Dumb question of the day! was just looking at the Cypress tutorial on Digital Filtering. Can anyone please tell me why the resistro connecting the Opamp Follower to the ADC is 100K? The opamp datasheet states that it can handle 10mA and if running at 2.5V, this results in 25uA. Is there something that I have missed or am I just being extra dumb today?
Understand the R/C filter and the level shifting - just wanted to make sure that was all that was going on before I vaporise the silicon in my 5LP. Have a great weekend!
There is no effective filtering action due to the 100K. It is strictly
there to provide a DC path to match the bias on the other input
Under ideal conditions, no current flows into the A/D, hence there
would be no drop across the R100k, its effectively not there. Even with
input bias current on A/D the drop is still negligible.
The only possible "filter" action would be from A/D input parasitic C
and that resulting pole would be so far out in the spectrum to provide
any appreciablke noise filtering from the OpAmp or Vref.
Its value of 100K is a compromise of drop/error due to leakage and
maintaining a resonable input Z, see datasheet specs.
Thanks for the clarification Dana!
I was looking at the output of the OpAmp and forgot to look at the A/D. Want to have a play with this circuit, so your input is very much appreciated.
Have a fantastic weekend!