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AC and DC Coupling: What's the Difference?

by Siemens Experimenter Siemens Experimenter ‎07-11-2016 08:33 AM - edited ‎03-20-2017 03:18 PM

Introduction

 

In LMS Test.Lab, when selecting InputMode have you ever noticed the "AC" or "DC" at the end of the line? What does that stand for? 

 

InputMode option under the Channel Setup tab in Test.LabInputMode option under the Channel Setup tab in Test.Lab

The InputMode option sets the kind of coupling needed for the transducers. In any data acquisition system, the transducers are either AC or DC coupled.  As an operator, you may need to change the coupling setting to get your desired data outcome. This document will explain how to choose the appropriate coupling setting!

 

What is meant by AC vs DC?  

 

AC and DC are abbreviations for Alternating (Capacitive) Coupling and Direct Coupling. This setting is important as it will affect the frequency content of your data.

Note: AC and DC do NOT refer to your favorite rock band bolt.PNG . 

 

 

Most signals are composed of AC and DC components. The DC component is the 0 Hz component that acts as an offset in the time domain. The AC component consists of all other frequencies.

 

Top Left: DC component of signal. Middle Left: AC component of signal. Bottom Left: Summation of AC and DC components. Right: Frequency content of net signal.Top Left: DC component of signal. Middle Left: AC component of signal. Bottom Left: Summation of AC and DC components. Right: Frequency content of net signal.

The graph above illustrates AC and DC signals. The AC signal fluctuates about the DC offset. After performing a Fourier transform on a signal that consists of both AC and DC components, the DC component will be at 0 Hz and the AC signal will be at its associated frequency.

 

What is Coupling? 

 

Coupling is the transfer of energy between two mediums by means of physical contact. For example, transferring electrical energy from a metallic wire to a terminal.  

 

 

Two couplings on SCADAS Mobile. One coupling between 4pin LEMO cable and first Tach channel and one coupling between 7pin LEMO cable and first dynamic channel.Two couplings on SCADAS Mobile. One coupling between 4pin LEMO cable and first Tach channel and one coupling between 7pin LEMO cable and first dynamic channel.

 Whenever a transducer wire is connected to your LMS SCADAS frontend, they are considered “coupled”.

 

AC Coupling:

 

AC (alternating coupling) allows only AC signals to pass through a connection. AC coupling removes the DC offset by making use of a DC-blocking capacitor in series with the signal. AC coupling effectively rejects the DC component of the signal normalizing the signal to a mean of zero. 

 

 

A strain gage reading using AC coupling. The reading fluctuates about 0 µE.A strain gage reading using AC coupling. The reading fluctuates about 0 µE.

 

DC Coupling:

 

DC (direct coupling) allows both AC and DC signals to pass through a connection. The DC component is a 0 Hz signal which acts as an offset about which the AC component of the signal fluctuates.

 

Strain measured with DC coupling. The reading fluctuates about a 55 µE offset.Strain measured with DC coupling. The reading fluctuates about a 55 µE offset.

Simultaneous Strain Measurement Example: AC and DC Coupling

 

In this experiment, two gauges were put on the same beam at the same location along the beam. Gauge1 was set to DC coupling and Gauge2 was set to AC coupling.

 

Experimental setup. Gauge one is set to DC coupling and gauge 2 is set to AC coupling.Experimental setup. Gauge one is set to DC coupling and gauge 2 is set to AC coupling.

 

AC and DC coupling of the strain gages.AC and DC coupling of the strain gages.

The gage that was set to AC coupling fluctuates about zero. The gage that was set to DC coupling fluctuates about 55µE.

 

What kind of coupling should I use for my transducers?

 

Below is a list of common sensors and the suggested coupling.

 

Remember, when using AC the resolution of the signal will be increased by removing the DC offset. Using DC is ideal for monitoring slowly changing signals such as thermocouples and strain gages or for signals where measuring the offset is key.

 

AC Coupling:

  • ICP Microphones (acoustic pressure)
  • ICP Accelerometers 
  • Strain gages (when interested in elastic / dynamic behavior only)
  • All ICP / IEPE transducers (the voltage supply for an ICP transducer is at 0 Hz and is removed by AC coupling)

 

DC Coupling:

  • Thermocouples
  • DC Accelerometer (not only measures fluctuations, but also measures offset caused by gravity)
  • Strain gages (when offsets may be important, i.e. when there is plastic deformation)

 

Does AC coupling only cutoff 0Hz? 

 

AC coupling removes the 0Hz component, but is that all it does?

 

No! The graph below demonstrates the filter shape of an AC coupling. The filter roll off point is at .707 of the amplitude of the signal which is equivalent to a -3dB attenuation of the signal at that point. The roll off point is a function of the coupling circuit (the DC-blocking capacitor). Depending on your application requirements, you can design the circuit to have the -3dB roll off point occur at different frequencies . 

 

For example, the graph below is for a .5 Hz coupling, meaning the roll off point occurs at .5 Hz.

 

So, the 0Hz component is removed from the signal, but additional low frequencies are also attenuated. 

 

 

Filter shape for 0.5 Hz AC coupling.Filter shape for 0.5 Hz AC coupling.

 

Where can I set AC or DC coupling?

 

To set the input mode of your sensor, go to the channel setup tab in Test.Lab. Under the InputMode dropdown there are options for AC and DC coupling.

 

Set ACDC.png

 Good luck and have fun! Questions? Contact Us!

 

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