I am looking at designing an amplifier for a phoptomultiplier tube.

I need a gain of 25 and a Fh of 300 MHz (to drive a 1GSPS 10 bit A to D with +/- 500mV input range).

Currently I'm investigating an amp consisting of 2 off cascaded AD8099 stages each with a gain of 5.

From the data sheets each stage will have a total output noise of 8.6 nV per root Hertz.

When I cascade two stages will the total noise be

1st stage noise x 2nd stage gain + 2nd stage noise, that is,

(5 x 8.6) + 8.6 = 51.6 nV per root Hertz?

OR

Is the noise statistical in nature and so needs different treatment, like taking the RMS value of 5 x 8.6 and 8.6 to give 43.8 nV per root Hertz.

Thanks,

Ian

Hi Ian,

The total noise at the output will be calculated using Root-Sum-Square method since the noise sources are uncorrelated (just like the case in considering the noise of the resistors). For your configuration,

Total output noise = sqrt( sq(8.6x5) + sq(8.6))

=43.85nV/rtHz

Here's a very good video regarding noise calculation:

http://videos.analog.com/video/products/amplifiers-and-linear/756529037001/Noise-of-a-Non-inverting-Operational-Amplifier/

Also, the noise spectral density is already expressed in Vrms per root Hertz.

You might also be interested in this video regarding Vrms noise:

http://videos.analog.com/video/products/amplifiers-and-linear/756529027001/Spectral-Noise-Density-to-RMS-Noise/

I hope this helps!

Neil