MPU6000 Low power and normal power

Hello,

I tried to compare the mpu6000 low and normal power mode. I have configured the following:
1. Low power, DLPF =5Hz, 40Hz sampling rate.
2. Normal power, DLPF =5Hz ,40 Hz sampling rate.

I used different vibration frequencies (SWEEP from 5 to 150 Hz)

In normal power mode, the graph was monotonically decreasing, having the -3dB frequency at approx DLPF Hz. Aliasing was noticeable. every now and then the sine wave became flat for a short period of time. Overall OK.

In low power mode, the graph was periodic!. In 40,80,120 the sine wave became flat (DC) with (what appeared to be) non filtered magnitude. Immediately after those frequencies, the magnitude decreased according to the DLPF until half the sampling rate, and then increasing towards the following DC aliased frequency.

It seems that in low power mode the mpu6000 somewhy first samples at 40 Hz and then filteres using the DLPF. In "normal mode" it seems exactly the opposite, like sampling should be- with LPF first.

Am I right? What am i doing wrong? Does in low power mode the mpu6000 first samples at 40 Hz and only then filters?

Thanks a lot!
John