PWM to analog Mesa 7i92

I have a feeling I'm missing something obvious and would appreciate any ideas here. 

I have a 4x8 cnc router with one of those Chinese vfds running the spindle. It wants 0-10v analog. I was trying to get away with just the 7i92 + a pwm -> analog adapter card.
I tried this one first: 
www.amazon.com/gp/product/B07XZ836QF/ref...02_s00?ie=UTF8&psc=1
And also this one: 
www.amazon.com/gp/product/B06XCVY3GM/ref...06_s00?ie=UTF8&psc=1

The problem I'm having is I can't get the calibration right. I have verified the mesa card and linux cnc are doing the right things. With an oscilloscope I can see when I command 1/2 speed (9k rpm on my 18k spindle) it does create a 50% duty cycle square wave. And at 100% speed it shows 100% duty cycle. 

But no matter what I do I can't get the full 10v at the top end & have the 50% anywhere near right. The adapter is wired to a 24v source. Grounds all seem reasonable. Both PWM -> analog cards behave the same way. 

I'm wondering if this is due to the PWM not being 5v on the mesa? Anybody run into this before? Is there a smarter / easier way to deal with this? 

Thanks!
-Jacob

If the PWM-->analog converter uses an opto isolator you may need to lower the PWM frequency to 1 KHz or so...

Thanks! I was at 2khz but I tried 1khz and 500hz but pretty much the same behavior.

So right now I have it so at 50% it runs at 150hz right on but if I run it up to 100% its only like 260hz max (or like 8.2v control voltage) I can't get it to pull up to 300hz / 10v.

I know this isn't exactly a linuxcnc / mesa issue since its making the correct square wave clearly on my oscilloscope but hoping somebody else has run into this.

If you can adjust for 10V at 100% PWM, you can use the lincurve
component to linearize the output.

You could check if the output is PWM level sensitive by adding a stiff pullup
resistor (say 470 Ohms to +5V) to the 7I92s PWM output pin so you get a full
5V swing.