada
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Thanks for the answers!

It was something that piqued my curiosity. I didn't expect it to be quite that easy, as I'm sure there would be a lot more on the subject if it was.

I do understand that decoupling caps as well as power line filters are the most effective way to reduce noise.

Hengy
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Old 28th November 2014, 06:54 PM   #6
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I once built a simple opamp rectifier and low pass, and i have been thinking about this myself. I considered using a power opamp for higher current capability, (opa541, LM12) but in not sure that i could get it to be stable at unity gain, or low (<10dB) gains, so i ditched the idea.
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