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Old 20th July 2013, 05:59 PM   #61
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hello,
try an Lt1468.
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Old 20th July 2013, 06:07 PM   #62
davada is online now davada  Canada
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Originally Posted by dadod View Post
With 5 pF it still have small oscilation, with 7 pF is table and distortion at 20kHz drops fro 0.037325% to 0.003628% and that is 20 dB still to little.
Hi dadod. The FB cap around the op amp is for balancing the input and stray capacitance.
This is typical with high GBP op amps, Try an LT1468. They're a bit higher in noise but the distortion is alarming low. If you use very low R for input and FB about the op amp you might get away with no FB cap. The 1468 is actually a 25MHz GBP part at gains below 30. It decompensates itself at gain above 35 to get the 90MHz GBP.

This quite neat what your doing here. Best of luck,

Cheers,
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Last edited by davada; 20th July 2013 at 06:14 PM.
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Old 20th July 2013, 06:08 PM   #63
davada is online now davada  Canada
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hello,
try an Lt1468.
I see you beat me to the recommendation. But of course.

Cheers,
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Old 20th July 2013, 06:19 PM   #64
davada is online now davada  Canada
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Thanks Manso, yes I understand important of opamp and I tried to use one from TI (the same as in the papers) but as mcd00uk said could not import yet to the LTspice.
BR Damir
The TI models don't run very well in LTspice. But if you're having trouble send me the TI model and I'll see if I can get it going for you.
A well behaved TI op amp is the OPA1611. Haven't had any trouble from this in the real world.

Cheers,
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Old 20th July 2013, 09:46 PM   #65
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Originally Posted by davada View Post
I see you beat me to the recommendation. But of course.

Cheers,
Thanks davada, here is zip file from TI for LME49710.
Damir
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File Type: zip snam001 (1).zip (38.4 KB, 24 views)
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Old 20th July 2013, 10:14 PM   #66
dadod is offline dadod  Croatia
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With LT1468 THD20k lowered by 26dB(at 8vpp output) and 10pF should be connected between -in and out to tame the opamp, but in this case clipping looks terible. To get better clipping behavior this cap should be increased to 47pF minimum(clipping still not satisfactory) but THD was lowered by 16dB only.
Damir
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Old 20th July 2013, 10:28 PM   #67
davada is online now davada  Canada
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Originally Posted by dadod View Post
With LT1468 THD20k lowered by 26dB(at 8vpp output) and 10pF should be connected between -in and out to tame the opamp, but in this case clipping looks terible. To get better clipping behavior this cap should be increased to 47pF minimum(clipping still not satisfactory) but THD was lowered by 16dB only.
Damir
Hi dadod,

Rather than increasing the fb C. Try putting a resistor at the output of the op amp.
47 - 100 ohm. This isolates external capacitance. The LT1468 is lightning fast settling and an instantaneous short, as capacitors, are cause the op amp to be continuously settling. Also you can try a zobel network at the op amp output. usually 1nF in series with a 100 ohm resistor is enough. You can experiment with the values.

Additionally, you can slow down the differentiation effect with a LP input filter. This should help to clean up the clipping behavior. Or it may be a case of having to add some lead compensation to correct for phase shift since your feed forwarding input signal. Also if the latter is the case you LP filter the feed forward signal to match the phase. I think what you have here is two signals arriving slightly out of phase which canl cause a bit of differentiation.

Can you post a plot of the effect so we can see it.

Cheers,
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Last edited by davada; 20th July 2013 at 10:39 PM.
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Old 20th July 2013, 10:35 PM   #68
dadod is offline dadod  Croatia
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Originally Posted by davada View Post
Hi dadod,

Rather than increasing the fb C. Try putting a resistor at the output of the op amp.
47 - 100 ohm. This isolates external capacitance. The LT1468 is lightning fast settling and an instantaneous short, as capacitors, are cause the op amp to be continuously settling. Also you can try a zobel network at the op amp output. usually 1nF in series with a 100 ohm resistor is enough. You can experiment with the values.

Cheers,
I don't think it will help, there is already 10k (P1) at the output, and zobel does not help either.
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Old 20th July 2013, 10:40 PM   #69
davada is online now davada  Canada
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I don't think it will help, there is already 10k (P1) at the output, and zobel does not help either.

Off course. I should have looked first.

Cheers,
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Old 20th July 2013, 10:50 PM   #70
davada is online now davada  Canada
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Default Sim files

Hi dadod,

I thought you were simulating with LTspice, These look more like pspice files.
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