I am relatively inexperienced with ADCs so my apologies if this question is a bit naive but I could not find any clear answer here.
I am using an SDR receiver build on the LTC2145-14 clocked at 125 Mhz in order to measure power of weak sinusoidal signals. The analogue frontend of the ADC is quite simple, it is the one in the datasheet of the device, a 50 ohm resistor is connected to the high impedance differential A+ and A- analogue inputs and a wideband 1:1 balun connects this 50 ohm resistor to the input sma port of the receiver. No analogue op amp in the way, just a very loss loss anti-aliasing filter with a 52 Mhz cut off frequency to prevent reception of signals above the Nyquist frequency of 62.5 Mhz. With this set-up the input impedance of the receiver is a nice and fla 50 ohm from 100khz to about 50 Mhz.
To measure power, I simply convert the signal received in dBFS into actual true world dBm. Using an external rf power meter, I have determined empirically that the relationship between "dBFS" and "dBM" in my set-up is -10 dB . It does not change much across the whole dynamic range of the SDR receiver and across its frequency range either. For instance a signal generator delivering -40 dBm into my 50 ohms input impedance rf power meter will correspond to a signal reading of -50 dBFS when it is connected to my 50 ohm input impedance SDR receiver.
The trouble is that I am unable to explain this result by looking at the datasheet of the LTC2145-14!! The SENSE pin 63 of the LTC2125-14 chip is set to "0V" which corresponds to 1v peak to peak full scale, so +4 dBm into a 50 ohm load. So if 0 dBFS is corresponding to +4 dBM then according to the same logic, a signal of -40dBm should correspond to a signal of -44 dBFS but as explained above, I measure -50 dBFS and not -44 dBFS !!! There is a 6dB difference that I can not explain in anyway!
I concluded it is the SDR software that is responsible for this difference but I am not too sure really. Can someone better explain me where this 6dB difference comes from ? Is it the SDR software that I use that explains it ? Is the reasoning I am making erroneous and if yes, can someone explain me why ?