u/DaddyDeno15 — reddlx

Hiii, I'm planning to make a signal generator (and adding custom waveforms function in the future to turn it into an AWG) and I'm currently still in the component selection stage, but I've decided on some basic specifications. The AWG should:

Achieve 10MHZ @ 20Vpp for sine waves
Achieve 1MHZ @ 20Vpp for square waves

My plan as of right now is that it would be using a STM32G4 MCU, AD9834 DDS and THS3091 as the amplifier and it would have a 4.3 inch LCD touch display (and some buttons). I'm new to PCB design so I have some questions regarding grounding: It would also be a four layer board: Analog (layer 1), Ground (Layer 2), Power Planes (3.3V and 5V, layer 3), Digital (layer 4).

I've been watching some videos on mixed signal PCB designs (specifically from Altium Academy) and they've recommended NOT splitting the analog and digital grounds. They've also specifically mentioned that splitting analog and digital grounds AND only keeping them connected via a mecca connection is pointless (and also causes issues with return current path) [Defining Ground | Mixed Signal PCB Design: Part Two]. However, in a different post that I've made, someone else specifically recommended the mecca connection instead. (link to comment) What's the correct/better way of grounding for my specific application?

Should I also still stick with my current layer layout? Or should I set layer 2 and 3 as ground?

Hi, so as the title says, I'm trying to clean up the output voltage from a MP1584 buck converter module (in the image), which supposedly switches at 1.5MHz (though some manufacturer runs it at 1MHz). This will be used for the 5V Vcc of a DDS chip so the input ripple would have to be really small.

The problem I'm running into now is that I've designed an LC filter circuit and soldered it onto a veroboard. The inductor used is a CD127R shielded inductor (can't really find a lot of datasheet on it, but the 10 microfarads inductor version has a sat. current of 3.2A), with ceramic capacitor + RC snubber circuit. The components are soldered as close as possible, and the wires connecting the buck converter output to the filter is also as short as possible. The load is a 27 ohm resistor, roughly 180mA.

With my oscilloscope (yellow trace is the output from the buck module, blue trace is the output of the filter, and the duration between the A and B cursors is 2us), I can verify that the filter is attenuating some of the 500kHz ringing, but the Vpp is still over 100mV, which isn't good enough for analogue ICs. I'm also thinking about adding a LDO (ADP7118, 50dB PSSR at 1MHz) so that it would further attenuate the high frequency ringing, though I haven't got my hands on one. I'm also thinking about adding a ferrite bead to remove the ripples as well.

So my question is:

Is it overkill to add the ferrite after the LDO? Would that add any beneficial difference
What is an acceptable Vpp when using analogue components, like DDS and op-amps
Does it still make sense to test the filter + LDO circuit on a veroboard? (since the copper traces have parasitic inductance and capacitance) Should I just test it on simulation, then proceed with the PCB design