Analog Techniques

The measurement of real world parameters such as temperature, pressure or strain is the job of transducers and sensors. Basically a transducer is a device that converts energy from one form to another. An example of this is the dynamic microphone which converts minute changes in air pressure (sound waves) to a varying electrical energy (voltage) analogous to the sound. Another example perhaps more applicable to Data Acquisition is the Thermocouple which converts heat into voltage.

Sensors, however, are not necessarily transducers such as in the case of a carbon microphone where the compression and rarefaction of the air varies the electrical resistance in its circuit creating the analog of the original sound. This requires an external electrical energy source and some processing or conversion of the variations to suitable voltages or currents.

What is Signal Conditioning?

What analog sensors and transducers have in common is an output of varying voltage or current that is analogous to the variation of the real world phenomena. They sometimes need to be conditioned by a transmitter to provide linearization and a standardised output format like the 4 to 20mA current loop or voltage outputs such as 0 to 10V and +/-5V.

Which Input type suits my Sensor?

Data Acquisition boards and modules can accept inputs either directly from the transducer or as conditioned from a transmitter. In the case of unconditioned transducers the inputs of DAQ devices must suit the type of sensor or the range of expected values. For example in temperature measurement, if using thermocouples (TCs), the input range must suit the millivolt output from the specific TC type as well as having Cold Junction Compensation. On the other hand the use of Resistive Temperature Devices (RTDs) requires a very accurate voltage source from the DAQ device to achieve a reliable and accurate reading. Using a specific input type of DAQ device can lead to an optimised and economical solution however the trade off is that you are limited to that sensor type.

When using conditioned outputs it is usually a matter of matching the sensor output and DAQ input ranges to optimise the native resolution of the system. If using a current loop transmitter a DAQ device with a current loop input is normally required however the use of a current sense resistor can convert the current to a suitable voltage for a Voltage input device.

For a DAQ device to be able to accept all these different types of sensor output types is not an easy task and could require additional expense for features not used. Firetail DAQ can supply Measurement Computing DAQ devices with inputs for specific sensors such as Thermocouples as well as multifunction and universal input modules. IOtech products take a slightly different approach specialising in mixed signal devices using signal conditioning expansion modules.

How do I connect my sensor to a DAQ Input?

We need to consider the nature of the sensor’s output signal or the transmitter’s output signal to determine the best connection type to the DAQ or Logging device to achieve maximum accuracy. There are two basic connection methods, Single-Ended and Differential.

What is a Single-Ended Input?

A single-Ended input is one where the signal is referenced to 0V or low level ground (LLGnd), that is, the voltage is measured between the signal wire and ground. This ground is common for all the Single-Ended inputs so we can use a single wire from each sensor or monitoring point. A benefit of this type of connection is the doubling of the number of inputs for a given amount of input hardware such as the input multiplexor (MUX) and Analog to Digital Convertors (ADC). The drawback of using this configuration is susceptibility to electrical noise on the signal wire due to induction (EMI) and at the signal source ground noise. There can be difference in the ground voltage at the source and at the input and this is called Common Mode Voltage.

What is a Differential Input?

A Differential Input measures the voltage difference between its high and low input and a differential senor output consists of a high and a low signal wire. This creates an immunity to electrical noise because any EMI induced into the high signal wire is also induced into the low signal wire. This EMI is cancelled out by the action of the input amplifier as it amplifies only the difference. Common Mode voltage issues are to a certain point overcome as the measurement is independent of ground potential variations between source and device. Common Mode voltages still need to be considered as the signal voltage plus any Common Mode voltage can not exceed the Common mode range of the device.

What is Grounding?

In addition to SE or Differential input considerations we need to consider system grounding and isolation when designing and connecting signal sources to DAQ devices. There are three different situations that need to be accounted for.

What is Common Ground?

Common ground is where there is no potential difference (PD) or voltage between the signal source and the DAQ device. Typically this would occur when the device is providing power or the excitation voltage to the Sensor. If the sensor or source is not directly connected to a Low Level Ground (LLGnd) then you should assume that there is no common ground and configure your inputs accordingly.

What is Ground Offset (Common Mode) Voltage?

This is the most frequently encountered grounding situation. The grounds are somehow connected but have a Ground Offset or Common Mode voltage between them. This can be an AC, DC or a combination and can be caused by many factors including a simple resistive path, EMI or Earth Potential Rise. As long as the total of the Common Mode Voltage is very low and that voltage plus the Signal Voltage does not exceed the common mode range of the device then no further conditioning would be required. If the common mode voltage plus the signal voltage is higher than the common mode range of your device then the signal source should not be connected to the device. Your system grounding configuration needs to be changed or Isolation modules such as the 5B series from Dataforth or Analog Devices need to be fitted.

What are Isolated Grounds?

Some Signal sources such as battery powered devices are already electrically isolated and can be connected as SE or differentially. Differential connection provides greater noise immunity especially when a 10kohm resistor is connected from one input side to the low level ground.
See the table below for recommended connection types or refer to the MCC guide to signal connection freely available from Measurement Computing.

http://www.mccdaq.com/pdfs/signals.pdf
Input Configuration Recommendations (based on a device with ±10V common mode range)

What is Resolution?

Resolution refers to the number of distinct values that can be quantified by the DAQ Device and is usually specified by the number of Bits. The higher the bit number, the greater the accuracy as the number of bits defines the number of discreet steps between the lowest and highest value in a given range. Resolution can also be described as a voltage which would be the smallest voltage difference that can be detected. The lower the voltage value the greater the resolution becomes as this defines the value of each step size. Typically, DAQ devices have resolutions of either 12, 14, 16 or 24 bits. It is usually desirable for the DAQ device to have a resolution or accuracy greater than that of the sensor.

What is Sample Rate?

Sample rate refers to the number of times the signal is sampled and quantified. This could be as many as millions of times per second or as few as once a day. It is important to select a sample rate appropriate to your sensor type and nature of the signal you are acquiring. There are two main types of sampling (1) Simultaneous where all input channels are sampled at the same time and (2) Sequential where each selected input channel is sampled in turn. If you wish to extract detailed information about the signal you are sampling then you need to sample significantly faster than the signal itself varies.

What is Bandwidth?

Bandwidth refers to the range of frequencies that can be applied to an input and be accurately or predictably quantified. It is a property of the analog input circuitry.