Atik 383L+ read noise increase with higher voltage?

[ogami]

This weekend i did some testing with new camera. I found one strange think ... with higher voltage i measured higher read noise. Im using 12 deep cycle battery with DC step-up converter.

DC converter with 13,8V on output -> read noise 13,2
DC converter with 13,3V on output -> read noise 12,2
Camera directly connected to full charged battery (12,9-13v) -> read noise 11,8
Camera powered with 220V power source (12v/4A on output) -> read noise 10,6

Is it some noise from power source? Or is it normal, that camera produce bigger read noise with higher voltage?

http://astrophoto.cz/temp/atik383_bias.rar

Here are 3 biases with different voltage (noted in file name), if someone want take a look. There is visible difference in histogram and standard deviation values.