oscilation at 75175 output - how to prevent ?

hello forum

in my servo driver project I am taking encoder signals A A* B B*
with 75175 and counting the output with microcontroller

however as shown in attached picture at the 75175 output
at the falling edge there occurs oscilation

this causes sometimes miscounting of encoder position
power supply is LM2575 and there is 100 nF bypass at power pins of 75175

my question how can I prevent this ?
thank you

Circuit for LM2575 is :

**broken link removed**

Did you make circuit like this?

yes I made this circuit
only I have 1000 uF + 100 nF at the output of LM2575

Looks like unsuitable circuit layout. A series resistor at the output (e.g. 100 ohm) can most likely dampen the oscillations.

Add 20uH (in line after feedback point) on output and add additional 100uF after that (+ and Gnd). This should be second filter.


http://www.piclist.com/images/www/hobby_elec/e_ckt22_7.htm


Try that and feedback results. ;-)

hello ,

I have found the problem
this is my servo driver project
I have observed the voltage between the motor PE cable ( connected to motor chasis )
and the uC board GND with oscilloskop

there are 100 V peaks every 100 uS ( my PWM frequency is 10 KHz )

this time I connected motor PE and uC board GND with a 1 mm2 cable ( about 10 cm ) and this conected to P.Earth of the shop
I again observed the voltage between the GND of uC board and the connection point to PE of motor ( 10 cm away from board )
there were 2 V oscillation every 100 uS

I mean there are about 2 V oscillation between the two end of 1 mm2 cable ?
how can this be ?

now I understand why they use multi layer boards in professional servo drives :-(
now what else can I do ?
thank you

To me it looks like you are get the high motor current flowing through the signal ground of the motor encoder generating the A/B incremental pulses. It should to be connected only with the control electronic ground and supply. The ground connection of the motor and the electronic should be a star connection. Maybe you need to think about a separate power supply for the motor encoder + control and the motor. Another option is to look for single chip encoder who delivers also the absolute information ( like in this application: https://ichaus.biz/wp_boost_performance ).

Enjoy your design work!