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# Question on Settling Time, Slew Rate & Rise/Fall Time

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#### Anachip

##### Member level 2
slew rate rise time

Hi Guys,

I'm confused on the mentioned parameters when designing an opamp. Can you explain me on my confusion?

1) What is the relation between 3 of them.
2) Why it is important to know the opamp settling time. Is that opamp settling time need to be faster than the rise/fall time of opamp?
3) Regarding, the slew rate, i know that is the rate at which output voltage change for a fiven max input (large current). So, what is the impotensy on knowing the parameters?

Pls guide me on all the above question so that i wil have some basic fundamental on opamp design.

Thanks,
Anachip

slew time

any signal given to opamp should have change in voltage lesser than slew rate otherwise wave form will be distorted...

crystalmh

### crystalmh

Points: 2
op amp settling time

Considering that the opamp can be roughly modeled as a second order system, the parameters for settling time (ts), rise time (tr) and fall time (tf) are related. I think slew rate is another term referring to rise/fall times and is not necessarily an additional term. Slew rate is usually given in V/us so it is the slope of the rising or falling edges of the opamp output waveform. So you can see how it could be related to the rise and fall times. Rise and fall times can be measured from the 10%-90% amplitude point of the waveforms or 20%-80%, it depends on who is measuring them.

Now back to the settling time etc. To see the relationship, you can consult any book on control systems and look at the transient time-domain behavior of second order systems.

Any second order system may be written as
$k \frac{\omega_n^2}{s^2 + 2 \zeta \omega_n s + \omega_n^2}$

where omega_n is the natural frequency and zeta is the damping coefficient. The settling time and rise time are usually estimated as a function of these variables.

$t_r = \frac{2.2}{\zeta \omega_n}, t_s = \frac{4.6}{\zeta \omega_n}$

You will find more information about these approximations in Benjamin C. Kuo, Automatic Control Systems, 7th ed., Prentice Hall, 1995.

I hope this information helps you out. [sorry about the formulas]

Best regards,
v_c

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mingli1

### mingli1

Points: 2
settling time op amp

Regarding the importance of Settling time.. I think it is important than the rise/fall time. Because only when an output signal from opamp settles to a point (after undershoot or overshoot) it becomes usable to the next stage..So, Settling time also gets into account while calculating the delay suffered by the signal..

The reason why all the parameters ..i.e Settling time ,rise time, fall time are all related is that, they all depend on the damping (δ )given in the circuit as quoted by V_C;

When the damping is high,undershoot occurs and viceversa which also has the effect in the Stability of the system.Thats why slew rate gets in connection with the fidelity of output..

Hope this helps.

GD

rise time second order system

Just to add on without all the math,

Slew rate is a large signal parameter. Settling time is a small signal parameter. Slew rate tells you how fast the system responds to a large signal change. Settling time tells you how fast the system settles to a corresponding value after the slewing (total settling time includes the slewing time+small signal settling time).

Slew rate depends on large signal parameters like Current(usually tail current) and Rate of charge (usually compensation and load capacitors).
Settling time depends on mainly the small signal parameters like phase margin. Better the phase marging smaller is the small signal settling time.

Why do you need to know all this?
Slew rate is a measure of how fast an input can be given. Slewing is a limiting factor on how fast the input can be. Settling time tells you how fast the output settles to desired value. Again this also can limit the frequency of the input signal.

mingli1 and harpv

Points: 2

Points: 2

### mingli1

Points: 2
second order system rise time

When u increase ur phase margin the system become overdamper r underdamped????
i study in books that if the phase margin is around 74 degree the the system becomed critically damped.
my question is if we increase the phase margin above 74 degree then our system becomes over damped r underdamped..???
manish

settling time opamp

Anachip said:
Hi Guys,

I'm confused on the mentioned parameters when designing an opamp. Can you explain me on my confusion?

1) What is the relation between 3 of them.
2) Why it is important to know the opamp settling time. Is that opamp settling time need to be faster than the rise/fall time of opamp?
3) Regarding, the slew rate, i know that is the rate at which output voltage change for a fiven max input (large current). So, what is the impotensy on knowing the parameters?

Pls guide me on all the above question so that i wil have some basic fundamental on opamp design.

Thanks,
Anachip

1. I believe the slew rate is measure how fast the output voltage of opamp rising up before the transistor move to saturation by giving large input swing signal.
(let say swing from 0 to 1.2v), below 1.2v, some transistor still in linear or cutoff.
It is not depend on phase margin. The output will rise linearly with time.

2. When the transistor in opamp go into saturation as input increase, the behaviour of the output depend on phase margin to determine wheather got overshoot or ringing or undershoot.

3. The rising time shd be measure the output rising time (from 20 to 80 %)

4. The setting time shd be measure from 0 to 98 or 99 % of actual output value.

5. So, the settling time shd be longer than rising time.

Pls comment....

Surianova

manissri said:
When u increase ur phase margin the system become overdamper r underdamped????
i study in books that if the phase margin is around 74 degree the the system becomed critically damped.
my question is if we increase the phase margin above 74 degree then our system becomes over damped r underdamped..???
manish

Remember the phase margin only valid when the system or transistor in saturation.

rise time second order

setting time is a large signal spec , setting time is totel spec which including large signal and small signal information, it determine the speed and resolution of a system

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