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# Why vil and vih has to be calculated through dc sweep ??

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#### MetalLord

##### Junior Member level 1
While designing ios why one has to calculate vil and vih using dc sweep ? And what will be the result if we calculate it through transient sweep ?

you can do the same thing through transient sweep also but keep the step size as small as possible.

amarnath

@ amarnath

As far as i know the step size has to be very large in order to do the same with a very slow ramp. ( dc )

why is that we have to keep the step size low..what is the reason behind it ?

iam talking about the transient step size not the ramp step size..

i do understand that u r talking abt transient step size, thing which i want to know is why we have to keep step size small

depending on the step size in the tran analysis, the values will be calculated at each step, so the vil vil values will yield more accurate results than when the step size is large.hope its clear.

amarnath

its jst a discussion

from your answer keeping the step size small we can get good result of vil and vih..but dont you think that if you measure vil, vih through dc analysis we can have a better result than this as it will take bit more time to rise or fall and will be more accurate than transient analysis ?..

yes dc sweep is the best way, i just told you an alternative to dc sweep and also a method by which your transient result can match the result of your dc sweep.

amarnath

oh ok ! Thanks a lot for the suggestion, will try with this approach.

MetalLord said:
oh ok ! Thanks a lot for the suggestion, will try with this approach.

Hi Metalord,

I am new in the i/o analog circuit domain. I am trying to calculate Vih/Vil & Voh/Vol values of receiver circuit. Since receiver was a bigger circuit, i decided to find out these values for a buffer first.

I dc sweeped input from 0 to 1 and plotted output. Now what to do? How to calculate Vil/Vih & Vol/Voh values for the buffer?

best regards!

Added after 2 hours 21 minutes:

amarnath said:
yes dc sweep is the best way, i just told you an alternative to dc sweep and also a method by which your transient result can match the result of your dc sweep.

amarnath

Can you help me please? I am new in the i/o analog circuit domain. I am trying to calculate Vih/Vil & Voh/Vol values of receiver circuit. Since receiver was a bigger circuit, i decided to find out these values for a buffer first.

I dc sweeped input from 0 to 1 and plotted output. Now what to do? How to calculate Vil/Vih & Vol/Voh values for the buffer?

best regards!

First of all, we talk about Vil/Vih or Vol/Voh only for Logic circuits.

DC sweep sometimes suffers convergence problem. So we can try tran.

When do tran sim, make sure the rising/falling time is long enough so that the results are close enough to DC results.

To onlymusic16:

plot your Vo vs Vi
the cross point is Vil or Vih, depends your input (ramp down or up).

For Vol or Voh, it defines with specific output driving. Fix your input, the output is either H or L, then source or sink a fixed current (per your spec), measure the Vo.

strennor said:
First of all, we talk about Vil/Vih or Vol/Voh only for Logic circuits.

DC sweep sometimes suffers convergence problem. So we can try tran.

When do tran sim, make sure the rising/falling time is long enough so that the results are close enough to DC results.

To onlymusic16:

plot your Vo vs Vi
the cross point is Vil or Vih, depends your input (ramp down or up).

For Vol or Voh, it defines with specific output driving. Fix your input, the output is either H or L, then source or sink a fixed current (per your spec), measure the Vo.

Thank you from the core of my heart! I must say it was a clear cut explanation. It helped me a lot in finding Vil/Vih values of the receiver circuit.

For Vol/Voh, i was able to understand the method but i do not understand why we attach a current source at output for finding Vol/Voh?

best regards

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