The gain was measured by Tee’ing a DDS output, post filter, to the oscilloscope (with 1M impedance), and into the ADC. The DDS attenuation was varied to produce different levels that were measured on the scope and with the ADC in dBFS and counts. The value of using counts is that the dbFS has small deviations from expected (0.9dB). I believe this is due to an error in my windowing amplitude correction. The ADC count levels are obtained using the peak picking on the time series with the offset found by “peaks” on a no-input signal. The values are all 16 bit normalized. The full scale is 1V. The following table captures measured values and those values processed to produce a voltage gain.
|ADC values with input using Rf=294,Rg=25 ohms.|
|ADC board FDA Spice Model|
The following is a spice AC Bode plot of the single ended input and output voltages. The single ended input is less than 0dB as it is referenced to the source (pre source output impedance). It is a little less than a 6dB drop so the input impedance of the amplifier is not exactly 50 ohms (which we knew from the calculations but close enough using standard resistor values). The output is +11.5dB from the reference with the difference between input and output being 15.6dB.
|SPICE AC Gain. SEin=-4.5dB, SEout=11.6dB, Voltage gain=15.6dB w/ 8 degree phase variance across the span.|
|SPICE model with output LPF and ADC input model.|
Magnitude and Phase of Vout-se/Vin-se with LPF and ADC model.