I'm doing a project on driving a solenoid valve in BOSCH. The input is a 0-10V sine wave (Which is actually an output from a D/A converter) and it is to be converted to a PWM signal of 300Hz with 0-100% range dutycycle. This duty cycle is to be used to inject current to drive a solenoid valve. For 100% dutycycle, the current should be ideally 1.4A to close the valve completely.
Would using a micro controller give a more accurate PWM signal?

That wasn't clear from what you wrote.

-You can use a simple voltage source. The voltage should be set to U= R . I.
-Switching the voltage on/off with a PWM signal: use the output of your circuit to drive a mosfet.
Fig 8: 8. Power MOSFET PWM Motor Driver

What you are trying to do is to convert a 0 ->100% PWM into a 0 -> 1.4A current source. Why?, why not convert your 0 -> 10V AC V into a 0 -> 1.4A current source. Or for a 1V ac input, are you are applying your 300Hz based PWM directly to the solenoid, so for 1V (10%), a pulse of 1.4A of .33mS length is applied every 3.3 mS?
If its just the 0 -> 10 V into a 0 ->1.4A, then build a precision rectifier as per inventor(y)'s suggestion, then use an op-amp with a power transistor using the solenoid as its collector load, and a low value resistor (1 ohm) in the emitter circuit. So now you have a monitoring point for the solenoids current. At 1.4A out, there will be 1.4V dropped across this resistor. So connect your feed back resistor from this point to the - input of the amp with another resistor to earth also from the - input. These resistors set your range in terms of Vin/Iout. Feed the actual voltage into the + input of the op-amp.

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