It is possible to use the SPI bus when the Vref of MCP3201 ADC is 2.5V? On the setup i just finished, I used two linear regulators: 5V -> 3.3V to power the MCP602 op-amps and MCP3201 ADC, and another one 5v -> 2.5V for the Vref. Everyone recommends to connect the Vref from MCP3201 to Arduino's 5V but this does not apply in my case.
The green connector is connected to GND and ADC's Dout where the measured voltage fluctuates from 0.180V when i touch the sensor and his shield, to 0.6V when i'm not touching those parts. I have no ideea if this is a normal behaviour but everything looks fine to me. Voltage measured at the output of A4 ( MCP3201 +IN ) is 0.85V and the current Iref is 1mA(through the yellow jumper) as they recommend.
Sorry if these details are a bit off-topic but i think they are relevant to the ADC role in this setup.