I suggest you to use one high current power supply for Flex and your electronics ( using the +V and gnd terminal of your power supply for powering separately your electronics and Flex).

...ok

Then you can use optocouplers for using the digital signals of Flex. The optocouplers will allow you to use different voltages for your electronic side.

What do you mean exactly ? Even if I use the 12V as power supply for the Flex and my power curcuit separatly, can I still use the 5V and 3.3V from the Flex for my digital devices ? Or it's better to use linear regulator direclty in my power circuit to obtain the 5V and 3.3V ?

The ground reference will be the same in any case (my power circuit, Flex, dsPIC etc..) ? I need to generate a PWM output is it that the low level is 0V and the high level 3.3V ?

Than you,
Pasquale.

Even if I use the 12V as power supply for the Flex and my power curcuit separatly, can I still use the 5V and 3.3V from the Flex for my digital devices ?

No, because you will have a different ground reference. Like an example, if you use the PWM output of the Flex board (0-3.3V) you will have less than 3.3V on your electronic because of the rectifier diode (GND_alim is different to GND_OUT). The gnd reference is not the same.

As a solution you can generate the PWM from Flex and than you have two solution:

1- Use the V_OUT and GND_OUT. In this case you do not have problems in terms of voltage references and so on and you can use directly the PWM to drive you electronics (I think that this can be a digital low-power IC) and use the V_OUT,GND_OUT for the power stage of your circuit.

2- Use the V_alim and GND_alim. In this case you will need an additional regulator (i.e. LD1117 3.3V). Your reference will be GND_alim so you will have to use a fast optocoupler to keep the isolation between the GND of the two circuits (see picture )

Sorry, this is the image...

Sorry...

Ok,
I understood what you mean. I'll just pilot the other optocoupler's pin otherwise I'll have a "NOT" PWM. I'll connect the diode LED's anode to the FLEX 3.3V and I'll pilot the cathode, of course I'll not have more the 16mA as current using the right resistor. Could be ok ?
Another question : I have also a current transducer. This one needs 5V and of course the 0V (ground). The maximum current that it absorbs is around 30mA. Where do you suggest me to connect it in order to have the right references and the right measures ?

Thank you,
Pasquale.

I'll just pilot the other optocoupler's pin otherwise I'll have a "NOT" PWM

Not properly, I mean that you will not have the right reference voltage. The 3.3V are 3.3V for the Flex board considering GND or GND_OUT. If you take the ground reference of the power supply GND_alim, this will be GND_OUT + or - 1,2V (the voltage across the rectifier diode).

I'll connect the diode LED's anode to the FLEX 3.3V and I'll pilot the cathode, of course I'll not have more the 16mA as current using the right resistor. Could be ok ?

You can drive the optocoupler with a buffer that will amplify a little bit the current from the dsPIC (es 74HCT244 search on RS) or you can simply drive the LED of the optocoupler with a BJT transistor (BC547) using the +5V output of Flex and a 1KOhm resistor for the _base_ and try with a 100Ohm on collector. You can connect the diode directly between +5V (anode) and the collector resistence.

The maximum current that it absorbs is around 30mA. Where do you suggest me to connect it in order to have the right references and the right measures ?

I think that the current transducer that you are going to use has a small resistance in order to measure current, I am thinking to the MAX471. This could be placed in series with your load while the logic part that is isolated from the measurement resistence can be directly connected to GND (Flex) and +3V (Flex) and the voltage output can be sampled with dsPIC.

Ciao,
Francesco.