15 Matching Annotations
  1. Oct 2018
    1. If the SM58 noise floor is calculated at room temperature, the voltage output is 0.00000032 volts.

      This is equal to -130 dBV. The SM58 Vocal Microphone Specification Sheet says:

      Sensitivity (at 1,000 Hz Open Circuit Voltage) –54.5 dBV/Pa (1.85 mV)

      At this sensitivity, the self-noise would be 94 - (-54.5 - -130) = 19 dB SPL, which is pretty typical and certainly not "lower than can be typically measured".

  2. May 2018
    1. plus the impedance of the path from the inverting input to ground i.e. R1 in parallel with R2.

      This is incorrect. If R1 is infinite and R2 is 0, the parallel impedance is 0 ohms, but the input impedance is much higher than the input impedance of the op-amp itself, due to feedback making the inputs very similar in voltage.

      The input impedance is actually

      $$(1 + A_0 B)\cdot Z_\mathrm{ino}$$


      • \(A_0\) is the gain without feedback (the open loop gain)
      • \(B\) is the fraction of the output which feeds back as a negative voltage at the input
      • \(Z_\mathrm{ino}\) is input impedance without negative feedback

      For the above buffer example, it would be close to \(A_0 Z_\mathrm{ino}\)

      See https://electronics.stackexchange.com/q/177007/142

      Simpson - Introductory electronics for scientists and engineers section 7.2 Negative Voltage Feedback explains this clearly

  3. Apr 2018
    1. 10–27

      Equation 10-27 is wrong:

      • Resistors are in parallel, their noise does not sum.
    2. Adding this and the 100-kΩ resistornoise to the amplifier noise

      This is 3 terms (10 MΩ noise, 100 kΩ noise, and amplifier noise), but the equation only includes 2.

    3. 10–25

      Equation 10-25 has several errors:

      • The left and right side are not equal.
      • Resistors are in parallel, so their noise does not sum.
      • Resistor noise needs to be multiplied by gain to be combined with 113.1 μV output noise of op-amp.
    4. 10–23)

      Equation 10-23 is incorrect:

      • The noise of the resistors does not sum, since the resistors are in parallel and load each other. It would be equal to a single 50 kΩ resistor producing 4.0 μV.
      • The 113.1 value is output noise. The resistor noise needs to be multiplied by the gain to get the output noise, too.
    5. 0.1 F

      Should be 0.1 μF

    6. 0.1 F

      Should be 0.1 μF

    7. The noise calculations have many errors. See annotations on https://via.hypothes.is/http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf for details

    8. which is 100

      actually should be multiplied by the non-inverting noise gain, which is 101

    9. TLE2201

      Should be TLC2201

  4. Mar 2018
  5. www.analog.com www.analog.com
    1. The poles of the Bessel filter can be determined by locating all of the poles on a circle and separating their imaginary parts byn2where nis the number of poles.

      This is incorrect:

      • The poles are not located on a circle
      • The imaginary parts of the poles are not equally spaced

      To generate the poles of a Bessel filter you need to use root-finding methods on the reverse Bessel polynomials. There's no other shortcut that I'm aware of.

    2. The step response shows no overshoot

      This is incorrect: There is a small amount of overshoot in Bessel filters.