OP-amp buffer problem

Discussion in 'The Projects Forum' started by Abusaleh, Mar 16, 2013.

  1. Abusaleh

    Thread Starter New Member

    Mar 16, 2013
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    0
    I have a problem with LM358 (dual op amp). It is cofigured as a unity gain buffer for ADC input of dspic30f6014a microcontrollar. That means I connected the negative input to the output terminal and to the ADC input, and the positive input to a current sensor output. The input of this buffer is a dc voltage from CSLA2CD current sensor ( http://datasheet.octopart.com/CSLA2CD-Honeywell-datasheet-9695429.pdf ). Therefore, the measurement of this voltage should be accurate because the current is calculated inside the uC using the following formulla: I=(V_ADC-V_offset)/0.0327. where V_offset=Vcc/2=4.0V. That means the V_ADC should be accurate to get good results. The problem is the unity gain buffer amplifies the voltage. i.e. when the input is 3.92 V, its output is 3.98 which leads to wrong readings. I want the output of buffer exactly equals the input. How to solve this problem?
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
    1,606
    The LM358 has huge offset voltages and input currents, as do most components designed 40-50 years ago.

    Try buying something designed THIS century.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    Post your circuit diagram.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Yeah, but not 60mV!

    What is the Vcc on the op amp? On the ADC?
    Maybe the op amp is oscillating. As Carl said, we need a schematic, including all power supply connections and decoupling caps.
     
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