[time-nuts] Looking for a low power very low noise DC/DC converter (100 - 200 ma 10VDC or 15VDC)

[ed breya]

I should mention that the input supply filtering to the DC-DC converter 
should have good attenuation at the switching frequency, but not at low 
frequencies, so there's no need to get carried away with the size of the 
filtering at the converter input. Too much filtering, especially 
inductance, can be detrimental, depending on the type of converter used.

If the converter is a simple chopper type without regulation, it will 
put out in proportion to input, so line rejection will be poor. If line 
ripple is an issue, then common solutions are brute force more filtering 
with capacitance, or linear regulation at the input or output.

If a self-regulating type DC-DC converter is used, and uses PWM, it will 
usually have pretty good line rejection, so eases the low frequency 
filtering requirement. However, since its input resistance is negative 
at low frequencies, it affects the choice of input filtering. Avoid 
overdoing it with L-C input filters that cut at too low a frequency or 
have high Q - it can result in oscillation. Lots of C is usually OK, but 
not too much L.

Ed