In Reply to: Re: Earth v.s. circuit ground posted by toysl on 08/15/00 at 10:50 PM:
Answering your questions in order:
1. It is desireable to have a signal ground seperate from chassis ground but in many cases for practical reasons they are not. A signal ground is designed to be electrically quiet so that it's connection to the safety (chassis) ground is isolated from current paths from other sources which would induce noise voltages due to several currents flowing through the common impedance of the wire to ground. The grounds ARE electrically connected but not at the same potential.
Eg. if an audio amp PCB has it's speaker return grounded to the chassis and the mic lead shield also connected to chassis, the wire from the amplifier PCB ground to chassis will carry both mic and speaker currents. The voltage developed from the much larger speaker current will be impressed on the mic and oscillation will result.
Move the mic shield connection from chassis to the amplifier PCB ground near the mic i/p and now it does not matter that the PCB ground is not the same as the chassis ground as the mic has no connection to the chassis. The PCB ground is now a signal ground for the low level audio. The chassis is the safety ground for the system and part of the EMI shield. Even though they are connected they are not at the same potential due to voltage drops across the wire connecting the two. Never mistake the DC resistance of a wire for it's impedance, they are vastly different. Eg a #10 wire 1m long and 25mm above ground has 3.1 milliohms resistance at DC, but has 4.7 ohms impedance at 1MHz. Even very short wires have significant impedance at 100MHz.
2. By Common mode capacitors in this context, I'm referring to capacitors that connect from each line of a pair to ground and a third betwen the lines. Noise voltages on the ground are connected equally and in phase to each line (called a common mode signal)and cancel at the load. Likewise noise received on one line is connected to the other.
3. If you have the signal ground totally isolated from any other ground then the only coupling to ground is via stray capacitance and my original statement statement does not apply. This is very uncommon not to have some ground connection somewhere though, and totally isolated systems are rare.
4. Many times the chassis earth and signal earth are common so it is prefferable to ground the suppressor at the signal entry point on the chassis to minimise EMC issues. Other times the designer didn't know what he was doing and connected it wrong. You have to look very carefully at what you're attempting to achieve and what is needed when installing suppressors.