AC coupled non-inverting amplifier circuit output latch-up on start

Discussion in 'Analog & Mixed-Signal Design' started by m zaid, Jan 9, 2016.

  1. m zaid

    Thread Starter Member

    Jan 9, 2016
    46
    5
    Hi guys,

    I wonder if any of you guys have accoutered this before. I built an AC coupled non-inverting amplifier using an LM358 as in:
    Ac coupled non-inverting amplifier circuit.jpg

    The problem I'm facing is output latch-up at the maximum output voltage upon power up. Only after a moment like in 10-15 seconds, the voltage gradually reduces to the output offset value I set by the voltage dividers at the non-inverting input.

    I tried connecting the voltage reference later a moment after power up after reading http://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/op-amp-practical-considerations/ . It says that I need to fire the inputs only after the main power has charged it's internal capacitors. However, upon connection, the output shoots to maximum and latches and only after 10-15 seconds it 'normalizes' just like before.

    I don't wan't the user of my device waiting for this time period. How can the latchups be prevented?

    Help please..thanks!
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    Post a schematic for your circuit, and the waveforms if you're able.
     
  3. m zaid

    Thread Starter Member

    Jan 9, 2016
    46
    5
    Hi,
    Thanks in advance!
    Well here is the actual circuit I'm using:
    Actual circuit.jpg
    System voltage is 3.3V.
    You could also appreciate a picture of my breadboard prototype:
    b_circuit.jpg
    Using the MCU's ADC to sample the output of the op amp, I was able plot this waveform:
    Wave forms.jpg
    Can you make it out from these?
    Thanks in advance!
     
    Last edited: Jan 9, 2016
  4. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    What is the supply voltage?
     
    Last edited: Jan 9, 2016
  5. m zaid

    Thread Starter Member

    Jan 9, 2016
    46
    5
    The system voltage is 3.3V.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,520
    1,247
    An LM358 can barely run on only 3.3 V. Look at the input common mode range and output voltage swing limits on the data sheet.
    You are asking for an AC gain of 4,300. That's 0ver 72 dB. Very few new opamps can do that. The 358 isn't new, and can't.
    The amp is saturating because of the enormous DC error gain before the 22 uF cap can charge up to the input offset voltage. The charging current comes from the output through the 4.3M resistor. It's a simple R-C time constant adjusted for the fact that the output cannot swing to the positive rail.

    Use a higher power supply voltage or change to a newer, rail-to-rail opamp rated for 3.3 V operation.
    Reduce the single-stage gain so that you have at least 20 dB of negative feedback *at the highest frequency of interest*.
    If you really need 73 dB of gain, you probably will have to have two amp stages in series.

    ak
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,001
    3,756
    Because it take about 15 seconds (or more) to charge the 22uF capacitor to the level of the voltage divider. You are charging it with a 4.3M ohm resistor so that is a time constant of 95 seconds. Luckily, your voltage divider on the non-inverting input is less than 63% of Vcc so you don't have to wait so long. Also, there is probably some residual charge in that 22uF cap from your previous use.

    I would dump that 22 uF cap and add a voltage divider there at your desired virtual ground. And, a cap from true ground to the virtual ground we are making with this voltage divider.

    EDIT: this time AK beat me!
     
    Last edited: Jan 9, 2016
  8. m zaid

    Thread Starter Member

    Jan 9, 2016
    46
    5
    Thx guys. The project is not finished but this chapter is.
    Well, did capacitor sizing, double stage amplifying (four actually), precision 5.5V opAmp (TLV2462). Now got clean signals. Please admire.
    fine sampling1.jpg
     
    Last edited: Jul 1, 2016
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