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?
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 .
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.
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.
Whenever a transducer wire is connected to your LMS SCADAS frontend, they are considered “coupled”.
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.
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.
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.
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.
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.
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.
Good luck and have fun! Questions? Contact Us!
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