adc circuit not working on pcb

Discussion in 'The Projects Forum' started by swty_todd, May 6, 2011.

  1. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    Hi,
    I am trying to make a project whereby room temperature is displayed in digital form on leds.For temperature sensing purposes I have used LM35 whose ouput is given to adc0804.The circuit works fine on breadboard but after mounting it on general purpose pcb ,it is showing 10 times more temperature.Thinking there might be a problem with my soldering, i got a second general purpose pcb mounted made by a professional.Again the displayed temperatute is 10 times more.So I am sure that the components have been connected properly on pcb. If i hold a matchstick near LM35 there is an increase in the digital ouput.I believe the problem could be due
    to parasitic capacitances being not present on pcb. I dont know much about these parasitic capacitances and although i know i need to add capacitors somewhere in the circuit ...i dont know where...:confused:
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    282
    Are you sure of this? The ADC804 has a serial output.
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    Are your AGND and DGND connected anywhere?
     
  4. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
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    Are those LEDs coming off the data lines? You should have current limiting resistors if they are.

    Also, what are you using to power the circuit? Does it measure correctly to 5 volts?
     
  5. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    I dont get you sir....
     
  6. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    Yes I have connected my analog and digital ground at one of the points(oops! think I forgot to show that in the diagram) and yes I am giving exactly 5 Volts from a normal power supply. I haven't used any resistors before the LEDs (yes, they are coming off the data lines ) because the circuit seemed to be working fine even without them (on breadboard).
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    I dropped the first zero. An ADC804 is a serial output 12 bit converter. The ADC0804 is an 8 bit parallel output.

    That said, pin 9 is not connected in a manner that changes the sensitivity. For the 5 volt span, one bit is 19.5 mv. The LM35 outputs 10 mv/°C.
     
  8. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    It was written in the datasheet that if we want a 5V span then no voltage should be given to pin 9. Am I missing something out here?
     
  9. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    Do I need to do some decoupling here using capacitors?
     
  10. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    First of all, just because the circuit worked without the current limiting resistors before does not mean it could not be causing a problem now or in the future.

    A couple of questions:

    1) Does the voltage coming out of the LM35 show the 10x gain?

    2) Does the 5 volt suppy stay constant throughout the entire temperature measurement?

    3) If the LM35 is sending out the correct voltage is it just the ADC0804 that is the source of the error. That is, the adc has 0.1 volts in and is reporting 1 volts.
     
  11. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    0
    Hello sir, I soldered the whole pcb carefully again, and this time I used resistors and it worked! I am still not sure what was causing the problem. Thanks a lot!
     
  12. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    Without the current limiting resistors, the output current can be much larger than what the outputs are rated for. It can disrupt voltages internal to the chip. Not knowing the chips internal design makes a definitive answer impossible. The design should ensure the chip remains in its correct operating ranges for all inputs and outputs.
     
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