Hello everyone, and thanks for looking at my project. I'm fairly new to electronics but I have attempted this circuit. It is an update to an older design which used parallel port input and bipolar supply (+/-12V). None of the bypass caps are shown on the schematic and I left out some of the details (resistance values etc.) which had no room. I made one on perfboard and used a 19.5v laptop power brick to power it and it seems to function properly.

My question is about creating a common reference between the USB-serial IC (cp2102 chip/computer) and the circuit. It doesn't seem right to connect the USB's ground to the "floating" virtual ground, I am concerned that there would be a potential difference. How can I create a common reference between this circuit and the computer it connects to?

Any other suggestions would also be welcomed!! Thanks

 

Is the earth ground symbol not earth ground?

 

Sorry, I used the ground symbol for the "floating" reference. It should be halfway between v+ and v-.

 

You could connect computer ground and circuit ground together via a resistor, if you feel squeamish about just joining them with a wire. But if your circuit truly is floating, there's nothing wrong with tying its 0v line to computer ground. Just be certain that anything else that you connect up has a compatible ground level!

If you're concerned about this, you could have a few components operating off the USB power supply, and connect to the rest of your circuit via optoisolators. Then you can have it at whatever voltage you want, but I'd be more comfortable with a ground voltage that matched the computer.

 

Thanks John. So, I could connect the power supply negative to the computer ground, and then I'd have to bias the data lines to the usb adapter (like bring them down 1/2 the supply volts) because the microcontroller is on the virtual ground? What would happen if I connected the circuits virtual ground to the computers ground? I changed the labels on the image to be less confusing. What I had as 20V and 0V are now V+ and V-.