cancel
Showing results for 
Search instead for 
Did you mean: 

PSoC 5, 3 & 1

ruha_285291
New Contributor

Hi All

   

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?

0 Likes
7 Replies
Bob_Marlowe
Expert II

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.

   

 

   

Bob

0 Likes
ruha_285291
New Contributor

 Thanks Bob.

   

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!

   

Roop

0 Likes
Bob_Marlowe
Expert II

" Have a great weekend!"

   

I'll do my very best!  😉

   

 

   

Bob

0 Likes
ETRO_SSN583
Esteemed Contributor

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

   

to A/D.

   

 

   

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.

   

 

   

Regards, Dana.

0 Likes
ruha_285291
New Contributor

 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!

   

Roop

0 Likes
ETRO_SSN583
Esteemed Contributor

You are always welcome !

   

 

   

Regards, Dana.

0 Likes
ETRO_SSN583
Esteemed Contributor

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

   

structure used.

   

 

   

Other approaches to doing offset which do not give up DC, attached.

   

 

   

Regards, Dana.

0 Likes