Electronic – Detecting a disconnected analog input


I am working with ADC on MCU SH7147 (Renesas).

I have a problem with a sensor as outlined below. How do I detect a disconnect of the sensor cable on the analog input pin on the MCU?

I tried to check it by cutting the the wire and seeing the analog data display in LCD, but it is random data, it is difficult to make a rule to detect disconnect.

Best Answer

Add a pull down or pull up resistor on the A2D input - it will then always read full scale or zero when the sensor is disconnected.

A value of about 100K should do the trick, A2D input impedance is typically 1M ohm or so. and sensor output is typically less than 100 ohm.