OPA828 datasheet(23/47 Pages) TI

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7.3.7 Settling Time

Settling time is a measure of an amplifier output to settle to within some percentage (error band) of the input

amplitude and is used to describe an amplifier response to a step input. An amplifier settling time is comprised

of both a large signal response and small signal response. The large signal response is characterized by the

rise and fall times, whereas the small signal response is characterized by overshoot and ringing. Figure 7-10

illustrates the concepts and terminology associated with an amplifier settling time. Specifically, the settling time is

defined as the time the output takes to settle to within a specified error band from the time the input signal was

applied.

Time

Output

Voltage

VOUT

0.1

0.9

1+0

1-0

Error Band

Overshoot

Settling Time

Figure 7-10. Settling Time

The OPAx828 minimize settling time for high-resolution systems in two ways. First, by incorporating an internal

slew-boost circuit that minimizes rise and fall times, Second, by having wide bandwidth with excellent phase

margin with low ringing, enabling small signal settling in minimal time. The OPAx828 are trimmed in laser

production that minimizes part-to-part variation in the device slew rate, bandwidth, and phase margin, thus

mainlining excellent unit-to-unit variations across all manufacturing lots.

7.3.8 Slew Rate

The parameter of an amplifier that best describes the large-signal dynamic behavior is the slew rate. Slew rate is

a measure of 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 for the output to swing

from 10% of the final value to 90% of the final value. The slew rate for the signal illustrated in Figure 7-10 is

given by Equation 3.

OUT

Slew Rate

(3)

The slew rate of an amplifier is limited by the internal architecture of the amplifier, the amplifier quiescent

power and internal capacitances. The OPAx828 maximize the slew rate by incorporating a slew-boost circuit.

The proprietary slew boost circuit used in the OPAx828 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

the +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. To provide optimal dynamic performance, place power-supply bypass capacitors close to the OPAx828.

If the inputs of the amplifier have a large static or dc differential voltage present, the OPAx828 recognize that

condition; not as an indicator of the need to slew faster, but rather as an overload condition. In this case, the

OPAx828 internal biasing currents do not increase, and the quiescent current remains unchanged from normal

operation.

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OPA828, OPA2828

SBOS671D – SEPTEMBER 2018 – REVISED DECEMBER 2022

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