Any body knows what is the Noise figure of LTC6560 (with an APD with 4pF total input capacitance)?
You could calculate the LTC6560 Noise Figure if you knew your APD's noise current density. However, from looking at the literature, the APD noise calculation is complicated and involves many operating characteristics such as APD Gain or Multiplication factor (M), excess Noise Factor (F), etc. as outlined in this document link below:
The Noise Figure equation is:
NF = 10log [1+Na/Ni]
where Na is the LTC6560 input-referred noise current density, and Ni is the APD noise density.
Na (from graph above) @ 10MHz =~5pA/RtHz
Assuming your APD noise density at 10MHz is 2pA/RtHz (I've just made-up a number to illustrate the computation) -->
NF (LTC6560) = 10*log[1+5pA/RtHz / 2pA/RtHz ] = 5.4dB
Hope this helps answer your question.
Hi Giora again,
That's great that you know your APD noise current density for your operating conditions.
Correction to my computation earlier: Both Na and Ni are Noise current power densities in A^2/Hz.
Here is the corrected computation of Noise Figure assuming the same conditions (Ni = (2pA/RtHz)^2 and Na = (5pA/RtHz)^2 both at 10MHz):
NF = 10log [1+Na/Ni] = 10*log [1 + (5pA/RtHz)^2 / (2pA/RtHz)^2 ] = 3.5dB (instead of 5.4dB I'd incorrectly shown earlier).