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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.
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.
What do yuo mean by that?
"Open-Drain" is usually used for outputs, not inputs...
None of this has anything to do with Keil software tools or products... http://www.keil.com/forum/
"i'm surprised to see the IN0 pin of ADC shows 2.5V. I have not connected any sensor to it"
How does it "show" 2.5V? How have you determined that?
It's not particularly surprising for an unconnected input to show any arbitrary value - it could be due to noise, leakages, deliberate bias, etc, etc.
You would need to show your circuit diagram for any meaningful comment!
Thanks Andy for ur kind reply
Thanks maaz. ill try it..