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.

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.

Scott

• Harry,

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

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. Scott

• Harry,

Also, Why 10nV/rt-Hz rather than 11nV/rt-Hz since the noise gain is x11 and the amp noise is 1nV?

Scott

• Harry,

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?

Scott

• Harry,

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