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The Two Most Important Criteria in Selecting Filter Op Amps

[1] Gain Bandwidth Product

The gain versus frequency response of a compensated op amp will roll off at 20dB/decade, and will have unity gain (0dB) at the Unity Gain Bandwidth. For such an op amp the Gain Bandwidth Product and the Unity Gain Bandwidth will be effectively the same.

gain_frequency

If the GBW of the op amp(s) used in a filter stage is not significantly higher than the maximum pass band signal frequency, then that signal may be distorted.

A general rule of thumb for the minimum op amp GBW is:

GBW = 100 x G x fp
Where:
G = gain of filter stage (V/V)
fp=pass band frequency of stage


Filter Wiz PRO provides two design aids to facilitate the determination of the minimum GBW requirement:

  1. an initial estimate of the required GBW for each stage topology (circuit)
  2. actual simulation of filter stage(s) to see the effect of different op amp GBWs. If the selected GBW is insufficient, then the filter stage gain-frequency response will not match the desired response.

[2] Slew Rate

Since an op amp must internally charge and discharge real capacitors, with finite currents passing through real resistances, there is a maximum rate of signal change (Slew Rate) beyond which a small signal will become distorted.

For a sinusoidal signal given by:

V = Vp x sin(2 x PI x f x t)
Where:
Vp is the peak voltage (not peak-to-peak)

The minimum op amp Slew Rate is given by:
Sr = 2 x PI x fmax x Vp
Where:
fmax is the maximum sustainable (undistorted) sine wave frequency

Required Slew Rate is most easily determined using a look-up graph. Find the maximum signal frequency on the bottom axis; find where it intersects the signal peak voltage (not peak-peak); read the minimum required Slew Rate on the left axis.

op_amp_slew_rate

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