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To Design a single ended to fully differential converter

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vlsi12

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I designed an folded cascode FDA. However when designing a single ended to differential converter using the same FDA the output swings from the 2 output ports are not symmetrical. Any suggestions as what to do?
The feedback resistance is 50K and the input resistance is 25Kpic.png
 

FvM

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What's this, resistor or no resistor?

res.PNG


If the resistor network is symmetrical, the output voltage difference is probably due to insufficient common mode balancer gain or bandwidth.
 

vlsi12

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Yes that is a resistor. Can you elaborate on the " insufficient common mode balancer gain or bandwidth." Any references in addition to the elaboration will really help.
 

frankrose

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Does it work in DC? Could you annotate the DC node voltages, and show some figures with different DC input values? If the common-mode feedback is good it should operate in DC.
 

vlsi12

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As you suggested I varied the input dc voltage. For a common mode voltage of 900m and signal dc vaue of 0 at the input the outputs are same Vocm value..How ever as I increase the input signal dc value, complementary dc outputs are not the same value over or below the Vocm.resultsdc3.pngresultsdc1.pngresultsdc2.png
 

FvM

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Why don't you report the DC levels instead of showing useless line plots? As far as I see, outp has gain of about 1 while outn has only -0.7. Suggests too low common mode loop gain.

We need to see substantial circuit information.
 

vlsi12

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Sorry for the late reply. As you suggested the loop gain was low. I made the necessary modifications and the Circuit works fine. However the signal swing in the positive side is getting clipped and the negative swing is increased when I use a conventional DDA CMFB instead of the resistive average which I am using now. The resistive CMFB doesnot have any problem for the swing except the fact that it lowers the gain by loading the output. Any thoughts on thatswingclip.png?
 

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You could increase the swing by decreasing the common-mode voltage. But no problem to use resistors if you have enough area and current consumption.. Be cautious with differential and common-mode stability, big resistors create pole with the error sense amplifier's input capacitance.
 

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