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The IN0 pin of ADC0808 shows 2.5volts with no connection to the sensor.

Kumara Guruparan

Hi,

i have interfaced ADC0808 with AT89c51 and i m surprised to see the IN0 pin of ADC shows 2.5V. I have not connected any sensor to it.

I like to know whether it usually shows this voltage or is there any problem with my circuit.

Moreover i see the digital values toggling randomly from 0-255 and if i ground that pin it shows 114.

can anybody help me rectifying this problem.

Parents
  • 0

    Dear Friend,
    All the input pins (IN0-IN7) will give such a behaviour i.e. their values will vary between 0-255 as all inputs are open drain and floating, therefore you'll need to connect them all with the ground.
    The other thing is that you can adjust 2.55V using a 50K pot at Vref(+) pin and connect Vref(-) to GND. in this way when IN0 is connected with GND the output will be 0 and similarly, if input is 1V, the output will be 100. and for 2.55V output will be 255. Means resolution will be set to 10mv / step.
    The other way is that, connect your Vref(+) to 5V and Vref(-) to GND, in this way your reading will be aprox. double, you can divide it in your controller and can use it.

    Regards.

Reply
  • 0

    Dear Friend,
    All the input pins (IN0-IN7) will give such a behaviour i.e. their values will vary between 0-255 as all inputs are open drain and floating, therefore you'll need to connect them all with the ground.
    The other thing is that you can adjust 2.55V using a 50K pot at Vref(+) pin and connect Vref(-) to GND. in this way when IN0 is connected with GND the output will be 0 and similarly, if input is 1V, the output will be 100. and for 2.55V output will be 255. Means resolution will be set to 10mv / step.
    The other way is that, connect your Vref(+) to 5V and Vref(-) to GND, in this way your reading will be aprox. double, you can divide it in your controller and can use it.

    Regards.

Children