Intellivox DDC Installation Manual Appendix | EN 15 |
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Although in terms of safety and immunity this is the most preferable set up, an unwanted ground loop is created which can cause audible noise or hum. The appearance of this effect is strongly depending on the cable routing, the design of the grounding scheme and the existence of local magnetic fields (caused by power transformers, railway chopper supplies etc.). If a ground loop occurs, the most straight forward solution to the problem is to interrupt the ground path from the shielding of the audio signal wiring. This situation is shown in Fig 10.
The audio input transformers feature a very high Common Mode Rejection Ratio (CMRR), this means that an interfering signal which is coupled onto both the audio + and - signal wires is strongly rejected compared to the differential audio signal. Unfortunately, this high CMRR is only achieved at low frequencies (<100 Hz) and deteriorates for higher frequencies. In order to restore the original grounding configuration for the HF part of the spectrum, it is good a practice to connect a 100 nF capacitor between the shielding of the audio cable and the GND pin of the terminal block (see Fig 11).
Fig 10 Grounding configuration with totally decoupled audio ground
Fig 11 Grounding configuration with low frequency decoupled audio ground
Bosch Security Systems July 2003