Help with isolated grounds

[kmike]

Dear members,

I am currently designing a board with isolated 24V digital inputs and +-10V analog inputs. The board would have several input blocks wich are isolated from each other and the microcontroller, but should still have a PE (earth) connection.

The current design is attached: 24V_DI and GND_DI are the power input and refernce for the digital inputs, 24V_DI2 is the power supply for the digital isolator chips.

As never having to design such a circuit, I have some questions:

1, is the attached way the proper way to do this?
2, should C3 be a ceramic or an electrolythic cap?
3, should I tie the grounds together with some high value resistors?

Thanks in advance!

[Terry Bites]

Right off the bat.

If both sides of the isolation barrier are connected to a common PE, then you've circumvented the isolation barrier.
Connecting individual input blocks to PE also defeats the inter isolation of these blocks.
You have to choose to ground one side of the barrier or the other or neither. Most rationaly at the micro side
It would be helpful if you drew a block diagram of your systems.

Tying the grounds together though high value resistors will might equalise static potentials, but it renders PE pointless, there will be no protection. You might think about connecting various grounds together via bidirectional TVSs to PE. The isogrounds only connect to PE during a fault condition, else just a very small leakage current flows between blocks and PE. The TVS has to be able to comfortably handle the full fault current until the line breaker or fuse pops. Vf drops won't matter in fault condition. A 6V device would keep all the grounds within 6V of PE for the duration of an event. 

You're aiming at multiport isolation including separate iso power supplies for each and every block except the micro?  A lot of DC-DC converters then.
What risks does your system need to mitigate against. Why is isolation needed? Why cant the inputs be differential pairs? 
Why do the inputs need to be tied to PE? 

What does C4 do? The Zener needs to be replaced with a TVS, it will clamp over-voltages much faster than a Zener. 

I played with some AMC1333M10s from Texas Inst recently. Powered from NXE1S0305MC-R7s. Easy. Next step, sort out digital filtering. Harder.
If I were more of a digital man I'd be looking at a ready rolled solution, ADE1201s @$10 a channel.