How to take samples of 300V signal
Expanding on Pyxzure's comment, the simplest solution is really just using resistors, in a voltage divider configuration:



Find how much current the circuit(s) you want to measure need, divide it by a factor (say 10 or 20) and that current is almost certainly negligible and won't affect your circuit in any significant way. Call this small current i. Then, the resistor values can be easily calculated with (assuming you want a max of 3V):

R2 = 3/i

R1 = 300/i - R2

You can make the resistor divider draw as little current as you wish, just use bigger resistors, lowering the i value.

Also make sure that the power rating for the resistors is compatible. If you are using 1/4W resistors, for instance, make sure that P2 and P1 are both less than 0.25, where:

P2 = R2*i^2

P1 = R1*i^2

(Just checking P1 is enough)

If you really really want to , then there are suitable power Op-amps that you can use in a voltage follower configuration (such as digikey PN: 598-1917-ND), but that is not necessary.