|ADC approximate grounded input spectrum at 10MSPS|
The next step is to add the amplifier and ground its input with a 50 ohm SMA terminator. The only filtering present is the shunt 75pF capacitor at C115. That spectrum is below.
|Prj137 50 ohm grounded input. Amplifier with ADC noise floor at 10MSPS.|
Finally, a histogram was taken of the grounded input configuration. Looking at the frequency spectrum and time series provides a lot of useful information in tracking down problems but I have also come to appreciate the value of a histogram. The initial efforts with this board over looked a software work around from the previous board where some of the LSB were incorrectly masked. This was not at all obvious in the grounded input time series or spectrum but was very clear in the histogram. The following is the histogram of the 50 ohm terminated board input with amplifier (same as previous spectrum).
As points of reference, the LSB is 1V / 2^12 = 1/4096 = 244uV. For noise evaluation using the processing gain of the FFT the -90 dBFS RMS voltage is dBFS = 20 * log10(Vnoise / 0.5V) or Vnoise = 0.5V * 10^( -90 dBFS / 20 ) = 16uV.