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Question about slew rate

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yonzzan

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If the spec of my fully differential opamp contains slew rate is 20 v/usec, does that mean the slew rate in my design has to be greater or less than the value?

For example, does slew rate 300 v/usec in my design satisfy the spec?

Thanks ahead.

Another question, I need to build a CMFB to have my op amp operate. However, I do not know how to design it. such as Current flow , Vov, width and length of the circuit. I have not been found any book which shows how to design it. I am looking forward to hearing your advice.

Best Regards
 

the slew rate you design should be higher than the spec.

as for cmfb, a resistor divider and a differential amplifier is work, you can refer to J. Baker's book - CMOS Mixed-Signal Circuit Design
 

The specification you are designing to should state for each parameter that it is a maximum or minimum allowable value. Generally, higher slew rates are desirable. That is, unless they increase the power drain and the power drain is an important parameter that needs to be below some limit.
 

For starting the design we take the minimum Slew rate which is needed.
Depending on it we set the value of current.
However based on other specs like bandwidth and power dissipation the current required may go up.

So depending on the current the slew rate can go higher. (It is anyways good for your design)

Regards
Sarfraz
 

flatulent said:
The specification you are designing to should state for each parameter that it is a maximum or minimum allowable value. Generally, higher slew rates are desirable. That is, unless they increase the power drain and the power drain is an important parameter that needs to be below some limit.
Click to expand...

High slew rates are not desirable, they cause distortion, depending on your application they may be acceptable, they are usually a byproduct of stability.
 

High slew rate will in fact avoid distortion. Distortion comes from slewing limitations because your sine wave can become triangular because of slewing.

The only limitation is that the power requirement goes gung-ho when you want high slewing. Hence the current devices become larger as well. These parasitics will increase non-linearity only to a minor extent.

yonzzan,

There is no need to take an amp of slew rate of 300V/us when the spec requires 20 V/us. Just take an amp which has a slewing of about 25 ~ 30 V/us. This will suffice. I am only thinking of corner conditions.
 

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