Voltage to frequency converter, input protection problem

[al_extreme]

I need some help about building à Voltage to frequency converter. I think I qill use the lm231 from National but I have some question for the input. I need to convert 0v to 40 v and output 0 to 800 hz that look easy. I need to protect the input to a maximum of 300 v without bugs, so I need someting that can bock the tension at 40v, can you help me plase

 

[flatulent]

voltage divider

You can use a 1/4 resistive voltage divider to get your 40V down to 10V which the chip can use. Then put a zener diode across the lower resistor to clamp the 10 V to something like 12 V or whatever is over 10 and less than Vcc. Have the resistor values such that putting 300 V on the input will not cause enough current to flow to overheat the resistors or the zener.