18V Motor, do i need isolation ?

I am facing, i think, an isolation problem.

I use arduino Uno R3 to control a motor, through an H-bridge of 4 mosfets and 2 transistors, and to analog read the BEMF of the motor every 1/10th of a second during stopping of PWM.

My experiments was with 5 Volt motor and now i must use a specific motor using 18V, powering the H-bridge from external power supply. I need to read the analog values, so i need usb connection.

How can i do this without damaging the usb ? Do i need an isolation ?

( I have read about ADUM3160, is there any other suggestion? )

Thank you

You will not need a usb isolator if you do the connections, especially the grounding, right. But measuring the back EMF with a h-bridge requires that you either turn on one of the transistors or that you make a differential measurement.

Thank you for your answer.

Before the measurement i have disabled the pwm and i have waited 1mS. Do you think that i need something else?

What do you mean differential measurement ?

Have a look at the diagram.
If you want to measure back EMF you can measure between ground and tp1. But in this case S2 must be closed. This is called a single ended measurement. Or you can have all switches off and measure between tp2 and tp1. This is called a differential measurement and requires 2 inputs.

Ok, thank you.

I use the first method.

And resistive divider(s) to scale the voltage down to the 0…5V range, of course…