I would like to convert the DAC output of a microcontroller from 0-3.3V to 0-5V. I will use the MCP6002 (rail to rail.) I have a power supply with 5V output.
Can I get exactly 5V on output (Vout) or should the supply voltage (VDD) be slightly above 5V (like 6V or more) combined with a pull-up resistor?
Alternatively, could this opamp achieve an output closer to VDD?
If everything above does not work, is the following approach correct?
No, 5V out is not possible with 5V supply.
The device datasheet says that typical output is 25mV less than the positive supply, so you do need a supply that is slightly above 5V. In theory 5.025V would be enough but allow for some margin.
Also, same applies to 0V output. 0V output is not possible with a 0V supply. You need a supply at least 25mV below 0V to output 0V.
When the output of a rail to rail opamp clips to VCC or GND, the corresponding MOSFET is fully ON.
(source)
So the RdsON of that FET is still in series. How close it will go to the rail depends on output current. If output current is very low, it can get pretty close, otherwise that depends on the FET RdsON.
Note a pullup resistor won't really help if current is drawn from the output.
But you have another problem: if you need 5V out, and you use a 5V power supply, it most likely comes from the usual 5V regulator which isn't that accurate. If it is +/-5% accurate, then expect 4.75 to 5.25V. If your regulator is a bit low, but still within spec, you won't get the 5V you want.
So personally, I'd use an adjustable regulator instead of fixed, and aim for the opamp supply voltage to be high enough to get the 5V you want, accounting for the opamp's maximum output voltage (which depends on output current) and also tolerance of regulator and voltage setting resistors.
Likewise the accuracy of your 5V output also depends on the voltage reference for your DAC, the gain setting resistors on the opamp, and the opamp's gain error.
So, time for an error budget calculation.
There are newer and better Rail-to-Rail OA’s now capable of 90 mA (TLV246x) or even 200 mA (TLV411_) which means at low load current your DAC can approach the rails with less voltage drop when accuracy counts at these limits. Consider and state all your requirements as there are dozens of specs to choose from before leaping to old standards.
