Comparator Circuit with Noisy Signal
In the previous chapter, we talk about the comparator circuit with only one threshold. Let’s consider this scenario, if we have noisy signal with our circuit, what the output will be. Look at the figure below, the output will be oscillate follow by the input signal. This should not happen in some system. For example, in the detection system, we need to turn on when we actually detect something and we will turn off when something actually disappear. This system, will not just turn on and just turn off in a short time. Thus, the schmitt trigger circuit is introduce.
Before we talk about schmitt trigger circuit, we need to understand the hysteresis because the schmitt trigger is hysteretic switch. In the figure below, the hysteresis threshold is mean that you have 2 threshold values: positive and negative threshold. The circuit will use the positive threshold when the signal is increasing, while it will use the negative threshold when the signal is decrease. In the example, we have the triangular signal, when it is increasing, it will use the the positive threshold(T). When the signal is decrease to T again, the output is not change until the signal decrease to -T. This is the meaning of hysteretic switch.
Now, we build the schmitt trigger circuit which we will have the positive and negative threshold as follow.
The experiment and result is shown below. The yellow is the input of the system. The blue is the voltage at the positive feedback at the op amp. Last but not least, the pink one is the output of the schmitt trigger. Because the input voltage is connect with the negative terminal of the op-amp. The output will be invert.
Thanks for reading.