APPLICATION NOTES CON'T

4

Rev. C 8/05

Definition of Settling Time

The time required for the output to come within a prede-

termined error band after application of a full scale step

input. This includes the time of delay, slew time and the

small signal settling of the amplifier.

Measuring Settling Time

The only accurate method of measuring settling time is by

the creation of a false summing junction and observing the

error band at that point.

The reasons for not using other methods are as follows:

Observation of settling at the actual summing junction adds

probe capacitance to the input and changes the entire re-

sponse of the system. (Probe capacitance almost doubles

the capacitance at the summing point.) Observing the out-

put is extremely difficult, as the 3% linearity of oscillo-

scopes, and reading inaccuracies, lead to a possible 5%

error. The false summing junction approach works well

bcause the amplifier is subtracting the output from the in-

put, and only 1/2 the actual error appears there.

False Summing Junction Circuit

Problems: Because the amplifier is to be overdriven, 1/2

the input voltage can be expected to appear at the false

summing junction. Therefore, it is necessary to clamp that

point with diodes to limit the voltage excursion to avoid

overdriving the oscilloscope with the consequent recovery

time of the scope itself. The scope probe has capacitance

which significantly affects the settling time measurement.

Keep the associated resistors as low as possible to mini-

mize the RC time constants, and take into account the added

time created by the false summing junction. On the ranges

used for settling time measurement even the best real-time

scopes suffer from reduced bandwidth and relatively slow

settling; a sampling scope is convenient for these measure-

ments.

Offset Null

Typically the MSK 801 has an input offset voltage of

less than ±1 mV. If it is desirable to "null" the offset volt-

age, the circuit below is recommended.

Ω

Output Short Circuit Protection

The collectors of the output devices have been brought

out to pins 10 and 12 for short circuit protection, if desired.

lected as follows:

The addition of the these resistors reduces output volt-

for transient pulses.

For normal operation and best overall response, short

Slew Rate vs. Slew Rate Limit

SLEW RATE:

S=2

πfVp; Slew rate is based upon the sinusoidal

linear response of the amplifier and is calculated from

the full power bandwidth frequency.

SLEW RATE LIMIT

dv/dt; The slew rate limit is based upon the amplifier's

response to a step input and is measured between 10%

and 90%. MSK measures Tr or Tf, whichever is greater

Ω.