Ethernet LED blinking changes DAC voltage

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LeNe
Posts: 6
Joined: Tue Dec 02, 2014 9:06 pm

Ethernet LED blinking changes DAC voltage

Post by LeNe » Wed Jan 13, 2021 6:18 pm

When the ethernet LED on a Stemlab125-14 blinks, this results in a change in DAC voltage of the order of ~0.5 mV. We have confirmed this correlation with a photodetector placed near the ethernet port LED. I believe the linux OS ethernet driver is responsible for this blinking. Is there a way to disable it to get the best possible noise performance out of a redpitaya?

See also https://github.com/RedPitaya/RedPitaya/issues/205 for the same text in a github issue, since it's not clear to me where to ask this.

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redpitaya
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Posts: 480
Joined: Wed Mar 26, 2014 7:04 pm

Re: Ethernet LED blinking changes DAC voltage

Post by redpitaya » Thu Mar 18, 2021 10:14 am

please share more information on our support email.

What power supply are you using? is it an original or a 5V 2A one?

lauser
Posts: 3
Joined: Tue Mar 16, 2021 1:53 am

Re: Ethernet LED blinking changes DAC voltage

Post by lauser » Sat Mar 20, 2021 10:59 pm

I think this is actually a known design limitation of the STEMLab 125-14 that has been known for years. The problem is that there is not a separate voltage regulator for the bias offset voltage for the DACs. It directly uses the same 3.3V rail as all other components on the board. So its not just the Ethernet, but everything on the entire board that causes the problem. It looks like RedPitaya has not resolved this problem in the v1.0.1 boards either.

In fact, if you look at the back of the board near the SMA connectors, you can actually see two depopulated positions for regulator ICs. Its like they know of this limitation, but to keep the cost down, or to drive sales to the higher end products, they did not populate them.

However, there is a solution described here:
https://ln1985blog.wordpress.com/2016/0 ... rformance/

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