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DP83TD510E: Calculated SNR values from DP83TD510E seem high

Part Number: DP83TD510E


We are using the DP83TD510E and have been reading the 0x0A85 register then converting it to SNR SNLA364A.  Here is the code that we are using:


               HAL_ETH_WritePHYRegister(&heth, 0x0D, 0x001F);

               HAL_ETH_WritePHYRegister(&heth, 0x0E, 0x0A85);

               HAL_ETH_WritePHYRegister(&heth, 0x0D, 0x401F);

               HAL_ETH_ReadPHYRegister(&heth, 0x0E, &mse);       // Read the PHY dsp_reg_72 register.



               Sig2Noise = -10.0 * log10((float)mse/131072.0) - 1.76;

But the SNR we get seems to high.  For example, we get the following results from one test:

From DP83TD510E register read and calculation, SNR = 28.5dB

A scope capture of the signals looks like this:

So we estimate the signal is actually about 1.2V (half of the 2.39V seen in the capture because of full duplex transmission), and noise is about 10mV.

By our estimate, SNR ~ 10 log (1.2/0.01) = 20.8dB

Why is our estimate so different from the SNR reported by the DP83TD510E?  The best SNR we expected was for a 2.4V signal = 23.8dB.

Thanks for your help,