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

• Scott,

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.

Also see:

Harry

• 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

• Sloppy thinking on my part.  Yes, the NG =11 and you are correct.

• 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