This application note discusses the discrepancy between the slew rate in applications and the slew rate specifications in op amp data sheets. The virtual ground (or virtual zero) concept simplifies op amp design by assuming the voltage difference between the inputs, VID, is zero. VID must increase to generate slew rate. Just over 100mV are needed to reach the maximum natural slew rate that is listed in many data sheets. Some op amps have a data sheet slew rate greater than natural slew rate by incorporating a slew boost circuit. The VID needed to reach boosted slew rate can be several hundreds of millivolts. Most data sheets do not state the presence of slew boost; however, there are methods to determine the presence of slew boost by examining the data sheet figures with waveforms.
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Slew rate is the fastest output voltage change over time in operational amplifiers, typically measured in units of volts per microsecond, V/µs. Some low-power op amps express slew rate in volts per millisecond, V/ms. The data sheet slew rate (SR) parameter is the rate of change of the output voltage with a large input signal and unity gain setup (unless specifically stated otherwise). This single value is often the only obvious information about slew rate in the data sheet. It is important to know that not all applications experience this magnitude of slew rate. Additionally, it is important to note that slew rate is not a constant value for all inputs and configurations.
Op amp circuits are easy to create and understand due to the ‘virtual ground’ mental concept. In closed loop circuits, the large gain of the op amp drives the input voltages (IN+ and IN-) to be the same. Input voltage difference (VID) is assumed to be zero. This assumption makes op amp circuit math simple. In reality, there are three effects that degrade this virtual ground concept. The first effect is DC offset voltage (VOS). The second effect is small signal gain. VID for a small signal is simply VOUT divided by AOL (open-loop gain). The third effect is slew rate generation. This application note focuses solely discussing on slew rate.
VID must be non-zero to generate slew rate. The greater the VID, the greater the slew rate. At some point, increasing VID no longer increases slew rate. The data sheet value is the slew rate where a larger VID has no effect.