It is important to note that the oscilloscope input amplifier may
be overdriven during a settling time measurement, so the os-
cilloscope must be capable of recovering from overdrive very
quickly. The signal generator used for this measurement must
be able to drive 50Ω with a very clean ±10VPP square wave.
The Slew Rate of LME49990 tells how fast it responses to a
transient or a step input. It may be measured by the test circuit
in Figure 7. The Slew Rate of LME49990 is specified in close-
Application Information
SETTLING TIME AND SLEW RATE MEASUREMENTS
The settling time of LME49990 may be verified using the test
circuit in Figure 6. The LME49990 is connected for inverting
operation, and the output voltage is summed with the input
voltage step. When the LME49990’s output voltage is equal
to the input voltage, the voltage on the PROBE 1 will be zero.
Any voltage appearing at this point will represent an error. And
the settling time is equal to the time required for the error sig-
nal displayed on the oscilloscope to decay to less than one-
half the necessary accuracy (See Settling Time – Output
Swing photo). For a 10V input signal, settling time to 0.01%
(1mV) will occur when the displayed error is less than 0.5mV.
Since settling time is strongly dependent on slew rate, settling
will be faster for smaller signal swings. The LME49990’s in-
verting slew rate is faster than its non-inverting slew rate, so
settling will be faster for inverting applications, as well.
loop gain = -1 when the output driving a 1kΩ load at 20VPP
.
The LME49990 behaves very stable in shape step response
and have a minimal ringing in both small and large signal step
response (See Typical Performance Characteristic). The slew
rate typical value reach as high as ±18V/μS was measured
when the output reach -20V refer to the start point when input
voltage equals to zero.
300597c1
FIGURE 6: Settling Time Test Circuit
300597c2
FIGURE 7: Slew Rate Test Circuit
11
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