I just received my order from digikey of MCP6002 which are supposed to be rail to rail, i popped the LM358 out of the circuit replacing it with the new chip and i was delighted to see the output going down to 0V but the high at the output is 4.5V and i was expecting 5V since that is my supply voltage to the circuit which is an astable oscillator running at ~ 1Hz, The output goes via 240R to an led to ground.

With the LM358 my output goes from .6V to 3.3V

With the MCP6002 my output goes from 0V to 4.5V

Why am i not getting rail to rail from the MCP6002?

 

Might be too much of a load.
See what it reads with a 1K resistor on the output.

 

Might be too much of a load.
See what it reads with a 1K resistor on the output.

That fixed it....i now output 0V to 5V square wave at 50% duty cycle which is what i needed to see.

thank you so much!

 

I just received my order from digikey of MCP6002 which are supposed to be rail to rail, i popped the LM358 out of the circuit replacing it with the new chip and i was delighted to see the output going down to 0V but the high at the output is 4.5V and i was expecting 5V since that is my supply voltage to the circuit which is an astable oscillator running at ~ 1Hz, The output goes via 240R to an led to ground.

With the LM358 my output goes from .6V to 3.3V

With the MCP6002 my output goes from 0V to 4.5V

Why am i not getting rail to rail from the MCP6002?

Because you are operating it well outside of it's specified operating range.



The specifications have a load of 10 kΩ, not 240 Ω and an LED. Depending on the LED, you are asking it to supply more than twenty times the current that it's rail-to-rail specifications are rated for.

Remove your load and see how close it gets.

To drive an LED, especially from a low-power opamp, use a transistor between them, either as a switch or as a voltage-follower (either should work for you, each has some pros and cons).

 

Note that the rail-to-rail output is for a very light load.
Any significant load will cause a small voltage drop, making the output less than the rails.