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How to design an inverter buffer to drive a 1pF capacitance?

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triquent

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how to calculate fan out of a buffer

If I want to design an inverter buffer to drive a 1pF capacitance. What is the minimum fan-out of the inverter to make the output waveform ok if don't consider the speed. how to calculate the needed fanout? or do it by simulation?

Added after 3 minutes:

is it usually the minimum fanout should be 1?
 

Re: how to design a buffer?

triquent said:
If I want to design an inverter buffer to drive a 1pF capacitance. What is the minimum fan-out of the inverter to make the output waveform ok if don't consider the speed. how to calculate the needed fanout? or do it by simulation?

Added after 3 minutes:

is it usually the minimum fanout should be 1?
Click to expand...

you want to design a std cell. or infer the buffer in your logic?
 

Re: how to design a buffer?

what's the difference?
1) output buffer fanout=?
2) internel buffer fanout=?
I mean a full customer design.
whizkid said:
triquent said:
If I want to design an inverter buffer to drive a 1pF capacitance. What is the minimum fan-out of the inverter to make the output waveform ok if don't consider the speed. how to calculate the needed fanout? or do it by simulation?

Added after 3 minutes:

is it usually the minimum fanout should be 1?
Click to expand...

you want to design a std cell. or infer the buffer in your logic?
Click to expand...
 

Re: how to design a buffer?

Since you are doing a full custom design, you are better off doing the simulation rather than trying to get the fan out no.. This fan out no. is normally only used in digital design where the gates are standard sizes. And if you don't want to consider the speed, then almost any reasonable gate size will do. Then again, if you are trying to drive something off-chip, you should go for a foundry provided buffer.
 

Re: how to design a buffer?

hi
read book by Rabey (digital integrated circuit).it has given good example about Buffer design
 

Re: how to design a buffer?

Hey,
generally...they fanout of any buffer is gain "1"....
and also it depends on the output capacitance of the transistor....
basically...you need to consider the input and output gain of the transistor.....which you are placing for designing the Buffer...and then....come the speed.......generally...if you consider...Buffer model....it may vary...depending on what kind of transistor model you are using...

all calculations etc.. you can refer.....digital integrated circuits
 

Re: how to design a buffer?

In theory, you can make an inverter chain with proper sizing to speed up the signal, if you know the loading capacitance. However, in reality, the calculated transistor size might be too large. So just use some approximation for the inverter chain sizing. Hope this helps.
Eric
 

Re: how to design a buffer?

Well regarding this I will say that you refer to LOGICAL EFFORT by David Harris...
It is the excellent book dealing with these aspects...
 

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