[sdiy] Do op amps need bypass caps?

[Tim Ressel]

I have to disagree. If your running an opamp capable of higher output currents, say 600 ohm drive capability, you may find those 47R resistors causing issues.

If you look at the PSRR curves (power supply rejection ratio) of my favorite amp, the LME49723, you'll see it is quite nice at low frequencies but drops to about -55dB at 10 KHz. This means high frequency crap on the power supply could leak into your nice clean audio path. Careful layout and bypass caps in the 10n-100n range are needed here. For more filtering consider using inductors and caps in a PI configuration to keep out the nasties.



Tim Ressel
Circuit Abbey
503-750-9331
timr at circuitabbey.com




________________________________
From: Roman Sowa <modular at go2.pl>
To: synth-diy at dropmix.xs4all.nl 
Sent: Tuesday, August 19, 2014 1:24 AM
Subject: Re: [sdiy] Do op amps need bypass caps?


0.1uF alone is tad too small for audio. Good practice is 47 ohms feeding
47uF cap optionally together with 0.1uF one. That gives corner frequency 
below 100Hz, which may be considered as DC quiet power line.

If you don't bypass, and 0.1u is like you don't, all those transient 
signals, caused by varying current of opamp's load, are travelling all 
the way via power and ground traces, and are picked up by other parts of 
the circuit, which sometimes makes nice oscillator out of an amplifier.

Roman

W dniu 2014-08-19 08:06, Chris McDowell pisze:
> Hi guys,
>
> I was taught to put 0.1uF-ish bypass caps on every IC, every time,
> but recently have come across statements declaring that op amps in
> the audio range and below don’t need them. Is this true? Is this
> controversial? What’s the real scoop here?
>
> Thanks, Chris _______________________________________________
> Synth-diy mailing list Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy



>
_______________________________________________
Synth-diy mailing list
Synth-diy at dropmix.xs4all.nl
http://dropmix.xs4all.nl/mailman/listinfo/synth-diy