I am building a power supply PCB board with Power In, Power Out (Drive Motor), 8V, 5V, 3.3V and a filtered 5V output to sourced to the 16 bit ADC and source voltage for the WAS.
The attached screenshot of my oscilloscope shows three traces, the Yellow trace is the output of the DC-DC buck converter converting from 13V to 8V, the Blue trace is the output of my Low Drop Out (LDO) 5V regulator, and the Pink trace is the output after my low pass filter (capacitance multiplier) circuit (6.7V).
The vertical scale is set to 50.0mV per division for all three traces. As you can see the output of the DC-DC converter adds a lot of switching noise, almost 100 mV peak to peak. For any analog to digital converter, 10 bit on Arduino or a 16 bit ADC this would be troublesome. Putting the output through a 5V regulator does reduce the noise quite a bit but it is still in the 30 mV range, which may be completely acceptable for most applications. I wanted to reduce the noise even further so I put the output of the DC-DC converted through an "Active" low pass filter. This virtually eliminated all of the switching noise from the DC-DC converter. The input voltage to my filter is 8V while the output was at 6.7V for my given load. Since I will only be driving my 16 ADC and WAS with this source I will still have plenty of head room for my LDO 5V regulator to give a rock solid 5V.
My "active" filter consist of 1 x 56 Ohm resistor, 1 x 47uF cap, and 1 x tip112 darlington pair transistor. Your mileage may vary but this setup should work for my application.
Last edited by Jays_Truck; 01-24-2019 at 09:30 PM.