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OPA2182: Signal distortion at bottom of sine wave

user1915296
Prodigy 100 points
Part Number: OPA2182
Other Parts Discussed in Thread: OPA2189, OPA189

Tool/software:

I use opa2182 / opa 182 as the  signal buffer,but  there some distortion at bottom of sine wave with the increasing of signal amptitude(PK >4V).

  • 0
    Guru 284090 points

    Please show the schematic, and specify the supply and signal voltage ranges and the signal frequency.

    It is likely that the desired output voltage is not far enough from the rails; see figures 7-12/7-13.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 1710 points

    Hi User, Clemens,

    I agree with Clemens in that we will need additional information about the circuit you are measuring. 

    Best,

    Alex Curtis

  • 0 in reply to user1915296
    Prodigy 100 points

    signal frequency: 80kHz，power tail is +/-5V

  • 0 in reply to user1915296
    Prodigy 100 points

    the input signal is from signal generator

  • 0 in reply to user1915296
    Guru 284090 points

    Figure 7-36 shows that this should work when there is no load. As shown in figures 7-12/13, output voltages 1 V from the rails should work with 20 mA output current.

    What is the exact voltage at which this happens? What is the load?

  • 0 in reply to user1915296
    TI__Mastermind 22197 points

    E2E user,

    1. Some of my comments here will reinforce what Clemens and Alex said.
    2. When you see distortion as the signal amplitude is increases, this is generally from slew-rate limitations.  There is a theoretical relationship VmaxPeak = SR/(2*pi*f), that shows the maximum signal amplitude without slew-induced distortion.  This relationship is generally a good indication of visible distortion (distortion that is obvious on the oscilloscope).  However, as you approach the limit you will get increased distortion that may not be obvious, and when you surpass the limit the distortion should be obvious.  Note: VmaxPeaktoPeak = 2VmaxPeak (i.e. sometimes it is specified as peak-to-peak).
    3. The equation doesn't take into account the small-signal versus large-signal transition point.  The equation also, does not consider the effects of slew boost.  This can introduce some inaccuracy in the curve (Figure 7-36 in this case).
    4. I recently have had a few questions on the the OPA2182 and OPA2189 regarding the full-power bandwidth curve.  I am still working on fully resolving the discrepancies that we have found.  For most op amp data sheets the full-power bandwidth curve is calculated using the equation: VmaxPeak = SR/(2*pi*f). For the OPA2182 the full-power-bandwidth curve is slightly different than what is indicated by VmaxPeak = SR/(2*pi*f) (off by a factor of 2).  The published curve is more conservative than the equation and may be a measured curve.  In any case, I measured the circuit in a buffer configuration at 200kHz and the data sheet curve did a better job of predicting distorted signal than the equation.
    5. Ultimately, the full-power bandwidth curves are a guideline and staying inside the "safe" limit does not mean zero distortion.  Keep in mind that the curve is a typical curve and may not include effects like slew-boost and also the small signal to large signal transition point.  I think a good practice is to make sure that your signal frequency / amplitude should be a decade from the boundary of the graph.
    6. Below is my measured results and a comparison of the ideal curve to the published curve.  I think your signal is at 4Vpk (8Vpp) and 100kHz.  If you check figure 7-36 you will see that you are inside the "safe" region but close to the curve.  I think you just need a device with higher slew-rate(OPA189 or OPA2888).  Note that measured results for OPA189 are also in the attachment below.  This device curve also doesn't match the expected equation but in this case I think it is a data-sheet error as the theoretical curve compares well to measurements.

    opa182-opa189-full-power-bw.pdf

    best regards, Art

  • 0 in reply to Clemens Ladisch
    Prodigy 100 points

    The load is a buffered  crosspoint switch.

  • 0 in reply to Art Kay
    Prodigy 100 points

    2.VmaxPeak = SR/(2*pi*f)= 10/(6.28*0.08)=20V or 2V?

  • 0 in reply to user1915296
    TI__Mastermind 22197 points

    User,

    The equation of Max Peak voltage can be expressed as follows:

    VmaxPeak = SR/(2*pi*f);

    if VmaxPeak >= Vpower_supply than VmaxPeak = Vpower_supply;

    Assuming: Vsupply = ±5V, SR = 10V/μs, and f = 80kHz,

    VmaxPeak = 10/(2*pi*f); = 10V/μs / (2*pi*80kHz) = 19.9V

    since 19.9V > 5V, VmaxPeak = 5V.

    Thus, at this frequency you should not expect slew induced distortion.  Any distortion you see is due to loading effects or some other issue.  You should disconnect the amplifier output to see if the distortion goes away.

    I hope this helps.

    Best regards, Art