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I slightly adjusted the diagram from TIDA-00765 with the realities, it’s easier than pulling it out in pieces from my diagram. For power supply, 24V is taken and supplied to DC/DC, then to the LDO stabilizer, where it is reduced to 3.3V. The power supply to the processor is decoupled using capacitors and FB (ferrite bead), just like the ADC.
As for the low-frequency noise, it seems like the display driver is guilty, I'll try to run a test without it tomorrow. It’s strange that it gives such a drawdown at 5V, although it is powered by 3.3. By the way, I did a test by supplying 5V to the AVDD from a laboratory power supply, the result was the same as from the “noisy” standard one in the circuit (DC/DC), I did not notice any improvements. Apparently the ratiometric measurement method allows fluctuations in the power supply of the excitation circuits to be compensated.
PS: Yes, that’s right, I disconnected the board with the display and the fluctuations in the 5V power supply disappeared (More precisely, it is correct to say that they became the same as with the supply of power from a laboratory power source), but the measurements still varied (jitter present), I don't think that was the problem was due to the the power supply. As I understand it, we have 10 digits and a dynamic display (one segment is drawn simultaneously on all digits).
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That is, somewhere around 100 mA there are current inrushes, "dot point" is not used yet, that’s where we see 7 oscillations (7 segments), then a pause on oscillogram.
Although, of course, in the future I need to think about how best to decouple the sources of power supply so that they (or consumers) do not interfere with each other. Probably it makes sense to install a separate DC/DC on the display?
By the way, I tried to increase the capacitance of the capacitor for powering the LED display driver, add 3300uF to 330uF, the ripple decreased slightly, but was still noticeable.
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