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Slew rate simulation for fully differential amplifier

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Junus2012

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Dear friends,

When I simulate the slew rate for each out of the fully differential amplifier indivisibly, it gives me different value when I measure the slew rate from the differential output signal (Vo1-Vo2), why this is happening and which is the right setup, from individual output signals or from the differential signal

Thank you very much
 

it gives me different value when I measure the slew rate from the differential output signal (Vo1-Vo2)
Sounds expectable. Datasheets of industry standard fully differential amplifiers are referring to differential output.
 
Thank you FvM for your reply

It means for every parameter nothing is measured with respect to the differential output only, I should follow this rule.

Thank you once again
 

Yes, it is expected. The currents coming from supply are usually different than the currents sunk to ground, so in slewing conditions the SR of the individual outputs can differ. But by careful design you can make them kind of the same.
 
Dear Suta,

Thank you very muchh

I am didn't mean the slew rate is different from each of other output, for the individual outputs they both have the same Slew rate, but the fully differential output (Vo1-Vo2) has different slew rate from Vo1 or Vo2 individually,

However, as FvM has mentioned, I should not wary because I only have to characterize my differential amplifier with respect to the differential output (Vo1-Vo2), not to Vo1 or Vo2
 

Hi,

I´d expect the differential slew rate to be about (SR_rising + SR_falling).
Means: If both are about equal: SR_diff is about 2x SR_single_ended.

Klaus
 
Dear Klaus,

Thank you a lot

Exactly, the fully differential output signal has twice the slew rate of individual output signal, as you said, this is the case I have

I should deal with the fact, that the fully differential output can not be treated as two single ended outputs otherwise I will loose the half of my properties values (gain, SR, etc), all my characteristics should be with respect to fully differential outputs to the fully differential inputs.

in the same time, the next stage to the fully differential amplifier must also read the output signals deferentially, again if the next stage is single ended in, then also I will loose the half values.

Dear friends, I have this question please,
In the past I was designing single ended opamp with load connected RL=100K, CL=5Pf
as you know I made a fully differential one based in the previous design, now I have two outputs on each I am connecting RL=100K, CL=5Pf, or do I need to change the load values (like half it) to make the same load condition of the single ended amp


Thank you very much once again
 

Hi,

differential is differential (with respect to each other), single ended is single ended (with respect to GND).
So if you have one signal rising with 1V/us and the other falling with 1V/us then the differential rate is 2V/us

I had similar discussions with my customers...

Klaus
 
Dear Klaus,

Thank you again for your confirmation,

The remaining thing is the load condition I asked about it before please

Thank once again
 

Your load depends on the external environment, not so much on the amplifier you're designing. 100K differential load corresponds to 50k to ground from each output. And 5pF of course is 10pF from each output to ground.
Having the load split like this for each single output will affect also the CMFB, while if you have the load connected differentially, the CMFB will not see it.
 
Dear Suta,

you notified me about one important me about one important issue, in my all simulation I am connecting two individual outputs, and may be this is the reason why I am suffering with the CMFB compensation,
So according to your explanation, if I am connecting 100 K ohm and 5 pF on each output separately, it means it is equivalent to 200 K ohm and 2.5 pF if would connect them deferentially.

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my next stage is fully differential anti aliasing filter then ADC comes
 

yes, that's correct but only in differential mode of operation.
 
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