UC400eth / HDBB2 PWM voltage error

[sgenevay]

Hi there.
I'm trying to search over the forum but don't find any answer.
i bought a UC400Eth with a HDBB2 Breakout board. all this is turning connected to UCCNC software.
I was trying to set the spindle speed via the analog output (pwm) of the HDBB2 board but i have a voltage problem.
I set the max rpm to 24000 in uccnc configuration. but when i set a speed of 24000, i've got a 4,66v voltage on the output pins instead of 5v. and if i set the speed to 12000, i've got 2,30v instead of 2,5v.
So at the end the spindle is not turning at the correct speed cause of the error.

Do you have any idea of how to fix this ?
is there a hardware manipulation or a software thing to obtain the correct voltage ?
If somebody have any idea...
Thanks a lot.
Regards.
Sylvain

[cncdrive]

You can expect the 5V output to be between 4.2V to 5.2V approximately.
The possible innacuracy, range comes from:

1.) The internal 5V 7805 voltage regulator 4% accuracy.
2.) The 74HC buffer chip's guaranteed high output level of 4.4V to 5V @ 5V Vcc voltage.

You can't really overcome this type of error with the board.
What you could do is you could use a digital open drain output instead and filter it yourself if and as neccessary.
The outputs have N-fets with the source pin of the Fet on the ground and the Drain floating.
You can use a resistor to pull the Drain to any voltage levels, e.g. to a 5V source.
You could even use a variable voltage source and adjust the voltage of the PSU as required for the application.

Then you could add an RC low pass filter if nessessary to smooth out the PWM coming out of the drain of the mosfet,
which may not be nessessary if your VFD has an analog input filter built in.
Most VFDs even have an adjustable time constant filter which you can adjust with a parameter of the VFD.

Another possible solution is to simply adjust the input Volts/output Hertz parameter of the VFD to make it output 24000 RPM at the 4.66Volts input voltage.