Full Differential and Single Ended

[firsttimedesigning]

fully differential comparator

I can understand why full differential circuits are better than the single ended ones but what I dont understand is how do you implement a fully differential circuit. Take comparator for example, all the comparators that I see in the book have single ended output. None of them has double ended outputs. So how do I actually make it a "fully differential" comparator?

 

[strabush]

fully differential to single ended

You can take a differential in and out OpAmp, remove its compensation capacitors (as no feedback will be used), and use it as a comparator.
If open loop of the OpAmp is high, it will function well as comparator.

 

[FvM]

single ended output ecl

Hello,

very fast comparators (Anlog Devices ADCMPxx et al) typically have differential outputs, cause they should be able to drive differential logic, e. g. LVDS. Although at the digital side the motivation for differential signaling isn't exactly the same as for the analog side, it shows similar advantages, e. g. lower interference to adjacent signals.

Regards,
Frank

 

[firsttimedesigning]

differential in single-end output

Ok thank you for replying. but i am still not sure on how to connect the op-amp. I have drawn a picture but i am not sure if i have connected it correctly. Also, I still dont understand, how is the op-amp(double ended output) that i have drawn better than the single ended one?

 

[FvM]

differential to single ended circuit

Hello,

shorting the differential op output to ground is no good idea, it would cause excessive output current and probably damage the device. However I don't see, why you give an integrator circuit but asked for a differential comparator. Integrator circuit would give no meaningful results with a comparator. So I wonder what you actually try to achieve?

Regards,
Frank

 

[firsttimedesigning]

single vs double ended outputs

Thanks for the reply.
Ok yeah so actually I am having trouble with both circuits, integrator and comparator. What I would like to achieve is that I want to connect the + end of the integrator to a comparator and then connect the output of the comparator to a control logic. I have drawn a new circuit, please tell me if it is right. Instead of shorting the - output to the ground, i just open it.

 

[FvM]

fully differential from single ended

Your circuit should work, but isn't using the fully differential feature anyhow. Why not extending it to true differential circuit?

 

[firsttimedesigning]

differential to single ended output

Thank you for the replying but it seems to me that if i make it true fully differential circuit, then the size of the circuit will increase. the capacitor, for example, i will need two of them. is there anyway to make it true fully differential without adding another cap and res. I guess not...

I have drawn a picture of the output of the integrator and comparator. not sure if they are correct. and so far I have not see the advatange of making the ADC differential. The -end output just seems like it can be eliminated...i dont get it, are all ADCs suppose to be differential? if so, why? i read papers and it seems to me that the ADCs presented are differential...but -end output, how do i use it to make the ADC better? i am really confused.....

 

[nicolas.borfigat]

fully differential signaling

Hello,
Maybe I make a mistake but one of the main reason for using a fully diff device is it ability to cut down the noise.

 

[FvM]

single ended signal to double ended signal

Yes, when noise is understood as common mode interferences, ground bounce and such. Additionally, linearity is improved, even harmonics are supressed, emission of interferences is reduced.

 

[firsttimedesigning]

singal end vs fully differential

Fvm

Actually, I just did a hand analysis and I found out that the circuit you have posted would not integrate at all. but thx for replying anyway...

 

[FvM]

differential to single+op

I just did a hand analysis and I found out that the circuit you have posted would not integrate at all.

Do you know a way, to convince the hardware versions of the circuit, that are performing well since years, they are simply doing wrong?

 

[firsttimedesigning]

differential to single end comparator

thx for the reply, but if you can, show me your hand analysis and prove me wrong. cuz i dont think i made a mistake...and i believe spice didnt either...

 

[wateror]

full diferential

refer to allen's book... you put a inverter at each of the opamp output, then you get differential outputs.

 

[kivvzhou]

single ended comparator

for diff opamp,guay's book explain very good!

 

[jecyhale]

dif.comparators

I can understand why full differential circuits are better than the single ended ones but what I dont understand is how do you implement a fully differential circuit. Take comparator for example, all the comparators that I see in the book have single ended output. None of them has double ended outputs. So how do I actually make it a "fully differential" comparator?

Differential signal is to redcue the noise, so it is good for small signals not for large siganl. comparator output is large signal.

 

[FvM]

single ended comparators

Really fast comparators usually have differential ECL or LVDS outputs. So actually differential signaling is suitable for digital signals or large signal as well.

 

[cretu]

fully differential comparator design

Really fast comparators usually have differential ECL or LVDS outputs. So actually differential signaling is suitable for digital signals or large signal as well.

It would also help if you don't realy need rail to rail signal through your high speed chain. You can have only at the output stage something that will be rail-to-rail.

 

[BackerShu]

fully differential comparator lecture

for diff opamp,guay's book explain very good!

Sorry,who is guay?
Do you mean gray?

 

[MicroElec]

differential to single-end integrator

Hello,
A Fully Differential comparator is a comparator with 2 inputs and 2 outputs....the aim is to reduce the Ground noise.
To make a Fully Differential comparator you can take for exemple un dif stage(with 4 transistors + current source).
The outputs are taken between each half part in the circuit (drains).
hope that i helped you.