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OPA1656: High-Performance CMOS Audio Op Amp
OPA1656: High-Performance CMOS Audio Op Amp
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Old Yesterday, 09:37 PM   #691
Tfive is offline Tfive  Germany
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Quote:
Originally Posted by JRA View Post
Don't think so. The scope is on the 1656's output albeit after a post roll-off R-C filter.
Yep, thinking about it you're right, you're measuring at a low impedance node - at the output of the opamp and not at the input.
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Old Yesterday, 09:53 PM   #692
MarcelvdG is online now MarcelvdG  Netherlands
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Quote:
Originally Posted by DSP_Geek View Post
This has bugged me for a while: since ceramic capacitor ESLs are more or less the same for a given size of capacitor independent of value, why not use microfarad scale bypass caps for electrolytics, for example 2.2 uF ceramic in parallel with 100 uF electrolytic? That way the Q of the created tank would be lower than it would be for smaller capacitor values. The cost difference isn't terribly great: 2.2 uF parts are maybe twice 0.22 uF, and perhaps thrice 0.022 uF, at about $0.75 vs $0.40 vs $0.25. Given electros are more than that anyway it's kind of a no-brainer.
The bigger values are usually class 2 ceramic capacitors, which are terribly non-linear. Whether that matters depends on the application. In this case capacitor non-linearity will cause distortion, but the signal voltage is very small, so it won't be much distortion. Then again, you don't need much to exceed the OPA1656's ultralow distortion levels.
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Old Today, 05:36 AM   #693
DSP_Geek is offline DSP_Geek  Canada
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I was thinking more of power supply caps not far from the opamp pins. Ceramics in the signal path? Apart from NP0, ew.

The point wrt resonant tanks still holds. I took off on a tangent wrt C4 in the schematic, which probably led to some confusion about whether the ceramic bypass was intended for that particular application. Mea culpa.

Speaking of which, I wonder whether the ESL of C4 would be biting into the phase margin since it would increase loop gain at high frequencies. Nichicon MUSE have something like ~100 nH ESL.
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Old Today, 05:43 AM   #694
MarcelvdG is online now MarcelvdG  Netherlands
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That's a lot. The ESL causes a left-half-plane zero in the loop gain, so if it does anything, it should increase the phase margin. Probably not by much, though, as the feedback capacitors are essentially short circuits at the frequency where C4's ESL kicks in.
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Old Today, 07:34 AM   #695
Hans Polak is online now Hans Polak  Netherlands
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In this very case, writing and thinking what effect C4 might have takes many minutes, while trying a shortcut and/or a par cap less then a minute.

Like with the damping, only taking the practical road and trying several combinations will lead to optimal results.

Hans
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Old Today, 12:40 PM   #696
Mark Johnson is offline Mark Johnson  United States
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OPA1656: High-Performance CMOS Audio Op Amp
Couple years ago I built a gizmo that measures electrolytic capacitor ESL. It creates a resonant circuit between the inductance of the big eCap, and an external discrete ceramic C (in the super low inductance 0612 SMD package (reverse geometry)). Resonant frequency was around 10 MHz, well above the SRF of the eCap. The gizmo worked reasonably well, but then I gave myself permission to buy an ET1092A L/C/R/Z meter which worked even better.

link: Measuring the "ESL" inductance of electrolytic caps (at 10 MHz!)

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Last edited by Mark Johnson; Today at 12:56 PM. Reason: moar linkz
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Old Today, 02:57 PM   #697
DSP_Geek is offline DSP_Geek  Canada
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Quote:
Originally Posted by MarcelvdG View Post
That's a lot. The ESL causes a left-half-plane zero in the loop gain, so if it does anything, it should increase the phase margin. Probably not by much, though, as the feedback capacitors are essentially short circuits at the frequency where C4's ESL kicks in.
You are of course correct. That'll teach me to post on no sleep
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