What is A??? And you forgot to attach img.tif and the "A" image, so I'm not going to do anything yet.
Since
PSNR=10log10(peakval^2/MSE)
and since you can't have negative values of PSNR unless MSE < peakValue^2, you must have A and ref on different scales, like you called im2double on one but not the other.
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