Ground
When we say we measure voltage we are actually measuring a voltage difference between two points. We often pick one point and assign the voltage of this point to be 0V and call it ground.
The voltage output of sensors may be relative to ground (in which case we call the signal a ground referenced signal) or not (in which case we say the signal is floating).
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Grounded Signal Source: a signal in which the voltage is referenced to a system ground, such as earth ground or building ground. The signal shares a common ground with the DAQ device. |
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Floating Signal Source: a signal in which the voltage is NOT referenced to a any common ground, such as earth ground or building ground. Examples include batteries, thermocouples, transformers. |
Measurement Systems
When we say we want to measure a voltage we must remember that we are actually measuring a potential difference. We can configure many DAQ systems to measure in any of the following three measurement modes:
- Differential (the myDAQ uses this mode solely)
- Reference single-ended
- Nonreferenced single-ended
Differential Measurement System
Many DAQ systems that have multiple analog inputs will actually have only instrumentation amplifier and one ADC. This is a cost savings measure. The ability to measure multiple analog signals while using only one ADC is accomplished using a multiplexer which is basically a switch which quickly cycles the signal going to the ADC through all of the required analog input channels. It is important to note that different channels will not actually be measured simultaneously in a multiplexed system but most of the time this is not an important issue. The rate at which the multiplexer changes from one channel to the next is called the slew rate. Very high end systems, often called simultaneous sampling systems, will have multiple ADCs and not use a multiplexer.
The system below shows a multiplexer in use in a differential measurement system. When using a differential measurement system the signal does not have to be referenced to ground. The myDAQ has two differential analog inputs, labeled ai0 and ai1. Each of them have two terminals for connecting to signal sources (i.e. analog input 0 measures the voltage between ai0- and ai0+).
In a differential measurement system neither, neither input is connected to a fixed reference such as earth or building ground. Figure 10-18 shows an eight channel differential measurement system.
Referenced Signal-Ended (RSE) Measurement System
A referenced single-ended (RSE) measurement system, also called grounded measurement system, is one in which the measurement is made with respect to earth ground. The figure below shows an eight channel RSE measurement system.
Non Referenced Signal-Ended (NRSE) Measurement System
A nonreferenced single-ended system is one in which all measurements are made relative to a common reference ground (AISENSE), but this reference can vary relative to the measurement system ground (AIGND).
Measurement System Summary
