Hey engineers!

I`m actually working on my Master Thesis which is concerned with an Laser Range Finder for Space proposes.

The AD640 logarithmic amplifier is used to measure the current from an avalanche photo diode (APD) and the AD8015 creates a current proportional voltage pulse..

Now i`m interested in the noise sources which are connected to the AD8015- trans impedance amplifier.

Significant noise sources through the AD8015 i can see are the APD with shot noise, thermic noise of resistors and noise load concerning to the noise current through the capacitys of the protection diodes D1 and D2 and the noise from the AD640.

In the picture you can see a simplified schematic plan and a noise schematic plan

My questions:

1.

The current noise density of the AD8015 is given with 3pA²/Hz. This is the shot noise Ib,s of the input with a load of 0.3pF on the input Stage.

The voltage noise density concerning to thermical noise of the input resistance Rb must be calculated by Un,in=sqrt(4 k T Rb B). This noise current effacts a load current through all capacitors by In,in=Un,in*C*B?

So, can i add Ib,s and In,in to get the resulting current noise density? -> as a value without noise of APD, R`s and AD640

2.

The voltage noise density Vn,in of the AD640 is given with 2nV²/Hz. What is the reason for this noise?

Can i use this voltage noise to calculate the resulting current noise by In,in²= Vn,in / Rin where Rin is the resistance of the input stage given with 500kOhms?

If not, what is the current noise i can expect from the AD640 that flows through the AD8015?

3.

Is it possible to calculate the Rb of the AD8015 or has anyone of the Analog engineers some explicit values like transition frequency of the input transistor or something like that for me to estimate?

Thank`s a lot for your help!!

With best regard

Tino

I can assist with your second question concerning the AD640. The 2nV/rtHz specified in the datasheet is the combined effect of voltage & current noise for a 50 ohm source. Figure 16 on page 7, of the datasheet shows the input structure. The typical Beta is about 120.

The equation you propose will provide a first order approximation of the current.