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Beginners / Re: LC filtering for combined Vref/VDD of ADC
« Last post by MrAl on Today at 03:55:05 pm »I am looking to use a Microchip MCP3202 ADC, but it does not have a separate Vref, only a combined Vref/VDD pin. I'll be powering it with 3.3V, but the power rail will be unavoidably noisy (around 100mV pk-pk noise), so I think it's a good idea to filter the power supply.
Because I don't want to greatly lower the impedance of the power supply then an RC filter is not suitable, right? So I'm thinking what I need is an LC filter. But still I'm a bit of a noob at all this kind of analog stuff and I don't really know what I'm doing, especially the maths.
I've been tinkering around simulating a low-pass filter that has a cut-off frequency of about 50 kHz with 10 uH inductor and 1uF cap, but it also seems to self-oscillate around that frequency, amplifying the noise greater than the input. If I add 10 ohms series resistance (i.e. turning it into RLC), it's fine. Should I be adding the DCR of the inductor as 'R' to the simulation? Most 10 uH inductors in 0805 package (what I plan to use) seem to have a DCR spec of 1.15 ohms. Or should I not be too concerned about needing to add 10 ohms, as the ADC only consumes about 550 uA worst case (according to datasheet)?
Hi,
It is difficult to predict what will happen with an LC filter like that. Luckily with that 10 Ohm series resistor it is overdamped. The lowest you can go on that is around 7 Ohms maybe a little less. But if you have to use a 10 Ohm resistor, why not go with just a large capacitor in parallel with a small ceramic like 0.1uf ? That's the more typical solution. Even more typical is just the large cap in parallel with a ceramic 0.1uf cap and no resistor.
It is interesting though that even a 2uH inductor and cap filter filters out a lot of ripple when used as a post filter on a buck circuit.