| Electronics > Projects, Designs, and Technical Stuff |
| 5V rail noise when updating 2x16 LCD |
| (1/5) > >> |
| txescientist:
Hi to all! Got a "little" problem. When PIC 18F4550 sends data to 2x16 LCD, +5V line gets noise (attached picture). PIC is decoupled with 100nF/10uF, LCD also. Tried adding PI filter and ferrite beads, no good. LCD is connected via ribbon cable, length is about 10 cm. Sorry, I have no pictures. Built on protoboard (soldered). Similar noise is present at the battery pack terminals. It is not bad probing, noise is real. Power is supplied from 9V battery pack and is regulated down to 5V by LF50CV. This level of noise is not a problem generally, but for analog part of electronics is making a lots of trouble. Anyone have any ideas how this can be solved? Please suggest some good filter, so I can power the analog part from the same battery pack. Thanks! TxeSci |
| SiliconWizard:
Does the 5V regulator have proper decoupling close to its input and output pins too? |
| txescientist:
Yes, forgot to mention that. Used electrolytic caps and ceramic for LF50CV. Analog part is connected to the shared battery pack and has its own LF50CV. Still, there is noise in the analog section also. |
| coppercone2:
does the PI filter operate within the noise spectrum of the noise signal? ferrite beads are hard to choose did you take measurements with and without filter to see magnitude change? Does it still occur with a lower impedance source then a 9V battery (which is 6 1.5 cells in series, it has high ESR kind of). Like, probe before and after the filter to see the effect that its transfer function has. And overlay the two signals. Then you can compare the expected attenuation (difficult because of unknown source and load impedance) and the actual attenuation if any. You might need a higher order filter or different topology. Do you daisy chain or star ground? Keep in mind there are Pi and T filters (which use inductors, but be careful because you need to known inductor SRF to get actual expected performance (since the parasitic capacitance can turn it into a partial pi filter). *I think they mean source not high on the top of the table* https://interferencetechnology.com/selecting-proper-emi-filter-circuit-military-defense-applications/ You need to think about the signal path. If you blocked it with filters its either bypassing or the filters are not working correctly for whatever reason. |
| SiliconWizard:
Yeah. Then again, if you're 100% certain it's not a probing issue, that much noise on the 5V supply coming for a linear reg, with just an MCU and an LCD display is not that normal IMO. Before adding a fancy filter, I would start with adding a beefier capacitor on the 5V supply, something like 100µF or so. |
| Navigation |
| Message Index |
| Next page |