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# How to test settling time of a fully differential opamps?

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#### wan

##### Junior Member level 2
thanks!

Re: How to test settling time of a fully differential opamps

Put a differential step on the input. There are some signal generators with differential outputs.

### wan

Points: 2
Re: How to test settling time of a fully differential opamps

Thank you, flatulent.
As what you said, the opamp is used as a comparator. Is it really?

Re: How to test settling time of a fully differential opamps

Op amps and comparators are designed totally differently. You should not use on for the other function unless you are willing to have poor performance. Sometimes the poor performance is acceptable.

Re: How to test settling time of a fully differential opamps

dear ..
put the network of capacitor on the fully differential opamp and apply a step signal on it .
as i describe..
let the in- and in+ is the input of opamp and Vo+ and Vo- is the output of opamp.and suppose u apply a siganl from vin+ and vin-
then put a capacitor
c1=1.0pf between Vin+ and in+
c2=0.5pf between in+ and Vo-
c3=1.0pf between Vin- and in-
C4=-0.5pf between in- and Vo+

then apply a small pulses of 0.5v with 1 ns rising and falling on Vin+ and Vin-
and check the output. then what curve u get at Vo+ and Vo- ,take difference of that curve that will be ur settling time of fully differential opamp
regards
manish

Points: 2

Points: 2

### han6716914

Points: 2
Re: How to test settling time of a fully differential opamps

Op Amp is acomplete design bg Amplifier, hence differential amplifier is the design for amplifyng signnal according to the input of V1 and V2.

There are many type of differential amplifier, so the time can be set according to your design. But remeber the time means the frequency and thus is has a realtion with the power dissipation of your amplifier.

p/s: Common-Mode differential amplifier tak sesuai untuk buat comparator :|

Re: How to test settling time of a fully differential opamps

Hi Flatulent,

Can you describe what the difference the Op-Amp and Comparator makes and how in detail they are different when we are designing them. May be you can suggest me any papers that desribe these.

Thanks,
Suria3

Re: How to test settling time of a fully differential opamps

Here are some major differences.

Comparators are run without feedback or with positive feedback. Op amps are almost always run closed loop negative feedback. This means that the comparator pole and zero locations of the gain are not important. For op amps they are very important and frequently a capacitor is added to move the lowest pole even lower. This capacitance affects the large signal slew rate. The comparator does not have this added capacitance and has a larger slew rate.

Comparator gain is usually just enough for the input offset voltage to swing the output to the logic level. Op amp open loop gain is very high so that the feedback will be accurate and wide band.

The output stage in a comparator is a logic type. In the op amp it is some form of linear circuit such as class B emitter or source followers.

Keep in mind that my IC design experience is from years ago when it was bipolar and metal gate NMOS and features were five microns in size.

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