If I’ve saved at least one of you from having to bin a batch of PCBs and do a redesign to filter supply noise then I’ll be happy.A capacitive filter is a circuit consisting of a capacitor and a resistor connected in parallel. I hope you enjoyed this short practical demonstration. Noise is reduced to a low, tolerable level that should not present any issues for sensitive components. The addition of a few small and cheap components can make a huge difference to the stability of the USB power supply. The noise patterns are also very similar which makes me think that maybe this residual noise is being picked up by the leads in my circuit. USB 3 filtered VBUSĪgain you can see that the filter has done its job very well with noise levels about the same as for my USB 2 port. Noise is infrequent and limited to 50mV when it does occur. I’m sure you’ll agree that this is a huge improvement over the unfiltered supply. Firstly here’s how the USB 2 supply looks with filtering: USB 2 filtered VBUS Now let’s take a look at how the power supply line has cleaned up with the filtering components in place. All those exposed wires will probably act as tiny little antenna picking up stray noise from the environment. The long leads and the breadboard interconnects are not ideal from a noise point of view. I have all of those components available in through-hole form. That’s the USB input on the left and the filtered output on the right If you do need significantly more capacitance than 10♟ then the FTDI application note shows one way that you can design a slow start into your circuit to limit the inrush current. I’ll admit to having broken this rule several times in the past and the world didn’t end but do bear this in mind if you’re designing a product for mass production. It’s a little known fact that the USB 2.0 standard mandates a maximum of 10♟ in parallel with VUSB to limit the inrush current. It’s basically an LC filter with a selection of additional capacitors chosen to respond to different frequency ranges. I’m going to filter out the noise using a selection of capacitors and a ferrite bead based on the values provided in this FTDI application note and if any significant noise is still present then I’ll tweak the values of the components until it’s gone. I’ve got spikes, harmonics of spikes and even a pretty decent sine wave in there! Maybe it’s because the cable has to travel all the way across the motherboard or maybe it’s just because I bought a cheap USB 3 card. The noise on my USB 3 port is even worse than on the USB 2 port. I don’t think your sensitive analogue components would thank you for connecting them to this supply. I’m sure you’ll agree the noise levels are pretty awful with spikes of +/- 100mV. The vertical resolution is 50mV/div in all the captures. Here’s a selection of screen grabs from the oscilloscope showing the noise on the supply from the two ports. Let’s get started by showing just how bad things are without any filtering. The cables from the back of these ports has to travel quite a long way across the case to the motherboard header for the USB 2 ports and across the board to the slots for the USB 3 ports. The remaining four ports connect back into the motherboard USB 2 headers.įor these tests I’ll be looking separately at VUSB on the USB 2 and 3 hubs on the left of the picture. The two blue ports on the left connect to a generic no-name PCIe USB 3 expansion card. I have a number of root hubs exposed through the front panel of my case. I should really be using a ground spring on the scope probe instead of a wire and clip but I haven’t got one those so I can’t.įirstly let’s establish the baseline by probing VUSB on a couple of USB ports. I’ll be using my trusty old HP 500MHz oscilloscope and I’ll be probing directly at the component legs so as not to introduce noise from jumper wires. I’l be using the hacked USB B connector pictured above and connected to a breadboard for testing. The result is a nice and stable VBUS line that you can use to feed your sensitive circuits. If you’re running a sensitive component such as an ADC or DAC then you need a clean power supply and all this noise will really ruin your day.Īnyway I thought I’d provide a practical example of how to clean up the power supply and eliminate all that noise. There’s persistent ripple and some mightily impressive spikes that sail through at seemingly random time intervals. There’s random noise at high and low frequencies. In fact I was surprised at just how noisy it is. I’m currently working on a new project that contains sensitive analogue components and have rediscovered what many before me already know, that is that the VUSB 5V line that comes out from your computer’s USB hub is noisy.
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