2014-05-27 08:06 AM
Hi
I'm having problems with noise on the STM32F103RB ADC.At the VDDA pin there is 20mV pk-pk noise from a300KHz
switch regulator.I am using the recommended 1uF || 10nF caps on the pin.I was going to add a series ferrite to block the switching noise but when I came to look for the design steps it looks like ferrites are for noise in the MHz range.Can anyone advise on options for putting a simple filter stage between my 3V3 rail and VDDA pin to attenuate the 300KHz switching noise.I would like to attenuate by an order of magnitude, it is currently 20mV pk-pkAny help would be highly appreciatedThanks2014-05-27 11:28 AM
Fix the source of the problem. TO-220 or D-Pak voltage regulators have less than a millivolt noise.
The ferrites will attenuate processor MHz switching noise ringing back up the VDD pins. Cheers, Hal2014-05-27 12:24 PM
Many buck regulators put a LOT of capacitance on the output side, on the order of 50-100uF. Try the brute force method, put a big cap on the output side of the regulator (and watch the ESR, use a low ESR like an aluminum cap). Doesn't hurt to have some larger caps on the input side too, depending on what kind of source for your power.
Jack Peacock2014-05-28 12:12 AM
Thanks to both of you for the info.
2014-05-28 01:15 AM
Hi
''Fix the source of the problem.'' Definitely! A further note: '' TO-220 or D-Pak voltage regulators have less than a millivolt noise.'' Note sure if he meant - replace your ''300KHz
switch regulator'' with a Voltage regualtor. If he mean put one in series with your switching regulator - be aware Voltage regulators can still pass high frequencey noise, I have been investigating building hybrid PSUs (Switching back end and Voltage Regulator front end - to reduce the power dissipation in the voltage regulators but have found that VRs do pass high freq noise). Also, check if the ADC has a separate supply/reference input. If so, you can use a separate supply/reference to the ADC from the rest of the processor. This is a common trick to reduce noise to ADC/DACs and keep PSU efficiency.