USB supply filtering
Hi, I've got a few different projects going on at the moment that are all USB powered so I thought I'd do some measurements of the USB supply noise and try some ways of improving it. Initially I added bulk capacitance, 4400uF, to reduce the noise. This worked but obviously isn't really a solution particularly when the USB 2.0 standard specifies a limit of 10uF in parallel with the supply line to limit inrush current. The specification states that bulk capacitance can be used if a soft start method is implemented so that the inrush is not exceeded. This information comes from an FTDI application note ( found here ) and in this they show an example circuit for filtering the supply line. This circuit consists of a few capacitors and a ferrite bead, specifically BK0603HS330-T ( Digikey link ). I don't have any of these on hand but I thought I'd try a couple of inductors that I have. Specifically the B82141A 33uH( Digikey Link) and 77F1R0K 1uH ( Digikey Link