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4 years ago.
LPC1768 : Voltage on AnalogIn pin with no input
Hello, I'm sorry if my tittle was not explicit.
I have a problem with my MBED, for a project I send a signal to pin17. This pin was set as an AnalogIn at the beginning of my code. When I only link the mbed to the computer by usb or provide energy to the mbed with a battery the pin17 send 2.4volt. The problem is that this output go to my circuit and false the signal I try to measure when i link the pin 17 to my measure circuit
Is it possible to set the output of pin17 to 0V and just keep him for reading ?
4 years ago.
What is the source of the signal you want to measure?
The analogIn pin won't be driven to any level (and certainly not to 2.4V, only the analog out pin could do that) but there will be a natural drift to some voltage level due to internal resistances. If the signal you are trying to measure is particularly high impedance then this bias could have an impact. Even if the bias could be set to 0V that would still have an influence, just towards a different voltage.
It is standard practice when connecting a very high impedance signal to an ADC input to add a buffer in order to prevent the ADC from influencing the measured signal. For most simple applications an op-amp in a non-inverting unity gain configuration will work fine.
Alternatively it may be possible to decrease the impedance of your driving circuit, it depends on what it is.