# How to use my bits to transmit this photodiode signal in the most efficient way possible? [closed]

Suppose I have photodiode with a Dark Current of 5 pA and that using the NEP of that photodiode at some frequency $$\\lambda\$$ I determined that the smallest signal I could detect was of 1 pA.

First of all, does it make sense that my signal is smaller than the Dark Current?, my understanding is that they are additive, so when I have no signal I would see Dark Current $$\\pm\$$ Noise, but that would always be smaller than 6 pA, thus when I get 6 pA I know I had a real signal.

Now, assuming this is the case, what should I do with my ADC in order to have all my bits carrying information from the signal?.

Again, my understanding is that if I have, for example, 16 bits, and the Dark Current uses 8 of those, then there are only 8 for my signal.

Should I set the ADC to ignore signals smaller than 6 pA?, 11 pA?.

Signal theory is really not my area and I am not sure where to start learning it.

## closed as unclear what you're asking by Eugene Sh., Edgar Brown, Dwayne Reid, Sparky256, BimpelrekkieFeb 12 at 14:32

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• No hope of working, learn more about communication theory, SNR , BER, signal vs Noise BW, BPF design, Q, group delay, detector jitter vs SNR, hysteresis, etc – Sunnyskyguy EE75 Feb 5 at 22:49
• Also the gap between your knowledge and reality is very large, many of the statements just don't make sense. Signal theory is really not my area that is clear. Start by reading a book and/or following a course. – Bimpelrekkie Feb 12 at 14:39

To get started, the NEP is the power of the signal that give you an SNR of 1 (not very useful for communication) at a bandwidth of 1 Hz (Damn slow). To do a quick back of the envelope calculation, pick a bandwidth, close to half your desired data rate, and a desired SNR, and get $$\P = SNR \cdot NEP \cdot \sqrt{BW}\$$. You will probavly get something higher than 1 pA.
• Thanks, first of all I should note that there are conflicting definitions of NEP, the link at the end explains them. The point is, I was using what you define as NEP $\times BW^{1/2}$, second, I don't just get values that are bellow my Dark Current, they are a whole order of magnitude bellow. But that is using a SNR of 1, should it be larger?, how large should it be?. Second, I want to know how big the signal should be and how to calculate it. Link to the paper about NEP: thorlabs.com/images/TabImages/… – Fernando Franco Félix Feb 5 at 23:16
• I'm not sure what you are referring to. NEP is in $W.Hz^{-1/2}$. Multiply by $\sqrt{BW}$ to get $W$. See equation (2) in your link. If your bandwidth is anything larger than $1 Hz$, $P$ is larger than the NEP. – pserra Feb 5 at 23:21