This Was Taken From the Logworks Datalogging Manual.
Most sensors requires a ground connection. The voltage output of the sensor is referenced to that
ground. If the device measuring the sensor is connected to a different ground, the voltage
difference between the grounds can shift the sensor output voltage relative to the measuring
device. Some measuring devices, like the LMA-3, have differential inputs to compensate for that
offset. The negative side of a differential input is NOT a ground. It is intended to be connected to
the same ground as the sensor. A measuring device with a differential input measures the
difference between its own ground and the sensor’s ground on the negative input and subtracts
the difference from the measured voltage on the positive input to compensate for the ground
offsets. The hookup from a differential input requires two wires. One for the signal itself and one
for the ground reference. The ground reference input (typically named CHx-) goes to the
grounding point of the sensor.
Ground offsets are especially problematic for resistive and voltage output sensors. They are not
so important for frequency or duty cycle based sensors. For a measuring device with common
ground, like the LMA-2, it is best to ground all sensors at the same point where the measuring
device itself is grounded.
Ground offsets can also affect frequency or duty cycle based sensors. Typically when measuring
a frequency, the signal is measured as ‘high’ or ‘low’ depending on the measurement device’s
high-low threshold. In the LMA-2 or LMA-3 the threshold is 1V for the LMA-2 or 2.5V for the LMA-
3. Ground offsets of 1V or 2.5V will not allow the device to measure correctly.
Another problem can be ‘ground bounce’. If the signal generating device (for example an ignition
system) creates very high current pulses, it’s ground can momentarily ‘jump’ over the threshold of
the measuring device because of the high current. These ‘ground-bounce’ pulses are measured
as false frequencies and can interfere with the measurement.
Another problem in sensor hookup is called ‘induced noise’. In an engine compartment very high
current pulses are routed many different places. These current pulses can come from the
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injectors, ignition system or alternator. Any (changing) current flowing through a conductor
creates a changing magnetic field. A changing magnetic field intersecting another conductor (for
example the sensor wires) will create a voltage along that conductor. This voltage is added as
noise to the sensor signal. This is how an antenna for radio signals works as well.
By twisting the sensor output and sensor ground wire together, that induced noise can be
minimized. Each twist creates a wire loop acting as small antenna to pick up the induced noise.
BUT, the noise induced in each loop changes polarity from loop to loop. This way each twist-loop
cancels out the noise from the previous loop. The tighter the twist, the more cancellation can be
achieved.
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Topic: Good Ground Info on sensor's ECT. (Read 6394 times)