Hi Guys! I need to use a symmetrical voltage source as an input in an amplifier project. My question is: in the connection with the top do I use the 'vssa' pin as input for the negative voltage?

Research single supply amplifiers. Let's say your VSSA pin is connected to 0 V and your VDDA pin to 3.3 V. The ground level should be something between those voltages. 1.65 V is a good point. Of course, depending on your amplifier topology, its common mode input range could also include the supply rails. https://mil.ufl.edu/4924/docs/TI_SingleSupply_OpAmp.pdf

Thank you for the suggested. I would like to know if the VSSA pin of the caravel can be connected to a negative voltage of -1.8V. I am simulating with +-1.8V symmetric source.

It depends on the kind of transistor you are using. If you're using the standard 1.8 V transistors, you can connect VSSA to 0 V and VDDA to 1.8 V. You could also connect VSSA to -0.9 V and VDDA to 0.9 V. For the thick oxide devices, the maximum VGS possible is 5 V, so you could use higher voltages.

Right. Thanks for the answers.

@User: You cannot connect any of the Caravel VSS* pads to a negative voltage relative to the digital ground (VSSIO) or else it will forward-bias the ESD diode. You could, I think, run a true -1.8V supply through one of the straight-through pins on the Caravan chip and run the negative voltage analog circuits in an isolated pwell. But Luis is right that your best option is to place your analog ground at the midpoint of the digital power supply; the digital can be run at 3.6V so you can have your analog ground equal to 1.8V on the digital supply, with analog -1.8V equal to digital ground and analog +1.8V equal to the 3.6V digital supply.