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Old 19th February 2014, 03:12 PM   #21
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You are missing the point that Calvin and myself are making. We both understand the circuit, but you are reading differently what we are writing.

Put 13.5k from U2 output to inverting input.
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Last edited by richie00boy; 19th February 2014 at 03:14 PM.
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Old 19th February 2014, 04:20 PM   #22
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Quote:
Originally Posted by richie00boy View Post
You are missing the point that Calvin and myself are making. We both understand the circuit, but you are reading differently what we are writing.

Put 13.5k from U2 output to inverting input.

Hi Richie00boy,

I am sorry if I was misunderstood both of you. So do you actually mean the feedback loop below doesn't work?

Adding 13.5k just can't realize the fully differential output capability. When only SE signal is provided, one output is larger than the other.
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Old 19th February 2014, 04:27 PM   #23
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That is a different circuit to what we have been discussing.
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Old 19th February 2014, 06:32 PM   #24
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same
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Old 19th February 2014, 08:31 PM   #25
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I wish people would learn to draw schematics in some sort of rational conventional way

I realise that the original schema in this thread is how one can fling it together to stuff it into a sim, but my old-fashioned brain finds it too confusing.

I have simplified it to MY tastes and experience. Sorry I have nothing but a pen to hand.

It seems to me that this circuit is very problematic.

There are two feedback paths for U2 - via U1 and U3 in parallel. I do not see how stability can be achieved. Because U2 is open loop (locally), its o/p will .... do what?

I it is not clear to me what is being attempted here!
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Old 19th February 2014, 08:33 PM   #26
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same
No it's not. The circuit we have been discussing has 3 op-amps. That has two.
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Old 20th February 2014, 05:04 AM   #27
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Originally Posted by richie00boy View Post
No it's not. The circuit we have been discussing has 3 op-amps. That has two.
Trace the signal in and the feedback path.


Finally I managed to stop the opamp from ocsillation by adding C3 to compensate the circuit. The SAW wave oscillation is gone completely. Zoomed in the scope to 2mV/div and only can read some "dirty noise" that is less than 1mV.

Both output is giving same output voltage amplitude when SE source/ Balanced source is used. So this is what I wanted in the earlier time, a fully differential design.
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Old 20th February 2014, 05:57 AM   #28
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Quote:
Originally Posted by cliffforrest View Post
I wish people would learn to draw schematics in some sort of rational conventional way

I realise that the original schema in this thread is how one can fling it together to stuff it into a sim, but my old-fashioned brain finds it too confusing.

I have simplified it to MY tastes and experience. Sorry I have nothing but a pen to hand.

It seems to me that this circuit is very problematic.

There are two feedback paths for U2 - via U1 and U3 in parallel. I do not see how stability can be achieved. Because U2 is open loop (locally), its o/p will .... do what?

I it is not clear to me what is being attempted here!

Ya, I am still learning to draw it nicely in the software, but sometimes I am too lazy for that

At very first, this circuit looks problematic on my eyes too, 'dual active unity gain feedback' loop( I don't know how to call it). But after look into the circuit, well it just worth to give it a try, and I managed to make it stable and works now

Performance wise, I have yet to measure it.
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Old 20th February 2014, 08:37 AM   #29
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Quote:
Originally Posted by chchyong89 View Post
Trace the signal in and the feedback path.
Yes, and yet again you totally miss the point I have been making. The circuits are not the same


Quote:
Originally Posted by chchyong89 View Post
Finally I managed to stop the opamp from ocsillation by adding C3 to compensate the circuit.
Glad you have a fix. What a surprise it is in essence very similar to what we were telling you to do...
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Old 20th February 2014, 08:49 AM   #30
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Originally Posted by richie00boy View Post
Yes, and yet again you totally miss the point I have been making. The circuits are not the same


Glad you have a fix. What a surprise it is in essence very similar to what we were telling you to do...

Oh is it?
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