D2 provides a rapid discharge path for capacitor C2 to reset at the end of the pulse. The diode also prevents the

non-inverting input from being driven below ground.

6.2.7 Bi-Stable Multivibrator

+

±

R3

100 NŸ

R1

100 NŸ

+V

SET

S

R

+V

R2

100 NŸ

RESET

R4

50 NŸ

Figure 6-16. Bi-Stable Multivibrator

A bi-stable multivibrator has two stable states. The reference voltage is set up by the voltage divider of R2 and

R3. A pulse applied to the SET terminal will switch the output of the comparator high. The resistor divider of

R1 and R4 now sets the non-inverting input to a voltage greater than the reference voltage. A pulse applied to

RESET will now toggle the output low.

6.2.8 Zero Crossing Detector

R4

100 NŸ

R1

5 NŸ

+V

R2

5 NŸ

VIN

R4

20 0Ÿ

+

±

R5

10 NŸ

R3

100 NŸ

D1

BAT54

VOUT

V1

V2

V3

R1 = R2 = (R5 / 2)

Figure 6-17. Zero Crossing Detector

A voltage divider of R4 and R5 establishes a reference voltage V1 at the non-inverting input. By making the

series resistance of R1 and R2 equal to R5, the comparator will switch when VIN = 0. Diode D1 insures that

V3 clamps near ground. The voltage divider of R2 and R3 then prevents V2 from going below ground. A small

amount of hysteresis is setup to ensure rapid output voltage transitions.

6.2.9 Pulse Slicer

A Pulse Slicer is a variation of the Zero Crossing Detector and is used to detect the zero crossings on an input

signal with a varying baseline level. This circuit works best with symmetrical waveforms. The RC network of R1

and C1 establishes an mean reference voltage VREF, which tracks the mean amplitude of the VIN signal. The

non-inverting input is directly connected to VREF through R2. R2 and R3 are used to produce hysteresis to keep

transitions free of spurious toggles. The time constant is a tradeoff between long-term symmetry and response

time to changes in amplitude.

If the waveform is data, it is recommended that the data be encoded in NRZ (Non-Return to Zero) format

to maintain proper average baseline. Asymmetrical inputs may suffer from timing distortions caused by the

changing VREF average voltage.

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