APPLICATION NOTE
2
The standard simulation methodology to test for settling time of an amplifier is to step the input voltage
over the relevant input range and measure the time taken for the output to settle to some defined value
close to its steady state value. The defined value depends upon the resolution of the system. For
example, a 12 bit system in a range of ten volts will probably need to settle to within 1.2 mV (1/2
lsb) of its final value. During the design of such an amplifier, many parameters are varied to optimize
the settling time. It can become extremely tedious moving along the response curves to find the exact
settling time. Performance Analysis by means of goal function definition, can facilitate this investigation.
In this application note, the settling time of an LF411 in unity gain configuration will be computed as a
function of load capacitance by implementing relevant goal functions.
Figure 1: Amplifier Schematic
Figure 2 shows the response of the system for 10 different load capacitors, to a one volt step at the input.
The method normally used to estimate the settling time from these curves is fairly straightforward. We
simply start at the end point and scan backwards along the curve until we find a point where the response
curve intersects the defined settled value
Figure 2: System response for 10 load capacitor values
The folowing goal function demonstrates the backwards search from the end of the run to where the
defined value (1.01 volts in this case) intersects the curve.
