In the first part of this blog, we started looking at the noise calculations for our amplifier, filter, and ADC combination. Let's continue the analysis.
The next step is to take this value and multiply by the bandwidth factor to remove the frequency component in the noise. This is done by taking the square root of the bandwidth (we'll use 110 MHz since that is the 3dB bandwidth of the filter). This results in a value 39.2μV for the noise of the amplifier. Next, this number is converted into watts using a load impedance in this example of 150Ω which gives a noise value of 10.2pW.
We are getting closer, but we still have some math to make it through before we can figure out the overall SNR of the system. After converting the noise to watts, we need to convert the number to dBm so we can then convert it over to dBFS. Once we have the dBm value, simply add the full scale power of the ADC in dBm to obtain the noise level in dBFS. In this case, the noise in dBm is -79.9dBm, the full scale level of the ADC is 4.07dBm (converted from 1.75V, p-p), which means the noise form the amplifier is -75.83dBFS.
Now we must convert the ADC noise (SNR in dBFS) and the amplifier noise (in dBFS) back over to watts so we can add them together. Once they are added together in watts they can be converted back to dBFS and the result ends up at 68.75dBFS. If you refer to circuit note CN-0242, you'll see that the final result of this example is 68.4dBFS at the center of the band.
The SNR in dBFS of the ADC by itself in this case (not shown in the circuit note) was measured at 69.7dBFS. Using this value the calculated value for the expected SNR is 68.752dBFS. This is quite a nice result to have the expected and actual SNR values line up so closely. As engineers we always like to see our calculations line up as close as possible to what we measure. Again, if you'd like to examine the calculations more closely, please send me an email and I can forward the calculation spreadsheet to you.
It is important to consider the noise impact when driving an ADC with an amplifier and an AAF. I hope you've found this useful and informative. Stay tuned as we continue to look at the other noise doorways and how they impact ADC performance.