hello all.i have connected the output D0-D7 of adc0804 to a trainer board 8 LEDs.the Vcc was given 5v.I have not given any voltage to the reference voltage input.now in the analog input Vin i have given a varying voltage.when i try to see the output i was surprised.i saw either all LEDs are on or a miscombination of on-LEDs.i thoght adc will give a exact digital converted signal.after sometimes adc was not working.now i am confused that if it is because i have not given any voltage to Vref?i also want to know if there is a relation between input volrage Vin and Vref?i have varied the voltage in Vin and gave a maximum voltage of 20 volt may be.is that the reason for which adc stops converting?

 

Have you looked at the datasheet for the ADC0804?

Perhaps not, since if you had, you would've realized that the absolute maximum rating for all other inputs/outputs besides logic was -0.3 to Vcc+0.3v, and in that case you would never have applied 20 volts to the input, because you would know that would cause the ADC to be destroyed.

Throw that particular IC away, since it is now ruined.

Or if you wish, you could try reading the Applications Section (page 6 in National Semiconductor's datasheet) about supplying Vref.

These IC's are very easy to let the smoke out of. You might not have destroyed it if you are very lucky, but it is very doubtful that the output will even be close to correct anymore, after being so abused.