Analog signal linearity

Hi, this is my first post
I'm working with mesa 7i77+5i25, i have everything connected and working, I'm in the tunning part,
i decided to test if the analog signal was at the correct scale, by opening a opened a halscope, and tested the commanded speed and encoder feedback, all the PID gains to zero just because i wanted to see the output without any PID correction, then i noticed that at low speeds i get a matching speed signal between commanded speed and encoder feedback, but when i increased the signal to get higher speed i got a noticeable difference.
Based on that i measured the voltage at the output,

Commanded - Measured
Voltage(VDC) - Voltage(VDC)
10 - - - - - - - - - - - - - 9.55
9 - - - - - - - - - - - - - - 8.5
8 - - - - - - - - - - - - - - 7.6
7 - - - - - - - - - - - - - - 6.7
6 - - - - - - - - - - - - - - 5.9
5 - - - - - - - - - - - - - - 5.2
4 - - - - - - - - - - - - - - 4.3
3 - - - - - - - - - - - - - - 3.00
2 - - - - - - - - - - - - - - 2.02
1 - - - - - - - - - - - - - - 1.02
0 - - - - - - - - - - - - - - 0.05



Measurements where made with a 24VDC power supply connected to field voltage input, i checked if there was some noise with a oscilloscope and everything was ok, i made the tests inside my house so i don't think that it was external noise from the shop, ground connection is ok.

i know that is not a big deal, perhaps PID can compensate that difference, but i think it will improve the performance of the system.
Is there a function to linearize analog output?
i can program my own, that's why i like linuxcnc but if it is already made i can save some time and go to the next step and see those chips flying around.

Best Regards

7I77 analog linearity should be better than .1 % and .05V offset at 0 is way too high

How are you measuring the output?

Also check the 7I77s 5V source. If this is low, you can have analog output issues

negative lead to GND (from the same terminal block ofc)
positive to AOUT0

i checked with multimeter and oscilloscope, both gave me same readings.

Regards

I would check the 5V, If that is OK and you are sure about you multimeter accuracy,
I would get an RMA for the card as it clearly broken.

Hi, thanks for your reply
I'm waiting for another multimeter from a friend and I was using the 5v power supply from 5i25 board (readings gave me 4.85v), I bought an independient adjustable 5v power supply to see if there are any improvement.

I will do tests today at noon.

Best Regards

Hi, sry for the delay with my reply, the independent 5v psu made a good improvement with the analog out, perhaps because the PC is an old one the PSU is about to die, i did some tests with good results so i will keep the external one.

The problem now are the servo motors, quite old, and no maintenance from the last owner damaged im some way the feedback devices.

Anyway thanks for your help.