What is the role of VREF3V0 and VREF1V5 in function Generator circuit?

[victory460 0]

Dear engineer！

     What is the role of VREF3V0 and VREF1V5 at the input of the IC10A op amp? 

     What effect of it?

[Fa-b 3]

They are to get a negative supply out of the positive digital output from the uC. Since the output of the uC is between 0 ... 3.3 V max.

VREF1V5 is the node determining at which point the IC10A will switch from positive output to negative and vice versa. The opamp will always attempt to keep the difference between inverting and non-inverting inputs zero.

VREF3V3 is a pullup of the inverting input, if the inverting input is pulled below 1.5V, the IC10A output will become positive in order to bring the inverting input back to 1.5V.

On the other hand, when the inverting input is above 1.5V, the output of the IC10A will become negative to bring the inverting input back to 1.5V.

 

I guess VREF3V0 could have been a higher voltage as well. But VREF1V5 should be as close to the center of the uC supply as possible in order to achieve symmetric output and the best resolution thereof.

I'm also guessing they were not willing to rely on a stable supply voltage from the linear 3.3V regulator and probably already required a precision 3V reference for other purposes.