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I'm looking for a solution to regulate power and provide isolation for a 3.3VDC system. I am using a 5VDC 500mA power supply that needs to be regulated to 3.3VDC with isolation to protect analog sensor signals from noise.

The circuit can draw up to 200mA and as little as 2mA. I've been looking at isolated regulators and it seems that there is a minimum load for the component to effectively regulate voltage based on my understanding of the datasheet. Is this accurate and if so is there a good solution for isolation and regulation of this circuit?

Example regulator: MEU1S0503ZC

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    \$\begingroup\$ Can I just add 80Ω 1/2 W resistor in series with the power supply to create the minimum 40 mA load? \$\endgroup\$ – neufuture Mar 9 '15 at 7:00
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    \$\begingroup\$ Is noise the only reason you want to isolate these sensors? Because isolated DC-DC converters will introduce a whole lot more noise that you then have to suppress; you're better off just suppressing any power supply noise without isolation. \$\endgroup\$ – Nick Johnson Mar 9 '15 at 7:33
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You put the horse in front of the cart by presuming that galvanic isolation is the only solution to your problem. Do you even know what your problem is? Have you measured the noise on the input? How did you measure it? Broadband measurements are hard!

There is no such thing as a perfect galvanic isolation. If you're dealing with bad high frequency common-mode noise, the primary-to-secondary capacitance in an isolation transformer will happily transfer that noise to your isolated circuit. The rest of the circuit, and its output cabling, will happily convert the common mode into differential mode, corrupting your signals.

The first step is determining what kind of a noise you're dealing with. If you're using a switching wall-wart, common mode noise is a real deal and should be addressed first. Typically, a good mid-frequency common-mode choke placed on the 5V input, followed by a high-frequency common-mode choke, followed by differential mode low-pass filter(s), will work quite well. The isolation can be used to break a ground loop, but it won't magically help with noise, and might be completely unnecessary.

The general idea, shown below, is to:

  1. Have common mode filtering first, as any asymmetric circuit will convert common mode into differential mode.

  2. Have high-frequency attenuating elements first, before the low-frequency element's parasitics get excited by the high frequencies.

L1 can be a high-frequency common mode choke, say with attenuation peak around 25MHz, followed by the choke L2 with attenuation peak of a MHz or less. L3 is to get rid of excessive ripple, and can be a part with high series resistance to lower the Q of the circuit. FB1 is a SMD ferrite bead. C1 and C2 are "small", on the order of 1-470nF. C3 and C4 can be 1-47u, depending on what U1 needs for stability.

Other parts of your circuit certainly can pollute the 3.3V supply - it's impossible to ascertain how bad of an effect they have without knowing, well, what's there.

schematic

simulate this circuit – Schematic created using CircuitLab

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