UNO Analog input at 1.7V without any sensor connected

Projects Programming
1

Hallo to all.

As written on the subject, i am on a UNO board project which uses A0 for analog input, printing the result on a nokia 5110. Without any software conversion it is for to read values between 0-1023. My issue is that even my sensor is not connected i still get values on the screen between 300-400. With the default 5V reference, this reading interprets to a voltage between 1.46V-1.95V. My question is why is that? Is it perhaps due to the high input impedance which picks capacitively this input ghost signal?

As a side note, is it possible for someone to suggest me a rock steady regulator at 1.1V for external reference?

Thanks

2

My question is why is that?

Because the input is floating and not held at GND potential. How about using a pulldown resistor to clamp it to GND ?

3

Thanks UKHeliBob. It sounds logical.

I'll test it to see its response.

4

Hallo again.

I have a sensor which outputs some few tenths of mV. I wonder if i can use the break down voltage of a diode or of some diodes in series to form a rock steady reference for external Aref feeding. What the implications would be? Is that a valid method? For example a 0.7V=700mV (silicon diode) divided by 1023 gives approximately a 0.6mV step. A 0.3V breakdown voltage (schottky) would give me a 0.3mV step. What is your opinion guys? Can this happen? Is diode's break down voltage steady enough to do the trick?

Regards

5

You can get 2.048V and 1.024V precision voltage references, and there will be some "bandgap reference"
chips too with an output of 1.15V or thereabouts (the bandgap of silicon at room temperature). There will
probably be some 1.000V references too.

6

Datasheet says 1.0 V is minimum Vref. Lesser reference may or may not work. Using op-amp to boost the measured voltage is probably better idea.

EDIT: you were speaking about FORWARD voltage, not breakdown. Only Zener diodes are built to survive breakdown voltage. (It is the voltage in reverse direction, where diode should be closed.)

7

Smajdalf:
EDIT: you were speaking about FORWARD voltage, not breakdown. Only Zener diodes are built to survive breakdown voltage. (It is the voltage in reverse direction, where diode should be closed.)

Oops! Yes. I meant forward voltage!

Thanks for the answers. I'll proceed with an opamp. My purpose is to translate an AC signal which vary in amplitude, to a value between 0-1023. I am interested in having an indication of an EM field's strength. Not absolute values. So i guess i have to rectify the signal which comes out of my antenna/preamp stage. Isn't that better in this case than offsetting my input signal? I find it more straight forward. Any comments on that?

Regards