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Isolated fixed low voltage power supply and serial interface

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Hi All.

I designed an isolated fixed low voltage power supply and serial interface board. Design idea is coming from Dave's uSupply board: http://eevblog.com/files/uSupplyUSBrevB.pdf He used optocoupler whereas I used a dedicated isolator IC. You can find the eagle based schematic and layout files attached.

Sometimes i need to send ADC results to the PC for further analysis but also I don't want to inject noise to my measurement system from PC ground. Isolated serial link is perfect for this. Also i used a very low noise LDO - LT1763 - which can be used to supply power to host circuit. Lastly, 1V65 supply can be used in single supply opamp based circuits.

I guess this will be a very usefull little tool which helps people to prototype their low noise measurement circuits.

What do you say? :) Comments are welcomed.


You should put component values (resistors and caps) on the schematic. You also might want to omit the text (email address?) on the bottom copper layer. It just reduces the effective isolation distance. Put it somewhere else, or make it a silkscreen instead.
Other than that, it looks great. :-+

Rotating the USB chip 90° CCW would make for a neater (shorter) traces in that area?

I don't understand why use the op-amp to get 1.65v and why you'd use such a weird regulator (with difficult to solder footprint).

A simple generic 1117-33 in to252/to-263/dpak form

for example

would work just the same and be easier to heatsink with just some copper pad under it. It's not like you won't waste just as much power as heat with that LT1763 getting 3.3v out of 5v, yet you just make it harder to solder it on the board.
The 1.65v could be done with a plain 1117 adjustable in to-261 form :

Basically , rather than using 3 separate parts with capacitors and resistors, you reuse same component, reduce bom, get better prices etc.

later edit:

move the traces for rx,tx,rs all the way to the side, move the 3.3v regulator just a bit more to the center with the heatsink tab kinda where those 4 capacitors are, and  your input and output capacitors by the insulation gap, away from heat. you'll have a larger area to act as a heatsink.

I'd probably put the rx,tx,rs, gnd, 3.3v on the far right side, aligned to a side and maybe with a 2row 10pin header (but 1 row, 5 pin is also ok) so that you could use commercially available cables (0.1" / 2.54mm spacing), or to just press fit it into a breadboard without blocking the whole breadboard.
Some space further to the right corner , put the gnd , 1.65v , 3 v .. again aligned at 0.1" (2.54mm, for breadboard) ... basically make it as if the holes are a DIP14 or something like that, with the middle pins missing.

Even if you don't care about breadboard, if the pins are all on one side it would be better because the pcb won't flip on the desk due to cable weight and it would use less desk space (the cables for serial won't get on the outside of the board, don't know if I explain it properly)

What is the 1.65V for anyway?  The only thing I saw connected to that was an LED which is unlikely to work at that voltage anyway unless it's red.


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