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+--- Thread: Why is it Called Ground or Earth, and What is Common Mode Rejection? (/showthread.php?tid=9)
Why is it Called Ground or Earth, and What is Common Mode Rejection? - Tom - 10-06-2020
Early telegraph systems used one wire, strung on poles. The circuit was completed at each end of the single wire by making a
connection to the earth, which is conductive. That connection was variously called earth, ground, common, or return. It was not
always easy to get a good ground connection. If a rod driven into the earth was not sufficient, a plate could be buried, to get more
surface area. Even with a good connection to the ground, the potential of the earth is not everywhere the same, so rather large
telegraphic signals were required for reliable communications.
These single wire systems worked well-enough until the introduction of the electric tram. The disturbances caused by trams in the
earth's potential swamped the largest telegraphic signals. The solution was to run a second wire, originally called a metalic return.
The return was also referred to as a ground wire, or simply, earth. Then, the telegraphic signals were detected across two wires,
instead of between one wire and the literal earth. One company that rose as a result of this improvement became A, T & T.
The difference between the single wire system and the two wire telegraph system reflects the distinction between "single-ended" and
"differential" and it matters because "ground" is not simply zero volts. In fact, any two points designated "ground" will have AC and
DC voltages present between them that can easily become non-trivial. In a switched power system with fast edges, we have seen 50
volt spikes developed across a 1/4 inch diameter, 1/2 inch long aluminium standoff. Those spikes were an AC effect due to
self-inductance, not resistance, but if you want to measure DC down to the uVolt, you need to pay attention to uOhms. If you want
high precision analog measurements, get used to the notion that "ground" is relative, not absolute.
We modify our wiring schemes to minimize these grounding effects. Star grounding brings many wires to a "single point" ground.
That approach certainly helps to minimize ground potential differences, but having a separate conductor from each ground to a
single point in a large system is physicaly impossible. Plus, ground wires have resistance and self-inductance in proportion to their
length. So, we make do, and "ground" ends up being a range of voltages centering around zero.
To measure a voltage with real accuracy, the measured voltage has to be a difference between the potential at two points.
Otherwise, the variation in "ground" potential adds to any other errors. That differential nature is more intuitive when you use a
battery-powered digital voltameter. The reading on the meter is the voltage difference between the potential at the black lead and
the red lead. Earth potential does not enter into it.
A precision analog-to-digital converter will have a true differential input, which amounts to the very much the same thing. There is
one additional factor for high input inpedance circuits. The meter may only have a few megohm input impedance, if you need to
not load down the voltage being measured, you need thousands, or millions of megohms input impedance. That requires a buffering
circuit, and the buffer in turn, requires a power supply. The buffer circuit will only operate properly when the input is between its
power supply voltages. Sometimes the buffer's input range is more limited than that. This specification is called the common mode
input range, and it is a voltage range referenced to the ground voltage as seen by the buffer circuit, which may, or may not, be
close to "earth" ground.
The other key specification is Common Mode Rejection Ratio (CMRR) which quantifies the insensitivity to voltages that appear on
both the input wires of a differential input. It is expressed as a ratio, in decibels (dB). It is logarithmic in nature. The formula is 20
times the log of the ratio. If a circuit passes 1/10 of any common mode voltage, it has a common mode rejection ratio of -20dB. If
a circuit passes 1/100 of any common mode voltage, it has a common mode rejection ratio of -40dB, etc.
To get best accuracy, you need to use differential inputs, you need to take care to minimize the common mode noise, and you need
an A/D converter with good common mode rejection. But first and foremost, you need to remember than "ground" is at a
different potential every place you drive your stake into the earth.
Tom Lawson
October 2020