ADC0804 conversion time

Discussion in 'General Electronics Chat' started by multiben, Jun 25, 2011.

  1. multiben

    Thread Starter New Member

    Jun 2, 2011
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    Hello,
    I have built the LED test circuit as described on page 12 of the ADC0804 datasheet. Everything is working correctly, but one thing is confusing me. When pin 6 is at 0 volts, the LEDs are all lit as expected. Then when I apply 5 volts to pin 6 it takes almost 1 second for the LEDs to cycle from all on to all off. The chip seems to pass through all 256 possible values before settling to 0.

    The datasheet claims that the chip runs at 8888 conversions/second when in free-running mode, so why does it take so long when changing the input voltage from 0 to 5?

    Cheers,
    Ben
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The datasheet for the ADC080x series on National Semiconductor's site does not have that schematic on page 12.
    http://www.national.com/mpf/DC/ADC0804.html#Overview

    There is a basic A/D tester on page 24.
    http://www.national.com/ds/DC/ADC0801.pdf
    If you don't have a cap on the Vref/2 input to ground, that could cause problems.
    If you don't have a stable power supply and/or caps across your Vcc/GND pins, that could also cause problems.

    Take a picture of your setup and post it.
     
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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Are you shorting the analog input to GND for the 0V test?
     
  4. SgtWookie

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    Jul 17, 2007
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    Hi Ron,
    It seems the OP had another thread open on this subject; I thought they had already resolved the problem in the other thread:
    http://forum.allaboutcircuits.com/showthread.php?t=56085
    but this thread was opened after the other one appeared to be resolved.

    They haven't been back, so I wonder if they resolved this new problem?
     
  5. Ron H

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    Apr 14, 2005
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    We may never know.:rolleyes:
    It just sounded to me like the response to a floating analog input, except he implied that the problem occurred when he went from 0V to +5V. The floating input symptom would only occur if he mistakenly assumed that 0V was equivalent to no input.
     
  6. multiben

    Thread Starter New Member

    Jun 2, 2011
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    Here I am!

    Sorry for the slow reply, but I have not had time to work on my pet projects because of busy work period.

    @SgtWookie,
    Yes the other problem was resolved after I finally figured out I had plugged my ZIF socket into the wrong holes (duh). This was a different issue I noticed once the circuit was working.

    I think the problem is related to Ron's question. Here's what happens:

    WHen I apply 5v through the output of a potentiometer to the ADC input and then turn the knob as quickly as i can to 0volts I get instant feedback on the LEDs. But when I remove the lead from the ADC input pin it displays the slow change behaviour I described. I thought this was the same as applying 0v.

    >The floating input symptom would only occur if he mistakenly assumed that 0V was equivalent to no input.

    That's exactly what I assumed. Would you mind explaining briefly how they differ?
    Cheers,
    Ben
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The ADC inputs are very high resistance, and they have a small capacitance. When you charge the input capacitance to 5V, then remove the 5V connection, the capacitance has to discharge through the high input resistance. When you cranked the pot to zero, the capacitance discharged through the pot (basically zero ohms) to ground. See this tutorial on RC time constants.
     
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  8. multiben

    Thread Starter New Member

    Jun 2, 2011
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    Brilliant. That explains it perfectly. Thanks very much for your help.
    Cheers,
    Ben
     
  9. Ron H

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    Apr 14, 2005
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    I should have mentioned that a floating input, such as the one you created when you removed the 5V, will be undefined. It might drift back to ground, or, if it started out low, it might drift high, or it might even stay at whatever voltage you charged it to for a very long time.
    Floating digital inputs can cause an IC to draw excess current, and can also cause havoc on the output(s). Never leave inputs floating.
     
  10. multiben

    Thread Starter New Member

    Jun 2, 2011
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    Could it ever potentially damage the chip itself?
     
  11. Ron H

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    Apr 14, 2005
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    I have never seen actual damage due to a floating input, other than possibly from electrostatic discharge. I have had chips get noticeably warm to the touch, when they should be dissipating (near) zero power if all the inputs were tied to a logic level.
    NXP says that floating inputs can damage a chip.
     
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