Error in the minimum value of the operational amplifier output

Question

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.

Answer 1

Page 3 of the spec sheet indicates that the output swing of that part is 1.3V to 22.7V nominal. If you want it to go all the way to the rail, you need a rail-to-rail part or a negative supply.

Your first stage is probably pegged at somewhere around 0.6V (it's allowed to perform better than the spec). The factor of 7 gain in the second stage brings it up to around 4V.

Answer 2

In this configuration, the minimum output voltage (no load) is approx. 0.5V. This is the first amplifier. The second amplifies this signal 7.3x, so its output will have a minimum of 3.65V. At least the first one should be replaced with a right one. Eg LM358, then the output voltage will be 5mV without load at first amplifier.