I would like a sanity check on my noise calculation and the actual result and any other comments.
Two ADA4895-1 op-amps in series each with a non-inverting gain of 10x. Rf = 226 ohms. Rg = 25.1 ohms, Rs = 51 ohms.
They are followed by an AD8021 with a non-inverting gain of 1. That is followed by a single pole R-C filter with 249 ohms and 18pF into an AD9240 ADC. The R-C filter frequency should be 35MHz. 55MHz was used due to the 1.57 single pole factor.
From the ADA4895-1 datasheet:
236 MHz, −3 dB bandwidth (G = +10)
voltage noise is 1nV / SQRT(Hz)
current noise is 1.6pA / SQRT(Hz)
With the input grounded what is the predicted amplifier output noise into the AD9240? For my first approximation I ignored the AD8021 and assumed the first stage ADA4895-1 would dominate.
I just noticed you haven't gotten a reply since 5/15, so we owe you something.
I'm lazy, so I won't calculate the whole thing, but remember Friis' equation:
Friis formulas for noise - Wikipedia, the free encyclopedia
Even though they are talking about microwaves, the same principle applies.
If you have enough gain in the first stage, the other stages matter very little.
So to a first approximation, it's simply 10nV/rt-Hz integrated over the bw.