The IN0 pin of ADC0808 shows 2.5volts with no connection to the sensor

Discussion in 'Embedded Systems and Microcontrollers' started by KumaraGuruparan, Aug 20, 2007.

  1. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
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    0
    hi,
    i have interfaced ADC0808 with AT89c51 and i see the IN0 pin of ADC shows 2.5V. I have not connected any sensor to it.Some say that the pin is open drain and hence there is 2.5V. I like to know whether it usually shows this voltage or is there any problem with my circuit.

    Also I have designed circuit with sensor. I use a platinum 100(PT100) RTD for sensing temperature and designed it such a way that the output voltage varies from 2.5V-5V. But the actual voltage range I need is 0-2.5V.can anybody help me how to scale the voltage from 2.5V-5V to 0-2.5V.
    Thanks
    Kumara.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    It would help if you could provide a schematic of the section of circuitry relating to you question.

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The IN0 pin is the first of the 8 inputs to the A-to-D convertor. Assuming that you have something connected to the input then the 2.5 volt signal is originating from that signal source.

    A schematic will help shed more light on an explanation.

    hgmjr
     
  4. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    I get the voltage without connecting any sensor.The pin is open.

    I have attached the circuit.Kindly check it
     
  5. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    I get the voltage without connecting any sensor.The pin is open.

    I have attached the circuit.Kindly check it.....

    If I connect LM35 sensor to this cicuit i get the correct output(0-100). But please help me in connecting a PT100 RTD with ADC0808.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I don't have the software to view your schematic.

    I can however say that the voltage level on an input that is not connected to a signal source is generally not a problem. It is a good engineering practice to tie all unused CMOS inputs to ground or the positive supply using a resistor between 10K and 100K.

    hgmjr
     
  7. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    Sir, I have attached the schematic in word format. Kindly check it.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    I would suggest that you put a 100K resistor to ground on all of the ADC0808 analog inputs just to avoid leaving them open when nothing is connected.

    What is the value of zener Z1?

    hgmjr
     
  9. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    You can use an instrument amplifier like INA118. How much resolution do you need? 8-bits may not be enough.

    edit: RTD is a pretty expensive way to go if you are going to feed it into an 8-bit A/D. What are your resolution, accuracy and range requirements? You might be better off w/NTC type.
     
  10. Distort10n

    Active Member

    Dec 25, 2006
    429
    1

    You can just tie the inputs to Vcc or to GND as long as those potentials are within the ADC's input common-mode range. A resistor is a bit of a waste since it will serve to drive up cost, and space. If it is noise that is a concern, a resistor will do nothing to help anyway.
     
  11. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    What is the value of zener Z1?

    The value of zener is 2.2V
     
  12. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    The value of zener is 2.2v
     
  13. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    I'd recommend using a voltage reference IC instead of a zener for Z1 unless you are accounting for the zener's tempco.
     
  14. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    sir,
    I have sent the sensor circuit. the (Vin Adj Gnd) in the ckt is LM317..
     
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  15. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    That circuit will cause the RTD to self heat thus killing a lot of your accuracy. What you want is to put a very small constant current through the RTD-- small so that you do not heat it up. The LM317 can be configured as a constant current current source. If I remember correctly the datasheet has an example circuit for using it as a current source.

    Here is a classic appnote that might help you:
    http://ww1.microchip.com/downloads/en/AppNotes/00687b.pdf

    I forgot to mention that you can use a simple Wheatstone bridge to interface your RTD also. Just feed it into an instrument amp like the AD627 or AD8555.
     
  16. hgmjr

    Moderator

    Jan 28, 2005
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    Did you purchase the 3-terminal or 4-terminal version of the PT100?

    hgmjr
     
  17. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    3 terminal version of PT100.
     
  18. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    I have tested the circuit in page 4 of the above link. but the voltage varies only between 0.78V to 1.04 volts. the resistances r3, r4, r5, r6 are not mentioned and i put 100 k for all the resistors. for c3 and c4 i put 0.1uf.

    Should i make any change in this resistances to get full voltage range.
     
  19. hgmjr

    Moderator

    Jan 28, 2005
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    Could not work with the link you provided in the title of the post.

    Here is a link to the document I found with google.

    hgmjr
     
  20. KumaraGuruparan

    Thread Starter Member

    Aug 20, 2007
    19
    0
    What value should i put for R3, R4, R5, R6, C3, C4.
     
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