AD9361 IIO Oscilloscope noise floor

More noise around fs/2 is due to sigma delta ADC noise shaping, When RF gain is very small, RF signal chain noise floor is dominated by the ADC Nosie floor, if the Over Sampling Ratio is big enough, (HB1/2/3 enable), It makes the K >=8, so the inband Nosie floor ( DC~fs/2 ) [fs=output DataRate] is flat. if K=1, the noise floor is the standard sigma-delta noise floor as observed.

More noise around fs/2 is due to sigma delta ADC noise shaping, When RF gain is very small, RF signal chain noise floor is dominated by the ADC Nosie floor, if the Over Sampling Ratio is big enough, (HB1/2/3 enable), It makes the K >=8, so the inband Nosie floor ( DC~fs/2 ) [fs=output DataRate] is flat. if K=1, the noise floor is the standard sigma-delta noise floor as observed.

Hello,

I hope it's not too late to have more informations!

You said " if the Over Sampling Ratio is big enough, (HB1/2/3 enable), It makes the K >=8, so the inband Noise floor ( DC~fs/2 ) [fs=output DataRate] is flat.". But in my configuration, HB1/2/3 are enabled as F_adc = 8 x F_rxsamp.

So, is it normal that the noise floor is not flat?

Thanks in advance.

Best regards,

Vincent

RFVS said:

You said " if the Over Sampling Ratio is big enough, (HB1/2/3 enable), It makes the K >=8, so the inband Noise floor ( DC~fs/2 ) [fs=output DataRate] is flat.". But in my configuration, HB1/2/3 are enabled as F_adc = 8 x F_rxsamp.

So, is it normal that the noise floor is not flat?

Yes, Your understanding is correct.

So, what are the conditions to have a flat noise floor?

Best regards.

I see that your are not using FIR, is there any reason? if not enable FIR and try.

You can use the Filter wizard to generate required config and filter.

https://wiki.analog.com/resources/eval/user-guides/ad-fmcomms2-ebz/software/filters

What is your sampling rate and BW requirement?

You can optimize your decimation such that ADC is at highest rate for your required data rate. In other way try to keep the OSR high enough.

Thanks for the answer.

Indeed, optimizing the decimation increases the performance.

We need a sampling rate of 55 MHz and a bandwidth of 50 MHz.

How would the FIR help us to reject the quantization noise ?

We recommend to create a custom profile using the filter wizard and verify on our eval board.