Noise reduction modification for DSO138

Noise reduction modification for DSO138

Postby gcwills » Sat Jul 25, 2015 8:21 am

Thanks to MorganFlint and nurbard at the Banggood forum http://forum.banggood.com/forum-topic-61279.html?page=3 I was able to apply those mods to my DSO138 to good effect.

I would estimate that this mod has reduced the noise evident in the display by 60-70% which is a significant reduction. For anyone who has built a DSO138 kit prior to the new release firmware, I would highly recommend this easy mod.

To recap on MorganFlint's work, it simply involves removing C26 from the board and laying it flat with its +ve lead soldered directly to the end of L4. The earthy end of C15 is lifted and a short cutoff component lead joins the negative of C26 to the lifted lead of C15. Then a short cutoff component lead joins the end of C15 to the earthy end of C8.

A picture paints a thousand words so here is a photo of my modified board:
Image
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Re: Noise reduction modification for DSO138

Postby davef » Sat Jul 25, 2015 10:12 am

Nice job. I tried mounting a .1uF low-ESR SMD cap in place of C15 with it's ground side actually connected to pin 8 on the IC (via some really fine wire), thus eliminating the wire back to C8. It made no difference.

BTW, your L2 problem you mentioned in the other post and looking at your photo especially around the V2 and V3 pads suggests a counterfeit board. Perhaps being supplied with the wrong value of L2 is another way to determine if they are real DSO138s or not!
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Re: Noise reduction modification for DSO138

Postby gcwills » Sat Jul 25, 2015 11:19 am

Thanks Dave, and thanks for your feedback on trying other "fixes" to the noise issue.
Yes, unfortunately, my kit turned out to be a counterfeit kit with the attendant issues of wrong components and dodgy layout :roll:
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Re: Noise reduction modification for DSO138

Postby gerass » Tue Aug 11, 2015 9:19 pm

Using some ferrite bead or low inuctance instead of soldered jumper JP3 can partialy remove spikes. I've used 4mm ferite ring with 3wounds of 0.4mm wire.
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Re: Noise reduction modification for DSO138

Postby lmantuano » Fri Aug 14, 2015 12:06 am

G'day!

I'm trying to find the specifics of the VSSA ground and 4.7uF tantalum cap mods to further reduce trace noise.. can someone point me to the info?

Would these mods be also relevant to the new version of the board (FW -050)?

thnx

Luigi M
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Re: Noise reduction modification for DSO138

Postby iugamarian » Tue Oct 24, 2017 4:08 pm

The noise can be reduced further if you replace the entire negative voltage generator with a cheap ICL7660 that DSO150 Shell uses. Requires two capacitors and three connections: VCC GND VOUTminus
Attachments
dso138-mod-2.png
entire negative voltage generator
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Re: Noise reduction modification for DSO138

Postby droppedonmy » Thu Sep 27, 2018 1:54 am

Hello GCwills, I have also isolated the analog ground with your modification, and can confirm that the noise reduction is significant. Thank you for the information.
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