# Input different from AnalogRead

I’m using a pH probe and a signal processing board (see attachment) that gives me an appropriate output voltage (e.g 3.7V for pH = 7 and 4.24 V for pH of about 5 (not sure exactly) the output voltage of the board is measured on the pins of the board between GND coming from the Arduino DUE and Vout (Po) of the board. The Analogpin (A0) on the DUE however gives me a numerical value of 4095 which is the equivalent of 5V max output. There is nothing else attached to the board. Just 5V supply and GND to the probe board and input of A0.
How is this possible?
Thanks for your time.

The Analogpin (A0) on the DUE however gives me a numerical value of 4095 which is the equivalent of 5V max output.

No, think about it. What voltage does a Due's chip run at?

Yes it is possible, you are getting it. It appears you have a wiring mistook mistake, post a schematic showing all power, ground, and interconnections, not a frizzy thing.

That maximum voltage allowed on any Due pin, analog or digital, is 3.3 volts. Anything beyond about 3.6 volts can permanently damage the board.

You need a voltage divider to lower the 5 volt analog signal to 3.3 volts. A 2.2K and 3.3K resistor will give 3 volts out for 5 volts input. It’s a simple matter to scale the input to an actual ph range using the map() function.

I was going to suggest 6K8 and 3K3.

There is a more-useful-than-most calculator here.

The Analogpin (A0) on the DUE however gives me a numerical value of 4095 which is the equivalent of 5V max output.

No it is not. What makes you think it is?

The oft quoted size of the steps applies only to a 5V reference signal.

You need to measure the voltage on the analogue pin to make sure it doesn’t exceed 3V3.
If it does then you need a potential divider as mentioned above.

The output from your Ph board is likely to be 5V if you have no probe attached to it.

OK, thanks for your comments. I’ve been using a DUE which has a maximum input voltage of 3.3V on the analog pins. I have overlooked that bit. I took most of my cues from other Arduino posts but they were based on the UNO boards. I’ll implement the voltage dividers and take it from there. Thanks for your time.