Analog Interfacing to Embedded Microprocessor Systems, 2nd Edition

For Vo high (5 V), Vr ¼ 2:5V, R1¼ 1 K, and R2 ¼ 100 K, V

þ

¼ 2:5247 V. So the out-

put will not switch until the input reaches 2.5247 V. Once the input reaches this value, the

output starts to change state. Since V

þ

is dependent on V

o

, it will change values as V

o

changes. When V

o

reaches its final value (0 V), V

þ

will have a new value. R1 and R2 are still

voltage dividers between V

o

and V

þ

, but now V

o

is 0 V. So if we calculate the new value for

V

þ

, we get 2.475 V. The input has to cross through this value to get the output to switch

back high. However, we got this value because the input reached the original value of

2.5247 V, so the input would have to swing 0.049 V (2.5247 – 2.475) to make the output

switch. This circuit has a hysteresis of 0.049 V.