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LM358-N: Help designing cascading amplifiers: voltage subtractor outputting to a non-inverting amplifier

Ryan Bayne
Prodigy 40 points
Part Number: LM358-N
Other Parts Discussed in Thread: LM358

I will preface this all with I am a mechanical engineering student that is working on an electronics project so I can use all the help you are willing to give me, ha! I have attached the two schematics. For purpose of this question I have separated the two op amps, however in the actual circuit the output of the voltage subtractor is meant to be the input for the amplifier. I have been running tests and running into problems. The voltage subtractor seems to be working well on its own, however when using Op Amp A I get a 6% error, and with B I get a 24 % error which is odd, any idea why this is? The larger question however lies with the amplifier configuration. When using this amplifier, for testing I put 50 mV in and expected a gain of 101 to 5 V, but instead I was getting a gain of 67 to an output of 3.75 V which is a 33% error. I also tried a configuration with 0.93 V in and received 3.71 V out which is a gain of 3.98, when I was expecting a gain of 5.7 which is a 20% error. For reference I am supplying the single mode op amp with 5V for Vcc. My power sources are arduino Uno's since they give a pretty constant voltage source. Again I have very minimal electronic background so any and all suggestions are greatly appreciated! Thanks in advance! 

  • 0
    TI__Guru*** 142006 points
    Ryan,

    With a 5V supply the maximum rated output is 3.5V (Vcc -1.5V). Typically it'll be a little higher like 3.7V, with a light load.
    The offset error assuming ideal resistors, for first stage is Vio * 2 where Vio is op amp offset error (between -7mV and +7mV)
    The offset error assuming ideal resistors, for second stage is Vio * 101 where Vio is op amp offset error (between -7mV and +7mV)

    The correct way to calculate gain is (Vout1 - Vout2) / (Vin1 - Vin2), using two data points. This gain should be as accurate as the resistors used.
  • 0 in reply to Ron Michallick
    Prodigy 40 points
    Thanks for the response Ron. I have a few follow up questions now if you wouldn't mind helping me further since I'm having trouble locating on the datasheet.
    1. What is the maximum rated output if both op amps are being used ie. I can power with 3.3 V. It seems odd to me that the output is maxed at such a low voltage?
    2. Where can I find the offset error on the data sheet?
    3. By first stage and second stage do you mean of a single op amp? As in I have to add the offset errors of both stages?
    4. Does the offset error change if I am using both op amps on the same chip?

    Thanks for your patience with my mechanical engineering brain!
    Ryan
  • 0 in reply to Ryan Bayne
    TI__Guru*** 142006 points
    Ryan,

    1 ) Both op amps in LM358 run independently. The souring current driver in LM358 is a Darlington transistor setup emitter so loses two base emitter forward voltages. That is about 1.4V at room temperature.
    2) Offset error is usually near the top of the electric characteristics table. It's 'symbol' is Vio or Vos
    3) If subtractor feeds the NI amp , then subtractor error which is 2x is amplifier by NI amp for a total potential error is now 202x (yes that is big). the NI error is 101x and that is it's toal error too. Both errors add at the end, sometimes they are opposite polarity and partially cancel each other.
    4) Each op amp has it own Vio error. Using just amp amp to do both jobs would reduce total error to 101x instead of 202x + 101x (each x is independent). the subtractor values change to 1k input and 101k feedback, so input load is heavier now.
  • 0 in reply to Ron Michallick
    Prodigy 40 points

    Thanks for all your help Ron! I seriously appreciate it.