Filter(s) before low-noise voltage regulator? - Page 1
Forum section: electronics / projects

author: awallin
author_rank: Frequent Contributor
timestamp: December 30, 2015, 07:27:39 pm
content: For my frequency distribution amplifier I want a low-noise dc-supply.For the first prototype I chose the LP38798 regulator. Someone then suggested LT3042 but that would require multiple in parallel for sufficient current.Analog seems to also have these low-noise RF/analog regulators e.g. ADM7150 or similar.Anyway the amplifier needs to work in a noisy environment with a non-ideal +12 VDC supply. So we need filtering - the more the better.I used a TDK common-mode choke and a Murata 3-terminal NFE61 filter on the prototype. Then someone suggested the Murata BNX-series.I plotted the PSRR of the regulator along with the filter insertion losses. To me this looks like:DC - 10 kHz: regulator has enough PSRR (?).10 kHz - 1MHz: doesn't look good - need a new filter here?1 MHz and up: BNX025 looks good. It seems I can completely remove the NFE61 as it doesn't contribute anything useful!??AFAIK all of these are for differential noise. The common-node choke might still be a good idea..Comments and links to working circuits would be appreciated!thanks,Anders

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2015, 08:29:31 pm
content: The filter function above 100 kHz depends very much on the layout. Also the filters are interacting and might well profit from an extra cap between the filters. The caps at the regualtor also adds some filter function, especially in the 10-100 kHz range. Even the comon mode filter will provide some differential mode inductance.Also PSRR of the regulator depends on the caps behind the regulator and the layout. The ouptut impedance of the regulator is also important: a high output impedance in the 100 kHz range also has a positive effect in providing extra filtering together with the ouptut caps.

author: awallin
author_rank: Frequent Contributor
timestamp: December 31, 2015, 07:08:22 pm
content: Thanks for these comments.Based on my previous plot I decided to add the BNX025 and remove the NFE61.To minimize interaction between the filters I added ferrite beads between them. These can be 0R resistors or inductors also in the prototyping stage.Here is what I have now as schematic and board layout. The board has the circuit-GND copper pour removed under all parts before the BNX025 output.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 03, 2016, 04:57:59 pm
content: capacitance multiplier is popular for this

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2016, 05:08:30 pm
content: NFEs are only for signals where you want to avoid the bother of discretes. They use ferrite beads and therefore saturate easily with DC current (over probably 20mA).And speaking of, if you want a discrete solution, to some extent you can combine the stray/leakage inductance of CMCs with parallel caps, to get some differential filtering, but otherwise, you're looking at adding a few power inductors, rated for the current and of suitable DCR. For picking values, you'll have sqrt(L/C) = Zo (whatever the characteristic impedance is -- for a PS filter, it might be arbitrary) and 1/(2*pi*sqrt(L*C)) = f_0 (the cutoff frequency).To solve the arbitrariness of impedance, you'll probably need some lossy capacitors -- usually electrolytic (ESR has a wide tempco, though), tantalum (stable; expensive, conflict minerals) or MLCC + resistor. Construct a "lossy capacitor" by using the desired capacitance, in parallel with a "bulk cap" having a capacitance of 2.5x the desired value and ESR = Zo.You can play with filters in any SPICE simulator, but keep in mind, the stop band results will be meaningless without a suitable model of ESR, ESL and complex (higher order) behavior. Likewise,commercial filter products are typically measured at a system impedance (like 50 ohms), so don't expect exact results in your circuit.To solve higher order attenuation, you should probably chain a few filters of different cutoff frequencies.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: January 03, 2016, 06:43:20 pm
content: Since you are dropping a few volts already, you can plop in a RC filter. Calculate R at max current, voltage drop.It is a dirty trick, no-one seems to use.

author: Marco
author_rank: Super Contributor
timestamp: January 04, 2016, 06:12:38 pm
content: Shouldn't you be using a proper feed through filter (ie. soldered/screwed into the shield) first and foremost?