The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> sigma delta digital output?
https://designers-guide.org/forum/YaBB.pl?num=1145534975

Message started by chungmnig on Apr 20^th, 2006, 5:09am

Title: sigma delta digital output?
Post by chungmnig on Apr 20^th, 2006, 5:09am

I has a question about the digital output of comparator.
Like attached file , the comparator has two output Vo+ and Vo-
Then i want to get the PCM signal , is only get Vo+ to do next step(FFT analysis)?
or get the differential output (Vo+) - (Vo-) to do next step?

I had already checked that , there are have some difference.
Which are the correct output , single ended or differential ?
or just see what application i used?
thanks~~!!

Title: Re: sigma delta digital output?
Post by vivkr on Apr 20^th, 2006, 8:25am

Hi,

How does it matter whether you use just one or both of them. There is no extra
information in the difference of the comparator outputs, when compared to
the single ended output.

The FFT you need to take is that of the digital data stream. I would be surprised if
you see different results.

Regards
Vivek

Title: Re: sigma delta digital output?
Post by ACWWong on Apr 20^th, 2006, 10:13am

i say differential, as only then will you be cancelling common-mode effects of oversampling clocks etc.
i think if you do the fft of single-ended vs differential (assumming you have real transistors below the symbols!) you WILL see a difference, especially in even-order harmonics.

Title: Re: sigma delta digital output?
Post by chungmnig on Apr 21^st, 2006, 12:04am

ACWWong wrote on Apr 20^th, 2006, 10:13am:

ACWWong:

What you said is totally right~~!!
My simulation result is the same as you said.
Is this normal or has some problem in circuit design?
thanks~~!!

Title: Re: sigma delta digital output?
Post by vivkr on Apr 21^st, 2006, 1:48am

Hi ACWWong,

I don't quite understand the point you make. It is clear that one needs to look at differential
outputs when dealing with analog signals such as the integrator outputs to cancel the common-mode
and even harmonics,

However, if a comparator is working correctly, then Vo+ is always the complement of Vo-, and these
2 signals are digital, and contain exactly the same information, except for an inversion. So, there can be
no common-mode or even harmonics on these. To have even harmonics on differential comparator outputs,
you would need a situation where sometimes Vo+ = Vo- and not its complement. This would imply
improper comparator operation. So I really wonder how one would see errors unless one was doing the
calculations incorrectly.

Could you elaborate your point further?

Regards
Vivek

Title: Re: sigma delta digital output?
Post by sheldon on Apr 21^st, 2006, 2:46am

Chungming,

The signal may represent digital information but it is still an
analog signal. For example, the dc offset of the single-ended
signal is Vdd/2. For purposes of the FFT, processing the
information differentially usually gives better results.

Best Regards,

Sheldon

Title: Re: sigma delta digital output?
Post by ACWWong on Apr 21^st, 2006, 5:15am

Yes, ideally Vout+ and Vout- are complementary, especially when it come to the signal being processed. Yes the signal is also digital.
Unfortunately other signals (unwanted) also appear, such as clock feedthrough, power supply/substrtae noise etc. and then can add to both Vout+ and Vout- simultaneously, therefore making Vout+ not the exact complement of Vout-. These effects also result due to non-ideal comparator design, rather than improper design.
Therefore to help cancel these unwanted (mostly even order, of which DC can be though of "0" order, whcih is the point sheldon made) effects, and get the best out of the fft, the fft should be done differentially.

Of course i am an RF/Analog IC person, so my judgement is always somewhat blurred :P

vivkr wrote on Apr 21^st, 2006, 1:48am:

Title: Re: sigma delta digital output?
Post by chungmnig on Apr 21^st, 2006, 7:42am

thanks sheldon and ACWWong.
Your suggestions are really helpful :D
very thanks~~~~!!!

Title: Re: sigma delta digital output?
Post by chase.ng on Apr 21^st, 2006, 5:46pm

Hi,

Most differential output signal is centered around near zero. However, the single ended output signal will be centered around a DC voltage. Therefore, my opinion is that, if you are doing FFT on the single ended output, you might found that it will has a very high DC component where the FFT of the differential signal will not shown. Depending on what kind of window you are using, you might find that the component at DC actually leak to other low frequency component, which might cause your output DFT to looks differently. Since FFT/DFT is actually sampling at a precise timing, it does not matter what your transient output looks like, what is important is that at the moment the FFT samples, your output is at the right value to be sampled, therefore unless the output is heavily contaminated with high frequency noise, otherwise it should give you the same result for using either kind of signals except for the DC part. For DS modulator kind of circuits, the absolute value of the output is somehow not important as long as they are in the range to properly define a logic state. For example, you can use an ideal amplifier to amplify the output of a modulator 100 times, so that your output is let say. 200V, take a DFT, yet you will find that the SNR, SFDR will remain the same. If your circuit will be working in baseband, I will suggest you to take FFT differentially since it is easier.

Regards,
Chase

Title: Re: sigma delta digital output?
Post by vivkr on Apr 23^rd, 2006, 12:50am

chase.ng wrote on Apr 21^st, 2006, 5:46pm:

Hi Chase,

Very well put ! Indeed, I had forgotten about the leakage from DC if one were using a "bad" window.

Regards
Vivek