Grounding control loop

instrumentation

#1

Proper grounding is an essential component for safely operating electrical systems. Improper grounding has the potential to bring disastrous results. There are many different categories and types of grounding principles.
grounding

Although grounding can cause a great source of a noise. Ground loops occur whenever the ground conductor of the electrical systems are connected to the ground plane at multiple points.

The three main causes of ground loops can be summarised as:

  • Differences in potential between the points of the ground plane to which the
    ground terminals have been connected.
  • Inductive coupling
  • Capacitive coupling

The three methods of correcting for the problems are:

  • Single Point grounding
  • Use of Differential Inputs
  • Use of input guarding
  • Use of battery powered instruments

Single point grounding:

single point grounding

If it is difficult to ensure single point grounding, some form of optical isolation should be employed. While this effectively isolates one circuit from the other and eliminates a ground loop, it does not necessarily prevent noise or interference being transmitted from one circuit to the other. Note although the diagram is intended for a digital signal it can be easily extended to optical isolation for analogue signals.

Differential Inputs:

With the single ended approach, the total voltage input to the amplifier consists of the Signal Voltage plus the common mode voltage. The differential approach has the common mode voltage applied to both inputs. These are cancelled out (because of the differential nature of the inputs); hence only the signal voltage is “seen".

Battery Powered Instruments:

Switching Direct Current (dc):

When switching a coil, there is often a back emf generated across the coil when the contact is opened. This can damage the contacts and also put nasty transients into the system and cause a spike on some of the instrumentation signals

Putting a flywheel diode across the coil can easily reduce this, allowing an easy path for the back emf energy to dissipate.

Switching Alternating Current (ac):

There is a similar situation when switching a coil but using ac. In this case a suppression RC network with 0.1 microfarads and 100 ohm resistor may remedy the problem.


Measuring Instrument signal Grounding techniques