The analog input channels of the dataTaker data loggers and Channel Expansion Modules (CEM-AD) each have four screw terminals comprising
Positive (+) input terminal for differential or single ended signal measurements
Negative (ñ) input terminal for differential or single ended signal measurements
Excite (S) output terminal for excitation voltages or currents, or input terminal for single ended measurements (dataTaker 500/600 series loggers only)
Analog Return ( R ) terminal for grounding and referencing signals, or input terminal (#) for measurement of current and current loop signals using the internal current shunt
Each channel of the dataTaker and Channel Expansion Module (CEM-AD) comprises a block of 4 screw terminals, which can be unplugged from the logger. This allows easy temporary disconnection of sensors, etc.
When installing a data logging system, there are two principal conditions which can become sources of measurement error, and which must be avoided.
These sources of error are
exceeding the common mode voltage - offset voltages present in the signal being measured may exceed the common mode range of the dataTaker
ground loops - connections between inputs of a grounded dataTaker and other grounded devices such as sensors, test instruments, the host computer, etc. may produce current return paths via ground.
Care must be taken to avoid establishing either of these error producing conditions when installing the dataTaker.
A sensor typically produces an output signal proportional to the magnitude of the physical parameter it is sensing.
However other voltage sources such as DC offset voltages, ground potential differences and electrical noise induced into sensor cables, may contribute additional voltage to the signal that is not due to the magnitude of the physical parameter being measured. This therefore is a source of measurement error.
This additional voltage is referred to as the common mode input voltage, and is defined as the average of the voltages between the measurement systemís ground and the two input terminals
Vcm = ( V+ + Vñ )/2
Vcm is the common mode voltage
V+ is the voltage on the +ve input terminal with respect to ground
Vñ is the voltage on the ñve input terminal with respect to ground
Some sources of common mode input voltages are summarized in Figure 24 overleaf.
Figure 24 ñ Sources of Common Mode Input Voltages
A measuring instrument must be able to tolerate or ërejectí the common mode voltages from differential inputs. However the extent to which the measuring instrument can reject common mode voltages is limited by the design of the instrument, and so varies between instruments. The ability to reject common mode voltages is referred to as the ëcommon mode voltage rangeí, ëcommon mode rangeí, common mode rejectioní.
The dataTaker 50, 500 and 600 data loggers have a common mode input voltage range of ñ3.5 Volts to +3.5 Volts.
The dataTaker 505, 515, 605 and 615 data loggers have a common mode input voltage range of ñ3.5 Volts to +3.5 Volts if the internal attenuators are not selected, or have a common mode input voltage range of ñ100 Volts to +100 Volts if the internal attenuators are selected.
The Channel Expansion Module (CEM-AD) has a common mode input voltage range which the same as the dataTaker model to which it is connected, as follows
if a Channel Expansion Module (CEM-AD) is connected to either a dataTaker 500 or600, the common mode voltage range is ñ3.5 Volts to +3.5 Volts
if a Channel Expansion Module (CEM-AD) is connected to either a dataTaker 505, 515, 605 or615, the common mode voltage range is ñ3.5 Volts to +3.5 Volts if the internal attenuators are not selected, or ñ100 Volts to +100 Volts if the internal attenuators are selected
If the total of the signal voltage, common mode voltage and noise voltages of an input are within the common mode range, there should be minimal measurement errors due to common mode voltages.
However if the total of the signal and noise voltages exceeds the common mode range, then measurement errors will occur. Excessive common mode voltages applied to a particular channel may also affect the accuracy of other input channels of the data logger.
In many of the illustrations provided throughout Section II of this manual, which detail connection of analog input signals to the dataTaker, the total of all common mode voltage sources is indicated as Vcm or NOISE.
Where the common mode voltages are likely to exceed the common mode voltage range of the dataTaker, then steps must be taken to reduce the common mode voltage. This can be done in various ways as follows
the dataTaker ground and sensor grounds should be connected to a common ground point, preferably at the data logger (see discussion relating to ground loops below)
where the DC offset voltages are large, the dataTaker can be isolated from true ground and the dataTaker ground connected to the negative pole of the input signal. This 'floating' of the dataTaker may introduce errors on other channels, and is not recommended
where large voltages are likely to be induced into signal cables by environmental noise, then shielded signal cables should be used and the shields grounded.
the input signal and common mode voltages can be attenuated to within the common mode voltage range of the logger, before being applied to an input channel.
Common mode voltages will more likely be a problem with those dataTakers which have solid state multiplexers, or if the internal attenuators are not selected for relay multiplexed dataTakers.
