I've read a ton of the awesome documentation that is avaible but I understand a fraction of it. I just want to confirm what I'm doing is correct and that I don't damage my U3 or overlook something silly.
The car is question has a signal wire from the throttle postion sensor that outputs 0.67V - 4.65V. My plan is to splice that wire back to AIN0 on the U3 and read the voltage in python with the getAIN(0) method.
The data sheet says that low voltage is 0-2.44V. If I'm reading the code correctly it looks like the getAIN method passes isLowVoltage=False to binaryToCalibratedAnalogVoltage(). I should be fine then yeah?
This means I can only use AIN0-3 for any signal wires > 2.44V? What about the FIO ports?
The attached graph looks right even if the numbers are slightly different than what my DMM reads. I'm mostly concerned about relative readings as in when the throttle is applied and how quickly I get to full throttle. I'm less concerned about the voltage reading being accurate.
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I can provide hardware help, but not much python expertise. Let us know if you need python help and I can mark this for someone else.
If you have a U3-HV, then AIN0-3 are high voltage analog inputs (+/-10V) and FIO4-EIO7 can be configured as low voltage analog inputs (0-2.4 or 0-3.6).
If your signal goes over 3.6V, you would need to use an LJTick-Divider to measure it with a low voltage AIN.
Even though you only care about relative values, it might be worth investigating the disagreement a little to make sure there is not some fundamental problem going on. You want to connect the DMM to AIN0/GND so you can compare DMM readings and U3 readings at the same time. Then put your system in some static condition so you have a steady fixed signal voltage. How much difference do you see?
The DMM reading between AIN0/GND reflect the lower voltage that laback is reporting.
Now if I move the DMM ground to a ground on the car I get the 0.67V-4.65V readings.
Should I run a ground wire from labjack to a ground on the car?
Yes, we recommend connecting a 100 ohm series resistor between grounds to avoid sinking large currents into LabJack GND. You could also connect to SGND instead. See this relevant datasheet page:
https://labjack.com/support/datasheets/u3/hardware-description/ain/analo...
Without a ground to SGND voltage reading was 0.26 - 4.16.
With a ground to SGND voltage reading is slightly higher. 0.5-4.45 which is slightly lower than the DMM between AIN0 and SGND.
Thanks for the replies btw. The electrical side of things is one of my weaker areas.
To be clear, you have now tested AIN0 to SGND with a DMM and this does not match the readings simultaneously obtained by the LabJack?