Is the hysteresis Vt(+)-V(-) of the AD7785 correct at 0.1V~0.17V?

Hi,

I will contact the product owner and get back to you.

Thanks,
Janine

Hi miky ,

It is not necessary that that difference between Vt(+) max and Vt(-) max is the hysteresis value. Hysteresis is the gray area wherein the switching of the logic output to HIGH (or to LOW) takes place, as seen on the figure below.

Typically, a 1.6V signal is used and the Vmax of hysteresis region is 0.17V. The Vt(+) and Vt(-) of the signal is computed by +/- half of the 0.17V as seen on the figure below. It shows that Vt(+) and Vt(-) of the 1.6V does not violate the datasheet specification for Vmin and Vmax voltages of Vt(+/-). Moreover, it is important to take not of the Vmin and Vmax voltages of the Vt(+/-) signal. The Vmin and Vmax voltages of Vt(+) still has to be followed based on the values specified on the datasheet, as well as Vmin and Vmax voltages of Vt(-).    

Thanks and regards, 

Rod

Hi, RAdante 

Thanks for your reply.

I have more questions about that.

What role does the hysteresis value 0.1V~0.17V in the AD7785 datasheet indicate?
The range of Vt(+) of AD7785 is 1.4V~2V and the range of Vt(-) is 0.8V~1.7V, so I don't understand what the hysteresis value 0.1V~0.17V is useful for.

Best regards.

Hi,

The minimum value of the hysteresis region (the gray area in the figure) should be at least 0.1V and only up to the maximum at 0.17V while considering the individual values you set at Vt(+) and Vt(-). 

From the figure, it is not advised to set Vt(+) to 1.7V since the hysteresis is already at maximum value (0.17V). However, if we were to use a 1.7V of Vt(+), the Vt(-) should be adjusted to 1.53V (still not violating the Vt(-) V max at 1.7V) to comply with the hysteresis Vmax at 0.17V. Moreover, Vt(-) can only be adjusted up to 1.6V since the V min for hysteresis is 0.1V. 

Thanks and regards,

Rod

HiRAdante ,

Thank you for your response.

I'm sorry. I have two more questions about that.

i)
Is there an identical AD7785 that has Vt(+)=2V and Vt(-)=0.8V?
Since the time varies depending on where the threshold of the Schmitt trigger input of the AD7785 is located, I would like to know how it may vary in the same unit.

ii)
The hysteresis seems to be specified as the width when 1.6V is used as a reference, but in reality, Vt(+) is 1.4V~2.0V and Vt(-) is 0.8V~1.7V, which may be out of the “1.6V±hysteresis” range. I am not sure how to utilize the hysteresis specified in the datasheet in my design.

Best regards

miky.

This query has been answered on this thread: (+) Please tell me about the hysteresis of the AD7785. - Q&A - Precision ADCs - EngineerZone