Buffering a DC voltage for the ADC

Which op amp do you use as a voltage follower when you need to buffer a DC voltage for the A/D converter of a PIC uC?
The op amp should work with single supply. My input voltage is about 1.5 V max and I have some LT1013 op amps. Should I choose another op amp with better "capacitive load capabilities"?
Thanks for any opinion.

eng1 said:

Which op amp do you use as a voltage follower when you need to buffer a DC voltage for the A/D converter of a PIC uC?
The op amp should work with single supply. My input voltage is about 1.5 V max and I have some LT1013 op amps. Should I choose another op amp with better "capacitive load capabilities"?
Thanks for any opinion.

Click to expand...

With single supply operation, you have to make sure that whatever opamp you have chosen, it would give zero volt output for zero volt input. This is a difficult criteria to fulfill.

I know that... and tha LT1013 op amp can go very near to ground (about 5 mV), but my input voltage will never go down to that level... also bandwith is not critical for this design.
What about the capacitive load? At the analog input of the PIC we have the input capacitance plus the sample capacitance, which is dinamically linked to the output of the op amp (even if it's not discharged after each sample).
I use a voltage follower to change the impedance for the ADC... I'd like to know if a general purpose single supply op amp is good...

PIC datasheet shows that he A/D input capacitance is only ~120PF and this is really a small capacitance for any opamp output stage to drive without problem especially you are not likely to have the opamp output changing at high frequency.

Thanks for your answers. Yes.. 120 pF is a small value, but it can be dangerous?
It is quite big, if compared to other ADCs. My doubts arose after reading this app note: https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1022,P1107,D4308

The time scale inside the quoted document are in 200nS/division, which is a really fast signal.

Ask yourself this question, how fast can one perform a new A/D conversion on a PIC running at 20MHz? Tens of microseconds I suppose?

You might try my PIC tutorials?, where I used a negative voltage generator to cure the 0V rail problem, and a 2.5V precision reference to cure the 5V rail problem.