# What inductor to use?

I'm working on a device utilizing Atmel's AT32UC3C microcontroller. The datasheet and design checklist documents provided by example schematics utilizing some type of RF/EMI inductor to isolate the digital and analog source circuits. However, they don't provide any indication as to what to use.

Atmel's recommended power supply schematic:

How do I calculate/figure out what inductor to use?

I'm utilizing the ADC on the chip and want to sample at the maximum 2MSPS (ADC clock frequency 2MHz) and want to operate the digital end at the maximum spec frequency of 66MHz. I plan on having the device USB powered and operating at 5V.

• The impedance of the 100 nF cap at 70 MHz is 0.02 $\Omega$, so with the 850 $\Omega;$ it divides the input noise roughly by 40000. So 40 mV noise will be reduced to 1 $\mu$V, which is extremely low. A 12-bit ADC at 5 V reference voltage has a resolution of 1 mV, and expect some counts of noise. So 40 mV noise would be reduced to 1/1000 of that. That's "acceptable" ;-). – stevenvh Aug 10 '12 at 6:31