The OPAx991-EP family features a slew-boosted architecture, to accelerate recovery from large-scale input steps. The rate of change of the device output scales according to the differential input voltage (V_ID), leading to three distinct modes of operation: a small-signal or linear region, a natural slew region, and a slew-boosted region. The effective slew rate of the device is therefore a function of the input signal, rather than having a fixed characteristic. During a large-signal input step, the output typically exhibits a fast rising edge while the inputs are held apart (V_ID > 150mV); then as the input voltages approach each other (V_ID < 150mV), the device output rate of change decreases and the output settles.

Note that the amplifier output rate of change in a given application depends on the circuit configuration, parastitics, and other factors. The data below reflect the open-loop step response of the device, while the values specified in Electrical Characteristics describe the closed-loop step response as expected in a typical circuit implementation.

When an amplifier enters a slew condition, the quiescent current typically increases slightly while the internal Miller capacitance is charging. When the output slew-boost circuitry of the OPAx991-EP is activated, the charging rate of the Miller capacitance is "boosted" and a temporary rise in supply current occurs, as illustrated in Figure 7-2. For applications and use cases where the inputs are pulled apart and the slew-boost feature activated, this higher supply current must be considered for power budgeting purposes.

The full-power bandwidth (FPBW) curve of the device describes the maximum peak to peak voltage (V_PP) at a given frequency that the amplifier can reproduce without significant visual distortion. Operating inside of the full power bandwidth curve with a pure sine wave input signal results in a visually undistorted output signal, while operating beyond the full power bandwidth boundary results in a visually highly distorted triangle wave that is caused by the slew rate limit. This type of output distortion is known as slew-induced distortion.

Because the device slew rate scales according to the differential input voltage (V_ID), the OPAx991-EP exhibits two distinct slew-rate limited FPBW boundaries: one for the slew-boosted mode of operation, and another for the natural slew region. Note that as the natural FPBW is exceeded, the expected total harmonic distortion (THD) increases. To reduce slew-induced distortion, operation within the natural FPBW curve is advised.

For detailed discussion of slew-boosted architectures and the associated effects on THD, including empirical measurements for the OPAx991 family, refer to the Full Power Bandwidth and Op Amp Distortion application note.