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Isolated Grounds

foTONICS

Sep 30, 2011
332
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Sep 30, 2011
Messages
332
Hey all,

So I'm putting together this circuit to test a prototype board and I'm unsure if connecting the grounds on each side of an isolated DC/DC converter is a bad idea. So here is a brief explanation of my circuit:

1. I will be powering this device from a 12V supply
2. The rotary encoder has to be isolated from the mcu which is powered with 3.3V
3. The 2 signals coming out of the rotary encoder need to be level shifted from 5V to 3.3V which are in turn pulled high to 3.3V via R1 and R2

My problem involves the jumper I have connecting the grounds below U3, Will shorting the grounds negatively affect my circuit? I feel like I need it since the signals coming out of U4 and U5 won't have a proper reference.

Clipboard01.jpg

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Sunnysky

Jul 15, 2016
541
Joined
Jul 15, 2016
Messages
541
The encoder signals might need be optoisolated, only if there is high voltage between encoder and mcu.
For long distances differential signals may get CM interference, so need to be balanced such as RS485 or a large ferrite balun or twisted pair.

It will not work if the optocoupler uses an output logic ground which is not shared with mcu.
schema. errors.
gnd is defined on U4,5 should be same as U2 out, and mcu.
if U2 =gnd then U3 can be an LDO instead.

Are U3,4,5,mcu on same board or close ?
If you have EMI issues , it would be easier to invite me to see than explain.
 

pgib8

Jul 26, 2015
107
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Jul 26, 2015
Messages
107
are those optocouplers? if so you already have two grounds on each.
one is the cathode of the led, the other is the GND for the logic output.
so what you have marked as GND should be the same one that goes to the MCU.
Thus you don't need to join the two grounds and you can keep them isolated. If you do want to keep them fairly lined up, just use a large resistor, like 10K or more to join them.
Also your optocoupler inputs, I don't understand your diode and TVS/zener setup, but I do think you should probably have a resistor in series to limit current through the photodiode in the optocoupler.
 
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