The resistorsy together with the C form an input filter and shift the input to Vdd/2. This shifting avoids voltages below GND and above Vdd.
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!
" Have a great weekend!"
I'll do my very best! ;-)
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!
You are always welcome !
Don't forget, the input to the A/D looks like HP filter, break at ~ 1.6Hz,
due to 100K and 1 uF. So measuring DC is not a feature of input
Other approaches to doing offset which do not give up DC, attached.
ADinput negative inputs24.zip 644.7 K