gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
110711-0755 EDT
Smart $:
Probably all high quality, meaning wide dynamic range, low output high impedance microphones will require electrostatic shielding and maybe magnetic shielding of the microphone and its cable. Almost always this will end up meaning the microphone is grounded at the amplifier input. In times past, before the 3 prong plug, ground might have meant simply the amplifier chassis.
Some microphones, like carbon button, are high voltage level devices in a low impedance circuit with low dynamic range and do not need shielding.
But back to instrumentation. Whether electrostatic shielding is required for an instrumentation cable is a function of many factors. Some of these are signal level, impedance, type of signal, redundancy of information, error correction, and sources of noise. Thus, one needs an understanding of the equipment and environment to make a decision on whether or not shielding or grounding are required.
In the case of a 20 mA current loop assume resolution is 0.1% and that noise should be limited to something on that order of magnitude. Then noise currents in the loop must be less than 0.02/1000 amperes or 20 microamperes. Suppose the loop resistance is 1000 ohms, then to produce 20 microamps requires an induced voltage in the loop of 20 mV. The twisted wire pair most likely will prevent this level from magnetic coupling and electrostatic shielding would not have prevented magnetic coupling. At 100 Hz to get a capacitive reactance of 1000 ohms requires 2.6 microfarads. If the voltage source to the capacitor is 200 V and this needs to be reduced to 20 mV, then approximately a coupling capacitance of 260 pfd would produce this signal level.
The geometry to producing this coupling would be very complex. Probably easily achieved if the current loop wire was in the same duct with a 480 V supply. But that is bad practice. If the signal wire was just strung across a room, then very unlikely much coupling. Shielding of the twisted pair does somewhat reduce then need to be concerned with the signal wire position relative to high power wiring.
But my guess is still that Ken9876's problem is not how the cable shielding has been done so long as the entire cable was shielded.
.
Smart $:
Probably all high quality, meaning wide dynamic range, low output high impedance microphones will require electrostatic shielding and maybe magnetic shielding of the microphone and its cable. Almost always this will end up meaning the microphone is grounded at the amplifier input. In times past, before the 3 prong plug, ground might have meant simply the amplifier chassis.
Some microphones, like carbon button, are high voltage level devices in a low impedance circuit with low dynamic range and do not need shielding.
But back to instrumentation. Whether electrostatic shielding is required for an instrumentation cable is a function of many factors. Some of these are signal level, impedance, type of signal, redundancy of information, error correction, and sources of noise. Thus, one needs an understanding of the equipment and environment to make a decision on whether or not shielding or grounding are required.
In the case of a 20 mA current loop assume resolution is 0.1% and that noise should be limited to something on that order of magnitude. Then noise currents in the loop must be less than 0.02/1000 amperes or 20 microamperes. Suppose the loop resistance is 1000 ohms, then to produce 20 microamps requires an induced voltage in the loop of 20 mV. The twisted wire pair most likely will prevent this level from magnetic coupling and electrostatic shielding would not have prevented magnetic coupling. At 100 Hz to get a capacitive reactance of 1000 ohms requires 2.6 microfarads. If the voltage source to the capacitor is 200 V and this needs to be reduced to 20 mV, then approximately a coupling capacitance of 260 pfd would produce this signal level.
The geometry to producing this coupling would be very complex. Probably easily achieved if the current loop wire was in the same duct with a 480 V supply. But that is bad practice. If the signal wire was just strung across a room, then very unlikely much coupling. Shielding of the twisted pair does somewhat reduce then need to be concerned with the signal wire position relative to high power wiring.
But my guess is still that Ken9876's problem is not how the cable shielding has been done so long as the entire cable was shielded.
.
