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 ?
I'm not familiar with the LTC2125 and the SDR software at all, but I thought I'd bring up a couple of semi-random points I use when doing a similar exercise with a different family of ADCs. A lot of this will be common sense but we might as well cover these for completeness and as a sanity check.
You are probably beyond this but I thought I'd mention some of these generic things in case they help.
I hope your project goes well.
Thanks for your reply.
Yes I have factored in all the points except maybe your last bullet point regarding the ADC adjustment gain. As I am not familiar with ADCs I am not sure if I factored this correctly or not. As you can read below, it may (?) be the explanation
As I explained it, the ADC2145-14 and I think many others ADCs of Analogue Devices have a sense PIN that allow to select different "input range of the ADC". When this sense pin is set to ground the input range is 1vpp corresponding to 4 dBm under 50 ohm, when it is set to VCC it is 2vPP corresponding to 10 dBm under 50 ohm and that is precisely a 6dB difference....
My expectation (but I am not sure) is that if I select the 1vPP range, the noise floor of the ADC that I measure in a 500 hz bandwidth should also raise e by 6dB ?, so raise from -123 dBFS (500 Hz bandwidth) to -117dBFS (500 Hz bandwidth) but it does not happen!!! the noise floor stays the same after I select 1Vpp instead of 2Vpp..... It maybe the problem in fact ??? Maybe the device or the PCB is somehow damaged and I am actually unable to select the 1Vpp to range for some reasons.
In the ADCs I'm familiar with, increasing the reference voltage, along with allowing for a greater input amplitude, also increases the noise density. You still get improved SNR but not as much as you might hope. I don't know if this comes into play with what you are looking at.
I'll ask one of my colleagues who knows about the LTC2145 and its sense function to jump in to this conversation.