ZHCSIQ4B September 2018 – December 2018 OPA828
The parameter of an amplifier that best describes the large signal dynamic behavior is the slew rate. Slew rate is a measure the maximum rate of change of the output voltage with respect to time and is generally expressed in units of volts-per-microsecond, (V/µs). Typically the slew rate is measured as the time it takes for the output to swing from 10% of its final value to 90% of its final value. The slew rate for the signal illustrated in Figure 50 is given by Equation 2.
The slew rate of an amplifier is limited by the internal architecture of the amplifier, the amplifiers quiescent power and internal capacitances. The OPA828 maximizes slew rate by incorporating a slew-boost circuit. The proprietary slew boost circuit used in the OPA828 results in a very high slew rate while maintaining low quiescent power levels. The internal slew boost circuit measures the input differential voltage present between +IN and –IN input pins. If this input differential voltage is sufficiently large enough, the internal slew boost circuit increases the internal biasing currents of the amplifier thereby increasing the ability of the output to slew faster. TI recommends placing power supply bypass capacitors close to the OPA828 to make sure of optimum dynamic performance.
Should the inputs of the amplifier have a large static or DC differential voltage present, the OPA828 recognizes that condition, not as an indicator of the need to slew faster, but rather as an overload condition. In this case the OPA828 internal biasing currents do not increase, and the quiescent current remains unchanged from normal operation.