# How to account for input noise on a zero crossing detector op amp?

To save potential confusion let me start by saying I just started learning opamp theory about a week ago. I'm currently familiar with inverting, non-inverting, summer, and difference opamps.

My (fairly new) understanding of zero crossing detectors is that when the input signal crosses from positive to negative or vice versa the polarity of output of the opamp will change as well. Essentially it's useful for converting something like a sine wave to a square signal.

If my understanding is correct then what happens if noise is introduced on the input line? I understand better with examples and numbers so let's say I know my noise oscillates over 0v at 100mv peak to peak. How would you account for that?

EDIT: If possible a schematic drawing of a before and after case for handling the noise would be extremely helpful!

• If the input signal is a triangle of 2 volts peakpeak and slewrate of 1 volt per second, what might happen? What is the bandwidth of the noise? Commented Sep 19, 2017 at 4:07
• @analogsystemsrf Every 4 second my output would change polarities right? Hmmm I'm not sure what the bandwidth is. But I'm guessing the way we filter it out is by kind of making a bandwidth filter that says "you must have a bandwidth of over X to trigger me"? For the sake of an example could you provide a common bandwidth of noise for me? Commented Sep 19, 2017 at 4:15
• Define the noise frequency then determine where it comes from to see if the source can be eliminated, filtered or shielded then use a low pass filter to improve the S/N ratio. Commented Sep 19, 2017 at 4:21
• @ Capn Jack All the time constants in the circuit affect the noise bandwidth, some with major effect. A 1Kohm resistor feeding 1nanoFarad (0.001uF) cap, together provide 1uS timeconstant and abandwidth of 1,000,000 radians/second / 2*pi = 159,000Hz Bandwidth. Commented Sep 19, 2017 at 17:39

Low signal to noise ratio may cause glitches if the device can support sufficient frequency transitions. Also additive noise causes phase noise on zero crossing limiters.

Hysteresis is generally added when the noise cannot be controlled using positive feedback R ratio. This doesn't eliminate the phase noise but will move the crossing threshold in the opposite away from "zero" each time the output hits the peak. Often 1% is considered reasonable for some analog signals 10% for others and 33% for noisy logic interfaces.

For single supply amps, the "zero" crossing is set to Vcc/2 by some method.

One can even use CMOS inverter logic as a limiter (aka slicer, aka zero crossing detector) if the signal is AC coupled and use a self-biasing high R value (1M) as negative feedback.

The other thing about Op Amps is since they have so much gain and high order delay effects they must put in a cap inside to make the Op Amp essentially a 1st order filter for open loop to make it stable with unity gain.

Comparators on the other hand do not have this compensation cap so they work much faster as zero crossing detectors and ECL comparators work over 1GHz as well as current mode logic but using differential current with zero crossing current and differential load resistors.

• A lot of what you're talking about is going over my head but I'm going to do my best here. Essentially the noise is filtered out through using a hysteresis loop that builds a threshold up to outscale the noise on the signal, correct? If so, this sounds like a solution for dynamic noise no? Is there not a more simple solution that can be employed given we know the Vpp of the noise? Also aren't comparators Op Amps as well? Sorry for the noob questions! Commented Sep 19, 2017 at 4:22
• Yes hysteresis is an effective way to prevent glitches . Comparators are different and many are open collector output. Much faster slew rates and no internal low pass filter near 10 Hz. We call the Op Amp or OA 's bandwidth by gain*BW or GBW so less gain with negative feedback, more bandwidth up to unity gain at GBW value. Unlike comparators which have much higher gain since no internal cap . Commented Sep 19, 2017 at 4:31