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Old 12th December 2012, 10:26 PM   #31
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Thanks
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Old 12th December 2012, 10:28 PM   #32
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Here is the second stage of my phono RIAA where you can see the principle. Ignore the name of the Opamp and the inbuilt EQ in the feedback loop.
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Old 12th December 2012, 10:30 PM   #33
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R21 is wrong but that does not matter in this discussion.
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Old 12th December 2012, 10:31 PM   #34
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Actually it says OPA1641 in blue.
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Old 12th December 2012, 10:39 PM   #35
jcx is online now jcx  United States
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Quote:
Originally Posted by smms73 View Post
jcx , to compensate the ada4637 that is compensate to gain >5 , I think that is need a 10 ohms resistence in series with c1 ,or at high frequencies c1 will bring gain below 5 . Or am I missing something ?

R3 and C3 forms a snubber, and snubbers are very good in limiting (dV/dt).

I think adding r3 and c3 is a good idea and can be use with compensated to unity gain opamp.
nominally I chose the "noise gain" as ~ C3/C1+1

I included R3, trimmed its value to show less ringing on the Vin - the exact value depends on what the op amp Zout looks like at the gain intercept frequency - which may not be accurately modeled

but you could look at R3 as being in the noise gain ratio with the op amp Zout


you really shouldn't try this noise gain campensation with such a fast chip (or the much faster AD8067) without a 100+ MHz 'scope to see if there are problems, adjust the values from the sim

Last edited by jcx; 12th December 2012 at 10:49 PM.
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Old 12th December 2012, 10:56 PM   #36
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Originally Posted by Joachim Gerhard View Post
Here is the second stage of my phono RIAA where you can see the principle. Ignore the name of the Opamp and the inbuilt EQ in the feedback loop.
Wow, thanks!
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Old 12th December 2012, 11:17 PM   #37
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My pleasure. If you try that please tell me how you like the result.
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Old 12th December 2012, 11:46 PM   #38
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Originally Posted by EUVL View Post
Not in the CEN IV circuit.

The added bonus is that Zin of the IV now equals Zout of the opamp.
And since the open loop gain is >120dB, and open loop output impedance is <1k,
you can argue that the OPA1641 Zout in follower mode would be <0.001 ohm.

And no, I don't have measurements to prove.
But yes, we have a PCB and will try it out some time.




Patrick
Don't get too excited about this circuit. It's actually worse than a "normal" IV converter using opamps, either in terms of noise or distortion. I believe what you would want if you do this is the lowest possible output impedance for the opamp which is why I added the buffer.
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Old 13th December 2012, 12:08 AM   #39
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Quote:
Originally Posted by jcx View Post
nominally I chose the "noise gain" as ~ C3/C1+1

I included R3, trimmed its value to show less ringing on the Vin - the exact value depends on what the op amp Zout looks like at the gain intercept frequency - which may not be accurately modeled
c3/c1+1 is right .
but if you add R3 in series with C3, you should also add a resistor in series with C1 in this case a value 5 times higher than R3.

I think that there is no problem using lead-lag compensation in faster opamps.

Adding R3 as I said before is a good idea.
Thanks for sharing
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Last edited by smms73; 13th December 2012 at 12:16 AM.
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Old 13th December 2012, 12:09 AM   #40
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What is this Dirk ? Positive input grounded and all currents lost in a loop.
I can not see how this animal can amplify but it will eat some current.
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