Computing signal to noise ratio (SNR) of two highly correlated time domain signals

I'm working in the space of EEG acquisition. I made a experiment as detailed below, and am now trying to obtain some results from the data.

I have a text file of an EEG signal in Matlab. I loaded the EEG signal onto an arbitrary waveform generator, then I monitored and recorded the waveform generator output on an oscilloscope. I imported the recorded signal from the oscilloscope back into Matlab. The correlation between the original signal and the oscilloscope signal is 0.9958 (i.e. very high).

I want to compute the SNR of the oscilloscope signal (what I'm calling my signal plus whatever noise is introduced through the digital-to-analog conversion and visa-versa). I have attached a snippet of the 2 signals for reference.

So my original signal is X and oscilloscope signal is X + N. I used the snr function to compute SNR as follows.

snr(original, (oscilloscope - original))

The result I got was 20.44 dB. This seems off to me as I would have thought with such a high correlation, that the SNR should be much higher?

Or is it not appropriate to try and compute SNR in this sort of situation?

All help is appreciated.

Thanks