The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> non-overlapping clock issue in a SC delta sigma modulator https://designers-guide.org/forum/YaBB.pl?num=1536452505 Message started by polyam on Sep 8th, 2018, 5:21pm |
Title: non-overlapping clock issue in a SC delta sigma modulator Post by polyam on Sep 8th, 2018, 5:21pm Hi, I've designed a 2nd order SC delta-sigma modulator to support 10kHz signal bandwidth. The OSR is 256 resulting in a 5.12MHz oversampling frequency. The OTAs used for the integrators are overdesigned to properly settle down even for a 20-MHz oversampling frequency. Just giving some numbers (DC-gain=120dB, UGBW=220MHz, phase margin=64). all switches and quantizer are transistorized. The fully transistorized modulator perfectly works with a sampling frequency of 5MHz when I apply an IDEAL non-overlapping clock signal. The modulator works also well when I increase the clock frequency to 20 MHz and integrating the noise over 40kHz (OSR still 246) again with an IDEAL non-overlapping clock frequency. Now here is the problem!! I replaced the ideal non-overlapping clock signal with a transistorized one. The modulator works well when the master clock is 5.12 MHz. BUT when I increase the master clock to 20MHz, I see a severe SNDR degradation. Does anyone have any idea to fix this issue? What causes this issue? (I'll post spectrum in different cases) Thanks |
Title: Re: non-overlapping clock issue in a SC delta sigma modulator Post by polyam on Sep 8th, 2018, 5:24pm When I use an ideal 20MHz non-overlapping clock and BW is 40kHz. The modulator works. |
Title: Re: non-overlapping clock issue in a SC delta sigma modulator Post by polyam on Sep 8th, 2018, 5:25pm When I use a transistorized non-overlapping clock signal. Master clock is 5.12MHz and BW is 10 kHz. |
Title: Re: non-overlapping clock issue in a SC delta sigma modulator Post by polyam on Sep 8th, 2018, 5:26pm When I use a transistorized non-overlapping clock signal. Master clock is 20 MHz over a BW of 40kHz. |
Title: Re: non-overlapping clock issue in a SC delta sigma modulator Post by polyam on Sep 8th, 2018, 5:30pm From my third spectrum, it's kind of weird. It looks like that I have a delay probably in the feedback path. The shape of the spectrum over fs/2 makes me conclude that! But anyway, I didn't change anything in the modulator. Just changing the master clock to 20 MHz and that's it! |
The Designer's Guide Community Forum » Powered by YaBB 2.2.2! YaBB © 2000-2008. All Rights Reserved. |