Comparator hysteresis resistors & toggle rate

In summary, the speaker is discussing a comparator circuit with external hysteresis and how it behaves when a square wave input is applied. They have observed that when the duty cycle of the square wave is increased beyond 75%, the output remains high. However, when the hysteresis resistor values are reduced, the output responds to higher duty cycles. This behavior is not limited to a specific device. The speaker suspects that the pin capacitance at the capacitor's pin may be causing this, as smaller feedback resistors could charge or discharge it faster and increase the system's bandwidth. They mention that the frequency is 1Mhz and the edge rate is 18ns.
  • #1
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I have a comparator ckt with external hysteresis. When I apply a square wave input everything works fine. The trip points are as designed, output goes high/low etc.
But when I increase the duty cycle of the square wave to say beyond 75%, the output just stays high.
I was playing with the hysteresis resistor values. When I reduce the resistor values (and keep the same ratio), the comparator output responds to duty cycles higher than 75%.
Same is true when i decrease the duty cycle.
The comparator I am using is Microchip 6564. However this behaviour not limited to just this device. (please ignore the part number in the attachment).
I don't understand what's causing this behaviour.
 

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  • #2
Since I don't know the frequencies or edge rates it's hard to say for sure but I suspect it is the pin capacitance at the pin of the capacitor. Smaller feedback resistors could charge or discharge this C faster increasing the bandwidth of the system.
 
  • #3
es1 said:
Since I don't know the frequencies or edge rates it's hard to say for sure but I suspect it is the pin capacitance at the pin of the capacitor. Smaller feedback resistors could charge or discharge this C faster increasing the bandwidth of the system.

Which capacitor?
The freq is 1Mhz,edge rate 18ns.
 
Last edited:

Related to Comparator hysteresis resistors & toggle rate

1. What is the purpose of using comparator hysteresis resistors?

Comparator hysteresis resistors are used to prevent unwanted oscillations or rapid changes in output voltage in a comparator circuit. They provide a stable reference voltage that helps to improve the accuracy of the comparator's output.

2. How do comparator hysteresis resistors work?

Comparator hysteresis resistors work by creating a small difference between the upper and lower threshold voltages of a comparator. This difference, known as hysteresis, ensures that the output of the comparator changes only when the input voltage exceeds the upper threshold by a certain amount, and not when it drops below the lower threshold. This helps to eliminate false triggering and produces a more stable output.

3. What is the importance of toggle rate in comparator hysteresis resistors?

Toggle rate refers to the speed at which the comparator output switches between high and low states. In comparator hysteresis resistors, a higher toggle rate means that the output will switch more quickly, which can be beneficial in certain applications where a fast response time is required.

4. Can the toggle rate be adjusted in comparator hysteresis resistors?

Yes, the toggle rate can be adjusted by changing the values of the hysteresis resistors. A lower resistance will result in a faster toggle rate, while a higher resistance will slow down the toggle rate. It is important to choose the right values for the resistors to achieve the desired toggle rate without affecting the stability of the circuit.

5. Are there any disadvantages to using comparator hysteresis resistors?

One potential disadvantage of using comparator hysteresis resistors is that they can introduce some delay in the output signal, which may be undesirable in certain applications. Additionally, choosing the wrong values for the resistors can lead to instability and false triggering of the comparator. Proper circuit design and selection of appropriate resistor values can help to overcome these potential drawbacks.

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