Ground loops can be inadvertently created in a data logging system in a number of ways. The two most common ways of creating ground loops are
connecting the dataTaker to a ground point which has a different potential to the ground of one or more of the sensors connected to the logger
connecting the dataTaker to a ground point which is at a different potential to the ground of the host computer (This is generally only a problem with Series 1 dataTakers, which did not have an isolated RS232 COMMS interface)
It is often assumed that all 'Ground' or 'Earth' points are always at the same potential of 0 Volts. However this is not the case, and differences of several volts can often occur between two or more ground points.
In some situations where there can be considerable electrical ground noise, due to the large electrical equipment found in factories and high rise buildings, the potential difference between two ground points can even be several tens of volts.
Ground potential differences are also a function of electrical conductivity of the soil. Therefore, in regions where soils are very dry or frozen, ground potential differences are likely to be greater than in wetter regions.
The potential difference between two or more ground points will cause a current to flow via the equipment, producing measurement errors and possibly even equipment failure.
The potential difference between all grounds in the data logging application should not be greater than ñ3.5 Volts to +3.5 Volts (solid state multiplexer versions), or ñ100 Volts to +100 Volts (relay multiplexer versions).
Ground Loops via the Communication Cable (Series 1 dataTakers)
Most data logging systems usually include a host computer, which is permanently connected to the serial interface of the dataTaker data logger. The host computer is normally connected to the local mains or line power supply ground, which may be different to the ground that the dataTaker is connected to.
Series 1 dataTakers do not have an electrically isolated RS232 COMMS interface, and so the communication cable can produce a connection or ground loop between the two grounds. If there is a potential difference between the computer ground and logger ground, then current will flow between them
if the ground potential difference is within the common mode range of ñ3.5 Volts
if the ground potential difference is outside the common mode range but within
if the ground potential difference is greater than these levels, then damage of the logger and computer will almost certainly occur.
Figure 25 ñ Ground Loops via the Communications Cable (Series 1 dataTakers)
The Series 2 and Series 3 dataTakers have an electrically isolated RS232 COMMS interface, and will not suffer these problems.
Ground Loops via the Sensor Cables
Data logging systems include a number of sensors or instruments, from which the readings are being taken. These may be simple sensors such as thermocouples that are grounded to the structure being monitored, or may be various instruments that are grounded via their mains or line power supply ground.
Often these sensors may be connected to a ground point that is at a different potential to that to which the data logger is connected. This ground potential difference can create ground loops via the sensor cables.
Figure 26 ñ Ground Loops via the Sensor Cables
Smaller ground potential differences can cause erroneous and/or erratic readings for only some analog input channels, however if the common mode voltage range of the data logger is exceeded then all readings will be effected.
Ground loops can be avoided by ensuring that the dataTaker and the sensors are all connected to the same ground point. In most cases this ground point should be at the data logger itself.
However when external and separate grounding of the sensors cannot be avoided, then other strategies can be used to avoid possible ground loops
signal inputs to the dataTaker should be as differential inputs, to take advantage of the loggerís common mode rejection range (see Section II ñ Differential Analog Input Mode)
a ground which is common to the sensors but separate from logger ground can be connected to the SE Ref terminal of the logger, and single ended measurements made with respect to this reference (see Section II ñ Single Ended Inputs Referenced to an External Common). This also takes advantage of the loggerís common mode rejection range.
install isolators between the sensors and the logger
Care must be taken to ensure that ground potential differences do not exceed common mode voltage range of the dataTaker.
With Series 1 dataTakers ground loops via the communication cable are avoided by isolating the RS232 COMMS serial interface, and powering the serial interface of the logger from an external power supply (See Section II ñ The RS232 COMMS Serial Interface).
If common mode or offset voltages are suspected of being the cause of measurement errors, then this can be confirmed and the cause identified as follows
using a multimeter, measure the voltage between dataTaker ground and each of the signal leads for each of the sensors connected to the dataTaker.
follow the procedures described below for identifying ground loops, which may be causing the excessive common mode voltages.
If ground loops are suspected of being the cause of measurement errors, then this can be confirmed and the cause identified as follows
Disconnect all sensors from the dataTaker, and perform the loggerís diagnostic test. If you are using DeTransfer, then issue a TEST command. If you are using DeLogger, then select dataTaker|Test from the main menu, or click on the Test button on the toolbar.
If you are using a Series 1 logger, then enter a Schedule into the logger and enable data logging. Log channel data for several minutes, then disconnect the computer and continue to log data for several more minutes.