Electronic – Arduino analog input pin impacts incoming signal

analogarduinoinputvoltage

I have a hall sensor throttle which outputs a signal that scales between 0.8 and 3.6V. The problem is that when I connect it to an Arduino MEGA2560 analog input pin, the voltage never goes over 2.3V, either when measuring it with a multimeter or using the Serial port. It's like something in the Arduino limits the voltage to this value. What could this be due to and how can I solve it?

schematic

simulate this circuit ā€“ Schematic created using CircuitLab

The 2.3V are measured between the A2 pin on the Arduino and the throttle

Best Answer

As we can read in the datasheet:

The ADC is optimized for analog signals with an output impedance of approximately 10kļ— or less. If such a source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling time will depend on how long time the source needs to charge the S/H capacitor, which can vary widely. The user is recom- mended to only use low impedant sources with slowly varying signals, since this minimizes the required charge transfer to the S/H capacitor.

If this is a case of impedance mismatch, then you need some kind of impedance buffer.

One of the possible solutions could be an op-amp in "voltage follower" configuration. enter image description here

http://www.atmel.com/Images/Atmel-2549-8-bit-AVR-Microcontroller-ATmega640-1280-1281-2560-2561_datasheet.pdf

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