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AD7761 minimum low time

Question asked by EngJunkie on Mar 3, 2018
Latest reply on Mar 14, 2018 by EngJunkie

Hey everyone,

 

I'm honestly not sure if I'm doing something wrong, but the formula for the minimum low time doesn't seem to make sense.

 

It's provided on page 54 of the datasheet and looks like this:

 

looking at Table 4 for the t_11 and t_4 specs I get the following:

 

 

I did work out my tp and tskew and I get the following:

 

tp = 0.3457 nS

tskew = 0.3755 nS

 

What I find odd is the following. Operating at a frequency of 32.768Mhz, the period time will be 1/32.768Mhz = 30,51757813 MHz.

 

This means if I ignore the skew and propogation delay, my minimum high/low time is 16 nanoseconds from table 4.

In my understanding from the datasheet it would mean that the device could never run at 32.768Mhz?

The minimum low/high times are just too high?

Rather it seems a maximum frequency of 29Mhz would be more realistic (including rise/fall times)?

 

Thanks in advance for any help, I'm not sure where my misunderstanding is.

 

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