Standard & Poor's raised its long-term issuer credit rating on AMP Bank to A+ from A yesterday. The short-term issuer credit rating remains at A-1. S&P also raised the issue ratings on the bank's senior subordinated debt to A, from A-. The outlook is stable.The ratings agency said its action "reflects our improved assessment of AMP Bank's stand-alone credit profile, reflecting the bank's revised capital and earnings assessment to 'very strong' from 'strong', based on our view that the bank's improved risk-adjusted capital ratio will remain above 15 per cent in the medium term."S&P also said it believed the bank "will stabilise and gradually improve its net interest margins, which should contribute to solidifying the current business position assessment."After reporting very strong earnings' growth in 2011, AMP Bank reported only a modest rise in operating profit in 2012, to A$62 million from $61 million.The cost of deposits hurt the bottom line in 2012. The bank's net interest margin fell from 1.54 per cent, in 2011, to 1.3 per cent in the first half of last year, and then fell further, to 1.23 per cent, in the December half. S&P expects this trend to reverse.In January, Moody's Investors Service affirmed AMP Bank's long term senior-debt rating at A2.