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The very low noise OPA211 and it's THD curve seems opposite of what I would expect

Grant3050
Prodigy 565 points
Other Parts Discussed in Thread: OPA211

The OPA211 datasheet shows that the THD gets WORSE with LOWER output voltage.  That is in Figure 4, page 6, The THD of Output voltage of .01V is -80dB down while at  3V output Vrms it is -130dB down.  I would expect THD to be better with smaller signals??  In my experience larger signals have more distortion due to the non-linearities of the amp.

I have a large environmental signal that I want to ignore and a small signal that I need to extract.  The small signal of interest may only by 1mV after a gain stage of 10, but my environmental signal will be about 8V.  For simplification let’s say my output voltage is an 8V sine wave with a 1mV signal of interest changing within that.

Is my small signal going to experience higher distortion (only -60dB down) because it isn’t large enough?   Or will it be fine because the 8V environmental signal on the output is providing minimum bias or loading or whatever is causing the poor performance at low output voltages.

  • 0
    TI__Guru 64530 points

    THD gets worse with lower output voltage because it actually is THD+Noise.  Thus as you lower the output voltage, the noise becomes dominant factor of THD+Noise spec - pure THD is a fixed percentage of the output signal while noise has a constant value (rms) regardless of the signal amplitude (at any given frequency).  THD+Noise measurement compares the distortion+noise at the op amp’s output to the ideal sinusoidal signal it applies at the input and reports it as a ratio of the two signals.  Since the THD contribution gets smaller for lower output voltage, the constant value of noise starts dominating the THD+Noise measurement for smaller output signal.

     

    Thus, your particular scenario will be worse than you anticipate. If you applied only 1mV signal of interest (without 8V environmental signal), you could expect around 60dB THD+Noise (dominated by noise).  But since your environmental 8V signal will generate distortion at the output (a fixed percentage of the input signal), this may dominate the THD+Noise when compared to 1mV signal of interest.

    For example, if we assume that the THD+Noise is 120dB for 8V environmental signal (for 10kHz signal), this will generate 8uV distortion which compared to 1mV signal of interest will result in 42dB THD+Noise.