Hi guys. I need some help, as my electronics skills are basic.

Well, I had a Cold cathod gauge from adixen with the appropriate gauge controller. Some days ago the gauge controller failed to detect which sensor is connected to it. I decided to use the arduino to read the voltage from the gauge and convert it to pressure following the equation released with the manual. In order to get the voltage from the gauge, I used a ADS1115 from adafruit. However, I still use the gauge controller to supply the 24V to the sensor electronics. From the sensor, I have two pins where I have 0-10V as response. The gauge manual says it has an impedance of 200 ohms on these pins.

As the gauge response is 0-10V and the ADS1115 only senses up to 6.1V I've made a voltage divider using 2 resistors of the same value.

From this point, without connecting the wires to the ADS1115, using 1kohm resistors, with a multimeter I measured 4.1V, in the divider, which is the half of the voltage between the two pins (8.2 V). So the voltage divider is OK.

Now comes the difficult part to me, which is to connect the divider to the ads1115. If I put the (-) from the gauge to the ground on ads1115 and the middle leg of the divider in one channel of the ADS1115, I never get the 4.1V, but extremely low voltages instead.

I have connected GND of ADS1115 ito GND from arduino.

Should I connect those grounds to the ground of the PS supply of the gauge controller ?

Manual:

What voltage do you measure AT THE PIN of the ADS1115?

The max input voltages are not those you mention in your post.

You certainly can't measure 6.1V with a 5V part. You can blow up the 5V part however... 1k resistor
is probably enough to prevent this, fortunately.

Use much higher divider resistors so you don't load the guage much, perhaps 100k and 68k (0.1%) which
maps 10V to 4.047V which is within the 4.096V range. A 200 ohm device driving 168k will see about 0.1%
droop, which you can live with. 200 ohm driving 2k will have 10% droop and will probably go non-linear