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Buffer input/output offset voltage

Moha Deeb1
Intellectual 270 points
Other Parts Discussed in Thread: REF200, LM324, TL084
Hi,
 
I have an issue with buffer circuit, the simple circuit is below
Inline image 1
I am using a constant current source (TI REF200) to measure resistance (NTC), the problem is with the buffer, at higher resistance >50K
 
I start to get an offset between input/output. In the simulation its 10mV, but in practical its ~35mV
 
I thought that this is caused by input bias current so I switched LM324 with JFET input TL084, in simulation its gone, but in practical its still 35mV.
 
 
any help?
 
Thanks
  • 0
    TI__Guru 60570 points

    Moha,

    Please use a rich formatting option on the bottom to Insert/Edit Media - we cannot see schematics that were pasted into the body of the forum post and without it it is difficult to address your question.

    Having said that, please make sure you meet the minimum 2.5V supply requirement (see PDS table below) plus the voltage drop across the resistor used; in case of using 50k resistor together with 100uA constant current source, you need a minimum of 7.5V voltage supply (2.5V+100uA*50k).

  • 0 in reply to Marek Lis
    Intellectual 270 points

    Hi Marek,

    Sorry for the schematics, please find it below.

    The application is for a maximum 100K NTC, so I am supplying the REF200 using +15V.

    Thanks.

  • 0 in reply to Moha Deeb1
    TI__Guru*** 140026 points

    Moha,

    Are you certain the buffer is faulty? Reading the input voltage at pin 3 with a standard meter (10M ohms) will lower the voltage at pin 3. If the same meter was used to measure pin 1 later, the voltage at pin 1 will be higher because pin 3 is not loaded with the meter anymore.
    You may measure the voltage between pins 1 and 3 to see the error.

    To use TL084 the -15V supply should be increased to -4V to cover the input common mode range for TL084.

  • 0 in reply to Ron Michallick
    Intellectual 270 points
    Hi Ronald,

    you are right, I connected the DMM to pin 3, and measured output using another DMM.

    Thanks.