# 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

## 2 Answers

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.

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.