AD9375 Full Scale Power

I've read the previous discussions on this matter and find the common answer seems to be:

0.707 mVpp which corresponds to +7 dBm based on a 50ohm system.

Also, commonly cited is a note about Sigma-Delta Converters exhibiting a soft overload characteristic, which is only important near maximum inputs.

We are using the AD9375 EVM connected to a custom FPGA EVM.  We have a RSSI calculation block built into our RX Signal Chain.  This block does a sum of I^2 + Q^2 of the received samples and we then use that to calculate RSSI of our input signal.

For testing and calibration we use a CW tone, 1MHz offset from the tuned frequency.  Applying the +7 dBm Full Scale Power, the calculation seems to be about 16.4 dB off expected.

To troubleshoot, I performed the following test.  Using a calibrated cable connected between the Sig Gen and the 9375 EVM RX1 port at 680 MHz, Rx tuned to 679 MHz, RX1 Gain index set to 255 (0 dB of attenuation), Logic Analyzer connected to capture the raw 14 bit JESD I/Q samples from the 9375, I see a very nice sinewave with  peak amplitude of 2414.  Since the samples are 14 bits, this is 2414/8191 in amplitude.  20 * log10(2414/8191) is -10.6 dB.  This says my signal is -10.6 dB below full scale.  My input is -20.0 dBm, so this says Full Scale Power is actually -9.4 dBm.

I understand the recommended max input to the 9375 is -14 dBm, but I was able to get up -11 dBm with no noticeable distortion in the observed sinewave samples, got a peak sample value of 6840, which using the same math as above yields -1.56 dB or a SP of -9.4 again. 

Can you please advise where I went wrong in my assessment of Full Scale Power?

Parents Reply Children
  • For the last sentence, "-14 dBm max input is with max gain (0 Attenuation) which corresponds to +7 dBm at ADC output.", did you mean +7 dBm at ADC "input"?

    Now that I read that statement again and I looked at the data sheet again, particularly Fig 13, which is Receiver Gain vs Receiver Attenuation, I've come to realize that there is between 16 and 17 dB of gain when there is 0 dB of attenuation.  Just for arguments sake, let's call it 16.4 dB of gain at 0 dB of attenuation.

    -20 dBm input + 16.4 dB of gain is -3.6 dBm at the ADC input.  If FSP is +7, this means I am -10.6 dB below FSP.

    -10.6 = 20 * log10(x/32767), solving for x gives 9643.  I measured 9656 (4 * 2414), so an exact match.

    So, my problem, and probably other peoples problem, is the 16.4 dB of gain which I think is only specifically called out in Figure 13.

    •  Analog Employees 
    on Sep 24, 2020 8:47 AM in reply to cpeters1

    Yes.All the calculations mentioned above is correct.