Intellivox DDC Installation Manual Appendix | EN 16 |
|
|
In order to restore the galvanic safety path of the signal wiring up to a certain level, it is possible to connect a 100 OHMS 1W resistor parallel with the 100nF capacitor. The isolation of the low frequency ground loop is partly canceled by this resistor, but the reduction of the induced noise can still be sufficient while at the same time some of the benefits of the original concept are restored. A diagram of this solution is shown in Fig 12.
The method described is also well suited for installations with multiple distributed units on one signal source. If only one chassis is connected, interruption of the loop near the signal source is theoretically a better option, although this method can cause problems if the output of the audio source is not equipped with a transformer (electronically differential output). Due to the capacitive coupling of the (remote grounded) shield, it is possible that a common mode
Fig 12 Ground configuration with audio ground partly decoupled for low frequencies
General strategy
The best approach is to interrupt the ground loop just far enough to get rid of the disturbing effects of it. Therefore it is advised to start with the full grounded configuration (Fig 9), if this causes problems try 100 OHMS // 100nF (Fig 12), next leave 100 OHM off (Fig 11) and the final solution might have to be the most rigorous method (Fig 10) which will introduce the danger of trading the hum for RFI problems.
For longer cable runs (> 50m)
Use low output impedance balanced signal sources, preferably transformer balanced. The CMRR for higher frequencies will be heavily degraded if the impedance with respect to ground of both + and - signal lines is not equal (as with unbalanced outputs). Degraded CMRR can also be caused by poor quality cable due to asymmetrical capacitive properties of the core.
interference signal is injected into the output terminals of the source that may cause misbehavior of the electronical driver circuit.
In case the chassis is connected to a 100 V or 70 V distributed system, no ground loop can occur. The configuration resembles the total decoupled ground configuration in Fig 10. To adapt the input transformer to the much higher 70 V / 100 V signal level, the input is internally attenuated by a differential mode divider network. This means that the common mode sensitivity of the attenuated 70 V / 100 V input is not different from the 0 dB input. If any problems occur caused by common mode induced RFI, a possible solution is to reduce the common mode impedance for high frequencies by adding two 100 pF capacitors as shown in Fig 13. For optimum CMRR the values of the capacitors should be matched as close as possible. An optional ferrite clamp can be used in combination with the capacitors to enhance the performance of the filter.
Bosch Security Systems July 2003