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.
Sorry, I didn't expect someone to do the full calculation. Was just looking for a sanity check.
The actual results measured using the AD9240 are below. The application is sensitive to peak voltages so that is shown along with the RMS conversion using 6.6, 99.9%. AD9240 is at the left of the list.
No buffer, AD9240 inputs shorted = 1.53mV pk-pk = 231uV RMS
AD8021 buffer = 2.14mV pk-pk = 324uV RMS
AD8021 buffer + AD8021 at x2.5, x5 and x10 all = 2.75mV pk-pk = 416uV RMS
AD8021 buffer + x5 AD8021 + x10 ADA4895-1 = 4.58mV pk-pk = 694uV RMS
AD8021 buffer + x10 AD8021 + x10 ADA4895-1 = 9.16mV pk-pk = 1.38mV RMS
The x10 noise from an AD8021 is the same as the noise from a 10x ADA4895-1 as the only amp.
The result is that gains over 50x do not help as the noise from 50x to 100x doubled.
Also, Why 10nV/rt-Hz rather than 11nV/rt-Hz since the noise gain is x11 and the amp noise is 1nV?
The Friis formula says to use the power gain. When the output of an op-amp is driving the high impedance input of another op-amp the resistance is near infinite. How does power gain relate to voltage gain in that situation?
I tried to make this clear. Hopefully it sums up why I am baffled.
All bit noise is peak to peak into the AD9240, +5V full scale. Each histogram has 2^32 samples (>4 billion). The highest and lowest values typically have less than 20 counts (they are very rare) but they matter in our application.
The board has several op-amps that can be switched into and out of the signal path. An ADA489595-1 configured for +10x produces 9 bits of noise. An AD8021 configured for +10x also produces 9 bits of noise..
Those same amplifiers cascaded with the ADA4895-1 first produce 28 bits of noise. If the noise from the first stage is supposed to dominate why do the +10x amps cascaded produce so much more noise than either do individually?
The graph previously posted has three grey dots. They are AD8021s with gains of +2.5x, +5x and +10x. Their noise levels are all the same individually, 9 bits.
Two AD8021s cascaded, the first set to +5x the second to +2.5x. produce 11 bits of noise.
Cascaded the AD8021 +10x and the +2.5x produce 12 bits of noise
Cascaded the AD8021 +10x and +5x produce 19 bits of noise.
Cascaded the ADA4895-1 +10x with the AD8021 +5x produces 15 bits of noise.