how to route a DC motor power supply

[Enrique Perez (Gast)]

Hello guys!

I need to supply a noise DC motor using +5v of my sensor board PCB. I am 
using GND plane and +5v plane to in the PCB and I am afraid that the 
noise effect of this motor affect to these power planes or other 
components of the sensor board.

My question is of anybody knows about pcb layout technique to isolated 
the signals of the motor and its power supply? Maybe should I use some 
kind of AO to isolate or any other idea?

Find attach a picture of the PCB.

[MaWin (Gast)]

Block most of the noise directly at the motor (M)

                 +--47uH--+---+---+
                 |        |   |   |
                 |        | 47nF  |
 ----------------+        |   |   |
twisted wire            47nF  +--(M)
 ----------------+        |   |   |
                 |        | 47nF  |
                 |        |   |   |
                 +--47uH--+---+---+

Buffer the supply at the connector using an electrolytic. Remember that 
a DC motor delivers voltage if braked, and if there is no one to take 
the braking power, the voltage rises.

Grab the FPGA VCC from the electrolytic using a choke and following 
electrolytic but use no choke in the GND connection.

Does your motor always run ? The is no motor control (full bridge or 
MOSFET) drawn.

If you isolate (optocoupler) you may also block GND to the FPGA using a 
choke.

[Enrique Perez (Gast)]

Thanks for your soon answer.

there is not any motor control, only the TTL signal provided by the FGPA 
trought a buffer/driver. The motor is always running.

On the other hand, having a look to your block diagram is not clear 
where should I place the 47nF caps. I have a PCB with a connector with 3 
pins. TTL, +5V and GND. Which components should I place in that 
connector and how to do it?

The FPGA is in other designed board, so all the solutions to remove the 
motor noise should be implemented in the Sensorboard PCB attached in the 
first post...So I don´t know if the solution of the choke in the FPGA 
side is valid anymore..

Thanks again for the answer!