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A/D-conversion module (ADM)
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The incoming signals from the intermediate current transformers are adapted to the
electronic voltage level with shunts. To gain dynamic range for the current inputs, two
shunts with separate A/D channels are used for each input current. By that a 16-bit dy-
namic range is obtained with a 12 bits A/D converter.
The next step in the signal flow is the analog filter of the first order, with a cut-off fre-
quency of 500 Hz. This filter is used to avoid aliasing problems.
The A/D converter has a 12-bit resolution. It samples each input signal (5 voltages and
2x5 currents) with a sampling frequency of 2 kHz.
Before the A/D-converted signals are transmitted to the signal processing module, the
signals are band-pass filtered and down-sampled to 1 kHz in a digital signal processor
(DSP).
The filter in the DSP is a numerical filter with a cut-off frequency of 250 Hz.
The transmission of data between the A/D-conversion module and the signal processing
module is done on a supervised serial link of RS485 type. This transmission is per-
formed once every millisecond and contains information about all incoming analog sig-
nals.
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