# Time constant of photodiode? parallel stray capacitance?

1. Aug 3, 2008

### cks

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.

2. Aug 3, 2008

### Redbelly98

Staff Emeritus
> "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.

3. Aug 3, 2008

### dlgoff

Yes. Here is what http://en.wikipedia.org/wiki/Parasitic_capacitance" [Broken] says.

Last edited by a moderator: May 3, 2017
4. Aug 5, 2008

### cks

Thank you for you guys help, I understand already. It just deserves a bit of thought. Thank you again.