Sun

While I agree fully with everyone who said to use isolation, I just thought you should know that the are some instances where you need an differential amplifier with a high common mode voltage and that such devices are available (for better or worse). I repeat, I don't think your application is the right application for these, but for anyone interested in the topic, you can check out the TI INA148 with a common mode voltage of
(+/-200VDC) and the Linear Tech LT1990 with a common mode range of (+/-250VDC)

See
http://focus.ti.com/docs/prod...na148.html and
http://www.linear.com/pc/downlo...7521,D5087


Depending on the accuracy and your budget there are many ways to isolate the input. You can buy a complete system from Analog Devices ( http://www.analog.com/en/subCat...5F,00.html )all the way through to using a cheap Voltage to Frequency converter across an optoisolator. In the latter case, you will still have to derive some power from your 300V, or transmit power across the barrier with some isolated power supply (of low current requirement). I might suggest something from C & D Electronics.
http://store.cd4power.com/cgi-bin/c...76/Catalog

Take extreme care
-Aubrey