LCD causing audio interference

Hi all

I got a DOGLCD interfaced to an ATMEGA328, and a PGA chip pre-amplifier. Everything works great, except there’s this interference noise that seems to be getting picked up by the preamplifier.

I can characterize the noise as “ticking/buzzing sound”. And it only seems to happen when the LCD screen is getting updated… for example, menu screen redraws, screen updates, and that sort of thing. The noise is faint, but when the gain of the preamp is set to 40dB and above without any other sound source, the interference is very audible.

All ICs have 0.1uf caps on their Vcc.

I’ve tried adding additional 0.1uf caps to the DOGLCD Vcc terminal but didn’t seem to help.

This problem is basically a show-stopper for this project and hope someone has any other ideas on how to get rid or reduce this noise.

Attached is the schematic of the DogLCD.

My guess is the interference is caused by the backlight charge pump? But then, the noise should be present all the time if that’s the case. It’s not. The noise only manifests when the LCD screen is updated.

Ahhhhhhhh I wasted a whole day on this!

Never mind... I am so stoooopid! There wasn't any "real' problem. The problem was caused by my own stupidity and laziness.

move along now...

Aw. Please tell us what caused it. Sounds like a trap others could fall into.

It's embarrassing, but here you go.

I'm trying to update my ATMEGA328 chip on my DIY Controller Board via ICSP (using USBASP v2.0 programmer and AVRFuses). So to do that, I had to hookup (6) wires between the USBASP and my Controller Board. Easy and simple.

I'm refining my program, edit, upload, edit, upload so due to my laziness.... I just left all the wires connected between the USBASP and my Controller Board. My Controller Board wouldn't run when powered on it's own supply, with the USBASP programmer board still connected to it, so I just disconnected the Vcc wire, leaving (5) wires still connected. If I need to re-program it, I just connect the 6th wire, the Vcc wire.

So everything seems dandy, the new firmware is working...

And that's when I noticed that an awful lot of RFI noise is happening... really loud. The preamp is only at 20dB gain and I can already hear it from my speaker monitor... and at 66dB gain (2000x voltage amplification), it's just awfully loud, really loud. The RFI noise is happening in coincidence with the GLCD updates.

I spent several hours trying to rewrite the firmware, change some logic to minimize noise, but then I realize that while I can minimize the occurrence of the noise, the problem is still there... and that's not good enough. I need to ELIMINATE this RFI noise. There should be no noise, even at 66dB gain (2000x voltage amplification). -- which means the problem is in the hardware?

Maybe I didn't do a good enough job of RFI filtering on the inputs, wrong cap values, etc?.... maybe my grounding scheme (ground fill - double sided, top and bottom) isn't good enough. Maybe I need to redesign the PCB and rethink/relayout my Digital Ground and Analog Ground.

The more I panic, the more I can't think straight. All I can think about is the back-to-square-one, and all the lost work and money I poured on this project.

I'm about to call it a day, and I saw another Controller Board prototype version I've made. It's basically the same as the newer Controller board, just minor revisions.

I hooked this old Controller Board to my Preamp Board and my preamp board was quiet! I'm a bit surprised. WAITTTTT...... that can't be! I compared the old Controller Board and the new Controller Board... what's the difference????? ---- you gotta be kidding me... tell me it's not true.... that stupid USBASP programmer board dangling with it's 5 wires connected to the ICSP port. Nooooooooo.... so I yanked the 5 wires off the board, powered it on, and ....... holy cow... sweet quietness even at max gain of 2000x amplification. So stupid stupid stupid of me.... why didn't I think of that 12 hours ago!

And that's it. Lesson for today is.... don't leave your USBASP programmer board connected to your project when you don't need it. -- I'm trying to save time from connecting these (6) ICSP wires, and I wasted 18+ hours of my life on a problem that wasn't there to begin with!

Well thank you for the heads up. I can easily imagine falling for that myself. So that’s something I’ve learned through your experience :slight_smile: