Oliver, Michael, Erik
yes i want to isolate the MCU circuit from the 4-20 mA circuit.
Actually i am making a liquid conductivity Indicator cum Controller cum Transmitter[4-20mA]. The sensors will measure the conductivity and temperature of the liquid and the instrument will indicate, give relay outputs and 4-20mA in proportion to the reading. Hence the traditional loop powering is not possible in this application.

@Kai Klaas:
Meanwhile today i enquired about the costing of HCNR201 and IL300; The prices are too high [ Rather using the existing MCU->Frequency->voltage->current system will be cheaper than that :)) ].

PWM or external DAC seems to be a most suitable ways. My silabs MCU has 8bit PWM [ i think atleast 10 bit resolution is required to express the conductivity range of 0 to 2000mS/cm ? ].

Will anybody suggest a a good DAC of 10+bit OR a silabs MCU having 10 /10+ bit PWM [] ?

Thanks