What's causing constant voltage on ADC pin?

[AScha.3]

• floating input pin on a cmos chip has random voltage
• the adc input is a switched capacity (about 5pF)
• after conversion (capacitive switching) this input cap has some remaining voltage
• if you start conversion at a high rate, you will get some voltage on this pin from the charged 5pF
• see manual for max. allowed input resistance for certain precision (no resistance = no precision )

[Riccardo Franceschetto]

I'm reading a 0-20 mA signal discharged on a 170 ohm resistance, so i get the 0-3.3V range values.

[ONadr.1]

You wrote that the pin is floating. Then it has a 170ohm resistor to ground. So how is it really?

[Riccardo Franceschetto]

When I disconnect the pin fom the resistor i get the voltage i mentioned.

The application want's the pin to be connected like in my reply.

[AScha.3]

>When I disconnect the pin fom the resistor i get the voltage i mentioned.

ok, i explained, why this happens.

floating cmos input gives floating value.

so you know now.

right?

[Riccardo Franceschetto]

ok... so far so good.

The problem is : from the 0-20 mA pin i get 0 when the value read from the temp sens is zero, with the ADC pin disconnected, but with the resistance on. Working properly then

When connecting the ADC input pin... the value of the 0 corresponds to a 1.3 V like constant signal. not to the 0 volt.

When i ground the ADC input pin. i get 0 as expected.

So the problem is on the ADC pin i assume.

[AScha.3]

so: adc input with 170 r to gnd . gives 1,3V at 170 r ? (test without adc running !)

this is > 7 mA coming from...? if this current is coming from adc pin, chip is defective or just this pin , so last chance: try other adc pin .

and when adc running (which sample rate ? ) voltage at 170 r ?

[S.Ma]

Maybe the ADC sampling cap is not reset (grounded) between channels.

Don't keep an analog pin floating, you can still apply a pull-up or pull-down when inactive.

[Riccardo Franceschetto]

You mean the other channels/pins of the adc to pulldown?

Not the one i'm reading values with right?