GND polygons/planes layout opinions for optoisolated digital inputs

[H. M.]

I am currently working on a design involving an ATmega48 12MHz MCU in
the digital area of the PCB and 8 galvanically isolated 24V opto digital
inputs located in another area of the PCB. The input isolator is a
standard Sharp PC847 general purpose photocoupler.

The layout will be a 2-layer PCB with two separate areas, digital
section on the left, input area on the right. I certainly will have as
much GND polygon and fill areas as possible on the digital side.

Now, is it recommendable to fill the non-copper areas on the isolated
input side with copper too? The 8 inputs have a common return which
could be used as the potential for the copper fill.

Ricky

 

[vasile]

I am currently working on a design involving an ATmega48 12MHz MCU in
the digital area of the PCB and 8 galvanically isolated 24V opto digital
inputs located in another area of the PCB. The input isolator is a
standard Sharp PC847 general purpose photocoupler.

The layout will be a 2-layer PCB with two separate areas, digital
section on the left, input area on the right. I certainly will have as
much GND polygon and fill areas as possible on the digital side.

Now, is it recommendable to fill the non-copper areas on the isolated
input side with copper too? The 8 inputs have a common return which
could be used as the potential for the copper fill.

If you need a high voltage isolation you can't fill with copper
everything just because you need isolation. If your design has
optocouplers just to be "interesting" and the optocoupler are isolating
low level voltage signals (24-50V) then you may fill everything with
copper (still using the 24V isolation between signal and ground area)

greetings,
Vasile

 

[H. M.]

If you need a high voltage isolation you can't fill with copper
everything just because you need isolation. If your design has
optocouplers just to be "interesting" and the optocoupler are isolating
low level voltage signals (24-50V) then you may fill everything with
copper (still using the 24V isolation between signal and ground area)

greetings,
Vasile
Vasile

The inputs are low voltage 24-30V and have a common return (COM).

There is no isolation between individual inputs on the card but between
the whole group of 8 inputs and the digital/microcontroller side. This
isolation shall withstand 500V and there is a 2.5mm gap between these
two sections on the PCB. Only the optocouplers are crossing this gap.

However the question to me is should I have also a copper pour on the
isolated input side connected to the COM potential which of course will
not be connected in any means to the GND of the digital side.

Ricky

 

[Rich Grise]

The inputs are low voltage 24-30V and have a common return (COM).

There is no isolation between individual inputs on the card but between
the whole group of 8 inputs and the digital/microcontroller side. This
isolation shall withstand 500V and there is a 2.5mm gap between these
two sections on the PCB. Only the optocouplers are crossing this gap.

However the question to me is should I have also a copper pour on the
isolated input side connected to the COM potential which of course will
not be connected in any means to the GND of the digital side.

Yes. A rule of thumb I always look at first is, you can't have too much
ground plane. (or too many bypass caps, but that's a different issue.)

Cheers!
Rich