Hi Mike,
PSR is how the ADS1220 will respond with respect to the noise of the supply. To measure the PSRR you need to modulate the supply with a known value so that you can adjust the calculation appropriately. Using a gain of 1 with the internal reference the full-scale range is 4.096V. If you modulate the supply with a peak to peak voltage some where in the range of 0.25 to 0.5V (remember that you must not exceed the absolute maximum voltage specifications and must stay within the proper operating voltage) you should be able to see the response on the FFT. You will need to adjust the response of the measured spur on the FFT with respect to the input signal. Let's say the input is 0.25V and the full-scale range is 4.096V, the input would be 20LOG(4.096/0.25)dB.
You can also measure in the time domain by measuring the signal based on the data results. You can measure the peak to peak variation of the data so that the calculation is 20LOG(input voltage/measured voltage).
To get above the noise floor you need to modulate the supply with a signal large enough to show up as either a noticable spur in the FFT or as measurable data.
Best regards,
Bob B