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problem with lm2903 on 3 bit flash adc circuit

hui jia
Prodigy 20 points
Other Parts Discussed in Thread: LM2903

Hi all,

I seen this website on the good answers and i would like to ask some question.I am building a 3 bit flash adc circuit using a reference of 8v with a string of resistor at 1k ohm each. I am using a Lm2903 comparator and although it is dual output and 4 inputs, i am only using 1 output and 2 input for each of the resistor. As we can see in between first and second resistor we should be feeding in 7v to the 1st comparator from the top and 1v to the last comparator.

It look something like this,

 

The input is lets say 3.5 volt,

 at the 4rd comparator from the output is near 0v due to 4v bigger than 3.5v but at the last 3rd comparator when input is larger than 3v, it outputs a 1.3v.The result of output 1.3v is same when compared at last 2nd comparator. I am feeding the VCC of the comparator a voltage of 18Volts. However at the last comparator, when 3.5v is larger than 1v, it now gives only a 0.4v. Why is that so? also Why is the output 1.3v and not 5v for logic '1'.

btw I am new on this. Sorry for any trouble

 

  • 0
    TI__Guru 53256 points

    Hello Hui,

    You have the concept - but you are missing one (seven) things - the pull-up resistors on the outputs.

    The LM2903 (and a lot of other comparators) have "Open Collector" outputs (just like the logic gates) and require a pull-up resistor to go "high". The comparator does not have a "upper" transistor to pull the output high. The output "shorts" to ground when the input is true.

    So you need to add individual 4.7K to 10K resistors from each comparator output to your logic supply voltage. The output does not have to be pulled to the same voltage as the comparator supply. It can be pulled-up to a lower voltage.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 20 points

    i tried using the pull up of 4.7k and it worked! thank you so much and tnks for speedy reply too =)