# Error in the minimum value of the operational amplifier output

I designed a circuit to control the speed of a 24V coller. For this I used the operational amplifier TCA0372DW.

As I am going to control the output voltage through a PWM signal that comes from the STM32, it was necessary to create an RC filter to convert the PWM signal into analog.

0% duty cycle = 0V

50% duty cycle = 1.65V

100% duty cycle = 3.3V

I determined the gain of the amplifier as follows: Vo / Vi = (R30 / R27) +1

With that I have a gain of approximately 7.3. Theoretically for:

Vi = 0V ------ Vo = 0V

Vi = 1.65V ------ Vo = 12.045

Vo = 3.3V ------ Vo = 24.09

But this is not happening for input voltages near 0V. With the minimum input value I have at the 4V output and this affects the functioning of my system.

I know that there is a saturation in the amplifier that makes it never reach the maximum value of 24V. But I couldn't understand why for values close to 0V the output remains at 4V.

• Return R27 to 1.2V (made by a resistor to +V and 2 diodes). Aug 5 '20 at 21:41