## 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.

**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:

- an initial estimate of the required GBW for each stage topology (circuit)
- 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.

**- Click image to enlarge -**