# Can I measure analog voltages for a circuit with multiple power supplies?

I have two components in a circuit: An Arduino Uno R3 and a Wind Sensor Rev. P. See diagram below. The wind sensor requires 12V to operate, so I powered it with an external power supply (provides 12V DC at 1A). The wind sensor outputs an analog signal up to 5V with the sensor reading.

I would like to use my arduino's analog-to-digital converter to read the sensor value. But can I do this if the arduino is powered with a different power supply? I have it connected to my laptop with a 5V USB cable.

I tried reading the value but I get fluctuating readings. To debug, I made a few more connections from the analog-in pins. As seen in the diagram, I can read voltages correctly if they are from the arduino. But if I try to read any voltage from the external power supply, the value is random.

I used a multimeter to check the voltages in the external power supply are 11.20V (close enough to 12V) and the voltage from the sensor OUT pin is 1.40V (within the 0V - 5V range I expected).

So why can my arduino not measure these voltages? I assume it's because the two different power supplies are changing the frame of reference for the ADC pin. But it's not clear to me, so I would appreciate a conceptual explanation.

My intuition tells me that both power supplies should share the same GND pin. But I didn't want to connect the GND between the two power supplies together, given they operate at different voltages and currents. Is this safe to do? Or should I just get a power supply module that will allow a single power supply to output both 12V and 5V?

• Trust your instincts. If the power supplies both isolated from high voltage nothing wrong to connect grounds. – user263983 Feb 21 at 21:41
• Ground means 0V where you choose it and all the 0V points must be common with good decoupling caps on Vdd close to the devices. When you measure V with a DMM are you aware of the impedances involved and noise ? This makes a difference to the Arduino. – Tony Stewart EE75 Feb 21 at 23:42
• Always verify all grounds read 0V and Supplies are correct . You were probably pulling down by ESD diode drop. to get 11.20 Is that really within spec of the regulator? – Tony Stewart EE75 Feb 21 at 23:49