LM358 input offset problem

Discussion in 'The Projects Forum' started by waiyang, Jun 13, 2016.

  1. waiyang

    Thread Starter New Member

    Jun 13, 2016

    Recently I working on the sensor MPS 3117 and found that Samsung used the same sensor on the blood pressure device (http://www.samsung.com/semiconducto...245_ARM_Blood_Pressure_Monitor_AN_REV00-0.pdf ).

    However, I found out that by using LM358 as differential amplifier there will be an input offset about 9mV. Is there any way to compensate this error and Samsung did add some extra circuit at the inverting input of the opamp is that a compensation for the error?
  2. dl324

    Distinguished Member

    Mar 30, 2015
    Welcome to AAC!

    This offset null can be applied to opamps that don't have explicit offset pins. The example shows a comparator, but works for opamps too. From Nat Semi Linear Applications Handbook 1994:
  3. waiyang

    Thread Starter New Member

    Jun 13, 2016
    Thanks for the suggestion @dl324. But I wish to know what is the function of the extra circuit and how they come out with the resistor value.
  4. DickCappels


    Aug 21, 2008
    Maybe the operation of this one is a little more obvious. There is some interaction between the setting of the "Zero" pot and gain, but that can be minimized by making the value of the pot as low as you can stand to have it (direct affect on power dissipation).

    You can make is so that the setting of the "Zero" post has no effect on the gain by putting a voltage follower between the wiper of the pot and the 2M resistor.