Time constant of photodiode? parallel stray capacitance?

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Discussion Overview

The discussion revolves around the time constant of a photodiode circuit, specifically focusing on the role of feedback resistance and parallel stray capacitance. Participants explore the implications of these concepts on circuit response and behavior, including definitions and distinctions between terms like stray capacitance and parasitic capacitance.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the meaning of the time constant of a resistor and its significance in the context of circuit response.
  • Another participant explains that the time constant is determined by the product of resistance and capacitance (RC), and that it affects how quickly the op-amp output responds to changes in the photodiode signal.
  • There is a discussion about whether parallel stray capacitance is equivalent to parasitic capacitance, with one participant suggesting that parasitic capacitance arises from the proximity of different circuit components.
  • A later reply provides a definition of parasitic capacitance, indicating it is an unavoidable capacitance due to the physical arrangement of circuit elements.

Areas of Agreement / Disagreement

Participants express some agreement on the definitions of stray and parasitic capacitance, but there is still uncertainty regarding the exact relationship between these terms and their implications in circuit behavior.

Contextual Notes

Some assumptions about the behavior of circuit elements and the definitions of capacitance types remain unresolved, particularly in how they apply to specific circuit configurations.

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Hihi,

http://sales.hamamatsu.com/assets/applications/SSD/si_pd_circuit_kspd1043e06.pdf
5. high-sensor

From the message there, I don't understand what is the meaning of time constant of resistor?

Why does it say that the time response of the circuit depends on the time constant?

Is parallel stray capacitance = parasitic capacitance?

Thank you.
 
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> "From the message there, I don't understand what is the meaning of time constant of resistor?"

The statement is "the time constant of the feedback resistance Rf and its parallel stray capacitance."

A resistor (or pretty much anything, for that matter) will have a capacitance. A reasonable model for this is to say there is a small capacitor in parallel with the resistor. The time constance is simply RC, as it is with any resistor-capacitor combination.


> "Why does it say that the time response of the circuit depends on the time constant?"

The effect of the resistor's parallel stray capacitance is to try to hold the op-amp output at the same level w.r.t. the input. This RC time affects how quickly the op-amp output responds to a change in the photodiode signal.


> "Is parallel stray capacitance = parasitic capacitance?"

I'm not entirely sure, but I think parasitic capacitance refers to different parts or wires in the circuit being near each other and creating a capacitance.
 
...I think parasitic capacitance refers to different parts or wires in the circuit being near each other and creating a capacitance.
Yes. Here is what http://en.wikipedia.org/wiki/Parasitic_capacitance" says.
In electrical circuits, parasitic capacitance is the unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit, simply because of their proximity to each other. All actual circuit elements such as inductors, diodes, and transistors have internal capacitance, which can cause their behavior to depart from that of 'ideal' circuit elements. In addition, parasitic capacitance can exist between closely spaced conductors, such as wires or printed circuit board traces.
 
Last edited by a moderator:
Thank you for you guys help, I understand already. It just deserves a bit of thought. Thank you again.
 

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