Causes of Ground Loops in Instrumentation & How to Avoid Them

Causes of Ground Loops In Instrumentation

We may have situations in instrumentation and control systems, where the sensor and the signal conditioning unit such as an amplifier or current-to-voltage converter are located some distance apart and are connected by a cable. This can lead to several problems that can reduce the accuracy or in extreme cases, totally obscure the desired signal. One of the problems that can occur is known as ground loops, which are created when the system is grounded at several points.

If we have a single-ended amplifier, one of its input terminals is connected to a ground wire of the electrical distribution system. These ground wires eventually lead to the ground bus in the electrical distribution panel, which in turn is connected to a conducting rod driven into the earth. In instrumentation systems, most often than not, different pieces of equipment are connected to ground through different wires. In an ideal world, the ground wires would have zero impedance, and all of the ground points would be at the same voltage. However, in reality, because of currents flowing through small but nonzero resistances of the various ground wires, small but significant voltages exist between various ground points.

Consider the following illustration where the sensor, an amplifier with a single-ended input and a cable connecting the sensor to the amplifier:

The cable wires have small resistances designated by R_cable. Two different ground wires are shown with their resistances R_g1 and R_g2. The current source I_g represents current flowing to ground. Normally, I_g stems from the 60 Hz/50 Hz line voltage through power supply circuits of the instruments. If we connect both the sensor and the amplifier input to the ground, part of I_g flows through the connecting cable and the input voltage is the sensor voltage minus the drop across R_cable.

V_input = V_sensor – I_g1R_cable

When the sensor voltage is very small, it can be totally obscured by the drop across the R_cable. On the other hand, if we break the sensor ground connections so only the amplifier is grounded, I_g1 becomes zero and the input voltage is the sensor voltage as desired. Hence, in connecting a sensor to an amplifier with a single-ended input, we should select an ungrounded or floating sensor.

Alternative Connections to Help Avoid Ground Loops

There are three connections that may be used in order to prevent ground loops in instrumentation systems:

• Floating sensor with single-ended amplifier.
• Grounded sensor with differential amplifier.
• Floating sensor with differential amplifier including resistors to provide a path for the input bias current.

Floating Sensor with Single-Ended Amplifier

This is illustrated below:

Grounded Sensor with Differential Amplifier

This system is demonstrated below:

Floating Sensor with Differential Amplifier Including Resistors

In most cases it is may be necessary to incorporate two high-valued resistors (much greater than the internal impedance of the sensor to prevent loading effects) to provide a path for the input bias current to the amplifier.

If the resistors are not included, the common-mode voltage of the source can become so large that the amplifier will not operate properly.

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• Features of Analog Signal Transmission
• Sources of Coherent Interference/Noise in Instrumentation Systems
• Pneumatic Signal Transmission System
• RTD 2, 3, 4 Wire Sensor Connections