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Single-ended and differential

Question

What is the difference between a "single-ended" accelerometer and a "differential" accelerometer?

Answer

The terms "single-ended" and "differential", when applied to piezoelectric charge (PE type) accelerometers, refers to the characteristic of the output signal of the accelerometer.

A single-ended output PE accelerometer uses a coaxial connector, where the center pin is the signal pin, and the connector shell is the signal common or ground. A differential output PE accelerometer (often referred to as a balanced, differential output accelerometer) uses a two pin connector. Both pins carry a signal, equal in amplitude but opposite in phase. The actual signal is the potential difference between these two pins. The connector shell may or may not be isolated from ground.

The key significant difference between the two is how they handle external noise sources, typically electro-magnetic interference (EMI). Piezoelectric sensing material is easily affected by EMI, by capacitive or inductive coupling. So it is incumbent upon the accelerometer manufacturer to design the sensing system (sensor, cable and signal conditioner) in a way that rejects, as much as possible, this noise coupling.

Single-ended accelerometers depend on shielding and a good ground to reject external noise sources. So the accelerometer's casing is grounded, and this ground is carried all the way through to the coaxial cable's shield. Somewhere in the system a good, reliable ground is needed, typically at the signal conditioner.

Differential output accelerometers depend on "common-mode" rejection to reject external noise sources. Since the output signal is differential (on two pins, equal in amplitude, but opposite in phase), any impinging external noise source will impact both signals equally. This is known as common-mode noise. With appropriate signal conditioning, this common-mode noise can be rejected to a great extent, while still passing valid signal. To ensure that this works, differential (or twisted-pair) cables must be used. Differential charge amplifiers/signal conditioners must also be used. Note that use of a single-ended input charge amplifier with this type of accelerometer will compromise the noise rejection system almost completely.

Note also that the differential output accelerometer does not depend largely on a good ground for noise rejection. Thus, differential output accelerometers find use in applications where good grounds may be difficult to achieve, and in proximity of high electromagnetic noise. Testing of gas turbines and jet engines is typically such as application.