As mentioned in th epost from Aug 22nd, we biult a Low Noise Preamplifier (D060205). When we measured its noise level, we did not use any anti-aliasing filter and therefore the noise level of the amplifier laid above its specs.
For better measurents, we have now implemented this filter in the data acquisition process (written in C): We use an inverse chebyshev filter with variable order, attenuation and stopband frequency. For the following results, we used the filter with an order of 10 ( which for numerical reasons is the highest stable order we achieved), a stopband frequency of 2048 Hz and an attenuation of 80 dB, which leads to a 3 dB-cutoff frequency of 1340 Hz.
This new noise spectrum comes very close to the specification, as the noise level now is about or below 1 nV/rtHz. Unfortunatly, you can still see the 60 Hz lines and its harmonics.