Dear Team,

I was seeing a comparator circuit designed with LM2903 .The circuit diagram is given below.It is working as expected in simulation.

In the feedback path you can see a 1Mega ohm resistor.

My question is,

1. Is it a good idea to put large resistors in feedback path.
2. Will this increase the noise pick up,because a large resistor in feedback path creates a high impedance node.

Everything works perfectly on a LTspice, but I wanted to check a few things before finalizing the board layout, and came across a TI application note which says in paragraph 6:

It is a standard procedure to use hysteresis (positive feedback) around a comparator, to prevent oscillation, and to avoid excessive noise on the output because the comparator is a good amplifier for its own noise. In the circuit of Figure 2, the feedback from the output to the positive input will cause about 3 mV of hysteresis. However, if the value of Rs is larger than 100Ω, such as 50 kΩ, it would not be reasonable to simply increase the value of the positive feedback resistor above 510 kΩ.

However, it doesn't say why a high value feedback resistor is "not reasonable".


I can definitely see good hysteresis behavior's with the 1M resistor in simulation

So, what is the drawback of High value resistor?

 

Sometimes it is a good idea. Notice that the feedback is to the positive input to the comparator. This means that once the comparator switches states it will take a larger change in input voltage get it to switch back. This usually means clean switching, improved because noise is largely ignores. The effect is referred to a Hysteresis.

 

Hi Hoy,
Checkout this PDF.
E

 

It depends on the intended use of the ciruit. If, for example, the circuit is for precise temperature control then the allowed temperature deviation would dictate the amount of hysteresis required. For something like an overload cut-off use the amount of hysteresis would likely be less critical.

 

A large resistor in the feedback path doesn‘t create a high impedance node. Impedance at the non-inverting input without the 1M resistor is 44.5k, with it is 42.6k.
Generally, hysteresis is a good thing as is stops multiple transitions as a slow waveform crosses the threshold, but it can lead to a loss of accuracy.

 

The word "Feedback" almost always refers to "Negative-Feedback",
so when referring to a "Positive-Feedback" Resistor,
the "Positive" qualifier should always be included so that others reading your question
will immediately know what You are referring to.
.
.
.

 

In the feedback path you can see a 1Mega ohm resistor.

My question is,

Your thread title and question are misleading. We don't normally have feedback resistors on comparators. What you have is more typically called hysteresis.

 

Your thread title and question are misleading. We don't normally have feedback resistors on comparators.

Disagree.
It is indeed positive feedback from output to input and is common in comparator circuits.

 

As everyone has said, putting a high value resistor from output to non-inverting input provides positive feedback.
This provides hysteresis which is very helpful in many comparator applications.

If you need hysteresis, this is how to do it. There is nothing bad about doing this.
If you do not want hysteresis, then don't include the feedback resistor. Then you have to deal with the consequences of "chatter", i.e. noise when the inverting and non-inverting input voltages are almost equal.

The positive feedback resistor makes the circuit less vulnerable to input and/or system noise or the effects of slowly changing input voltages.