I came across this circuit (http://www.linear.com/solutions/1185) and I was wondering how the voltage across the 10meg resistor, always remains 1/1000 of the input voltage. Any ideas?

 

I came across this circuit (http://www.linear.com/solutions/1185) and I was wondering how the voltage across the 10meg resistor, always remains 1/1000 of the input voltage. Any ideas?

Uhhh... look carefully: the very first thing Vin is connected to is a 1000:1 voltage divider. There's your answer.

 

Oh ok, got it. Moderator, you can delete this if you want...(it's pointless)

 

Oh ok, got it. Moderator, you can delete this if you want...(it's pointless)

Circuit in question:


There's a little more to it than that. The output of the top opamp is always going to be greater than 1/1000 of the input whenever there's a load.

If you look at the feedback loop, the bottom opamp is a voltage follower sampling the voltage across the load. It's output is fed into a difference amplifier that's subtracting the voltage across the load from the voltage across the 10M resistor+load. That output is fed back to the top opamp so it can keep the voltage across the 10M resistor 1/1000 of the input voltage.