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DC and detection of very low frequencies
Audio AD-converters always have a DC filter, because the ADCs operate with a fixed bias voltage
(VCOM), and any deviation from this leads to a degradation of their performance. An externally
supplied DC voltage would do exactly that. In all Pro, two unipolar Nichicon audio electrolytic
capacitors, directly at the input jack, prevent external DC voltage from entering. The rest of the
analog signal path is DC-coupled to maintain the lowest possible cutoff frequency and phase
deviation.
The combination of relatively high input impedance (47 kOhm) with 47 µF coupling capacitors
results in an extremely low cutoff frequency of (calculated) 0.07 Hz.
The ADI-2/4 Pro SE can actually use this low cutoff frequency effectively - many other converters
could not, because the analog input section has a low DC offset, which is detected by the ADC,
and shows up in the recorded audio signal as a fixed DC component. If this offset is too high, click
noises occur at the beginning, at the end, during editing etc.. The ADI-2/4 Pro SE is factory ad-
justed to minimum DC offset, typically -90 dBFS - problem solved.
The DC offset is usually eliminated in the digital domain via a high-pass with 1 Hz cut-off frequency
within the ADC. However, the ADC of the ADI-2/4 Pro SE does not provide such a filter. Therefore
a filter developed by RME is used, in the FPGA, which has a cut-off frequency below 0.5 Hz and
very low phase deviation. This filter is active by default. The DC component is thus reduced to
below -130 dBFS.
For the measurement of extremely low frequency signals, the RME filter can be deactivated to
obtain the lowest possible cutoff frequency. An example is the measurement of these two states.
The cutoff frequency at -3 dB, measured with the APx555B, is actually in the calculated range,
below 100 mHz.
Noise and Out-of-band signals
See next chapter
User's Guide ADI-2/4 Pro SE – v 1.0
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