Example - 1

Design a voltage level detector with noise immunity that indicates when an input signal crosses the nominal threshold of – 2.5 V. The output is to switch from high to low when the signal crosses the threshold in the positive direction, and vice versa. Noise level expected is 0.2 V_PP, maximum. Assume the output levels are V_H = 10 V and V_L = 0V.

Solution:

For the triggering action required an inverting configuration is required. Let the hysteresis voltage be 20% larger that the maximum pp noise voltage, that is, V_hys = 0.24V.

Thus, the upper and lower trigger level voltages are -2.5 ± 0.12, or

UTP = 2.38 V and LTP = -2.62 V

Since the output levels are V_H and V_L instead of +V_sat and –V_sat, therefore, hysteresis voltage is given by

or        
and

The reference voltage V­ R can be obtained from the expression of LTP.

Given that V_L = 0, and LTP= -262, we obtain

V_R = (1 + R₂ / R₁) LTP = (1 + 1 / 40.7) (-2.62) = - 2.68 V

We can select any values for R₂ and R₁ that satisfy the ratio of 40.7. It is a good practice to have more than 100 kΩ for the sum of R₁ and R₂ and 1 kΩ to 3kΩ for the pull up resistor on the output. The circuit shown in fig. 7 shows a possible final design. The potentiometer serves as a fine adjustment for V_R, while the voltage follower makes V_R to appear as an almost ideal voltage source.

Fig. 7

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