ferrite beads vs RLC?

[CopperCone]

If the frequencies are limited, can a RLC filter be used instead (easy to design), so long the SRF of the L is high (these are more expensive then beads, but I don't care), and the inductor is high quality.

This is to filter noise from a DDS chip, after the signal went from differential to single ended. The frequencies are ultrasonic, but I don't  want to contaminate the up-stream components with microwaves.

I have seen something like this used in a Wavetek generator, it filters the signal through a little board of HF parts before it goes to the filter.

Do the little LPF modules have better performance? I found that the drift added(simulation) to the AC level is sub 1ppm/degree C, so long I design it like 1 octave above my frequency of interest.

[T3sl4co1l]

Sure, why not?  Or just RL as the C is usually undesired and incidental.

It's not necessary for general signal applications, but if you need more accurate filtering, it's better to use real inductors than ferrite beads.  In power supplies, it's required -- ferrite beads saturate over (depending on size) 10-200mA.

Tim

[CopperCone]

I need to see the emissions of the DDS to decide filter order. Annoying to have to prototype that.

I wonder if the emissions are temperature sensitive. A bit I bet. And different between batches. Could make things hard.

I guess I should put the DDS in its own shield. And around the op amp, so the signal goes strait into the filter outside the shield, as its 200MHz clock. I feel like noise is going to get blasted into the power rails though, through the op-amp. I guess best bet would be to put another HF filter at the output, I guess a LCL filter to also block transients some, and external noise.

Why all this? To do HP justice.

The DAC output on my wavetek function generator is pretty bad, the upstream filtering makes it reasonable. (when I was repairing it I looked).

Perhaps some filters on the power pins of the instrumentation amplifier (like 2 orders of magnitude above the frequency its doing) would help? Don't care about DC here, goes through a BPF
How much headroom does the power rail of a IA require in terms of impedance  vs its operating frequency