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Old 20th February 2014, 07:54 AM   #31
jcx is offline jcx  United States
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another point shown wrong in the earlier pics: using tihs type of full diff circuit requires both + and - inputs to be tied to low, matching Z source - can't leave one floating for single ended

in fact it would behave poorly with most single ended audio sources which have series R in the signal lead

you need to buffer both inputs for a truely general purpose circuit
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Old 20th February 2014, 08:53 AM   #32
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Quote:
Originally Posted by jcx View Post
another point shown wrong in the earlier pics: using tihs type of full diff circuit requires both + and - inputs to be tied to low, matching Z source - can't leave one floating for single ended

in fact it would behave poorly with most single ended audio sources which have series R in the signal lead

you need to buffer both inputs for a truely general purpose circuit

Thanks JCX, will look into this problem and try to come back with a better schematic.
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Old 21st February 2014, 08:10 AM   #33
Calvin is offline Calvin  Germany
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Hi,

is it?
Yes it is! Adding the cap is nothing else but giving U2 its own feedback loop.
Im a bit surprised only by the fact that the cap seems sufficient here, as Id expected that a resistor would have been needed too to get U2s gain down.
I admit, that I didnt analyze the circuit as thorough as needed regarding the function of U2 not only supplying for a DC-Bias, but also to feed the input signal of one input to the second input to achieve balanced output under all circumstances.
The doubt about U2 running openloop beeing the source of oscillation seems to be correct, eyh?

jauu
Calvin
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Old 21st February 2014, 08:38 AM   #34
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Quote:
Originally Posted by Calvin View Post
Hi,

is it?
Yes it is! Adding the cap is nothing else but giving U2 its own feedback loop.
Im a bit surprised only by the fact that the cap seems sufficient here, as Id expected that a resistor would have been needed too to get U2s gain down.
I admit, that I didnt analyze the circuit as thorough as needed regarding the function of U2 not only supplying for a DC-Bias, but also to feed the input signal of one input to the second input to achieve balanced output under all circumstances.
The doubt about U2 running openloop beeing the source of oscillation seems to be correct, eyh?

jauu
Calvin
Is U1 running in complete Open loop?
*JPG1

I am using a very ordinary way to compensate the opamp, connecting a resistor from the output back to the inverting input to reduce or increase the gain is just a complete different story.
*JPG2

Back to the very basic circuit to compensate an oscillated basic opamp. Now you can illustrate what I was actually doing that.
*JPG3
Attached Images
File Type: jpg CLLP.JPG (28.2 KB, 20 views)
File Type: jpg CLLPC.JPG (36.2 KB, 20 views)
File Type: jpg CLLPO.JPG (32.1 KB, 20 views)
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Last edited by chchyong89; 21st February 2014 at 08:57 AM.
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Old 21st February 2014, 09:13 AM   #35
Calvin is offline Calvin  Germany
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Hi,

I now hacked the circuit from #1 into the sim.
Using OPA134 Model the sim showed oscillation ~6.5MHz.
A increase of the feedback caps C6 and C7 from 12p to 27p and adding a local feedback cap of just 4p7 to U2 cleaned up things.
Simming with a 10kHz rectangle the tiny cap cleaned up U2s response.
Larger Caps for U2 worsened the Step response of the whole circuit, looking like typical slewrate induced distortion.

jauu
Calvin
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File Type: txt fully differential singlesupply 3-OPamp.asc.txt (6.0 KB, 9 views)
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