high impedence +5V buffer(opamp)

Discussion in 'General Electronics Chat' started by sundeep, Sep 28, 2005.

  1. sundeep

    Thread Starter New Member

    Sep 28, 2005

    I need the part no. which is suitable for my application if anyone can help me out i will be really thankfull

    i require an isolation between a thermistor and a microcontroller

    isolation can be in the form of a buffer(or unity gain Opamp)

    input to the opamp will be a potential divider arrangement using a thermistor and a resistance, where thermistor value will range from 1K Ω to 275K Ω

    output of opamp will be connected to the input of microcontroller.

    the specs of opamp should be:-
    1) single supply +5VDC
    2) high input impedence in the range of mega ohms as the input impedence of thermistor is upto 275K Ω
    3) input voltage will vary from .25V DC to 4.75V DC, ouptput should be exactly the same

    Presently i was using LM324 but the problem was the output ranges from .25 to 3.6V and not upto 4.75 as required.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Hi sundeep,

    It would be helpful if you could post a schematic of your present opamp interface circuit.

  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
  4. hgmjr

    Retired Moderator

    Jan 28, 2005
    Hi sundeep,

    I believe the suggestion by RonH to consider using a rail-to-rail opamp will most likely correct your problem. The LM324 opamp predates the newer generation rail-to-rail opamps and so its positive swing is limited. Your need for the output to reach 4.75V coupled with the use of a +5V supply are requirements that the LM324 will have difficulty meeting.

    It would still be helpful if you could post your schematic so that a clearer determination could be made. There may be additional factors responsible for your lower than expected output level.

  5. sanjeev203

    New Member

    Sep 19, 2005
    Hi Sundeep,

    Texas Instruments offers very good RRIO OPAMPS like OPA350, OPA355 which have very high input impedence and can drive the microcontroller also.

    Best regards,