The pins 0 and 0 can act as dedicated pin for SAR bypass capacitor connection and as the Opamp negative input. These connections are not programmable. Unfortunately it is if you want to use the same piar of opamp and bypass option in SAR.The solution out of this(if you are using only 2 opamps) is to use the next pair of opamps Opamp2 and Opamp3 which make use of port3.
Thank you for the answer.
If I use a Psoc with only two OPA the SAR bypass capacitor pin are not more available when the OPA are activated.
Then I am looking for a backup plan.
If Vref is output to a analog pin where an external bypass capacitor is connected, will it improve the SAR behavior and Vref noise level?
You need to make sure that the opamp pair that do not use the 0,0 pins are used in your project. To make sure that you are using the right opamps, you can go to CYDWR -> Analog tab and check where the opamps are placed. You can right click on the placed component and relocate to the position you want.
Your second statement is right. The bypass capacitor enable SAR to work in higher datarate with better noise immunity. Without bypass capacitor the conversion rate will be limited.
Even though you are forced to place the Vref bypass caps on the dedicated pins, you are not also forced to do the same with the OpAmp pins. The latter ones are routable in the PSoC routing matrix.
I just places two OpAmps and a SAR ADC, and let Creator route anything by itself:
Routing for Vref:
Routing for OpAmp2 positive input (which is on P4_0):
The only drawback is that these routed connections now have an higher internal resistance (the Ohmmeter in the Analog tab says 1350 ohm from pin to Opamp input, vs. 450 ohm for the other OpAmps connection).
Thank you for the advice.
On Friday I tried to move the +input pin of the opamp and it seems suitable.
It brings around 800 Ohms additional serial resistor when the opamp input current is 15pA typical.
12 nV unbalace is neglectible in our design where the opamp gain is 1.
This is a good way to have the SAR Vref connected to a bypass capacitor and still use the opamp.