Transducer Input/Output

Understanding Transducer Input and Output

An electrical input, called the excitation voltage, is required to operate a pressure transducer. This DC excitation can vary from 5-10VDC for unamplified transducers with millivolt output, to 8-36VDC for amplified transducers producing voltage or current output. Taber pressure transducers offer three electrical output options:

• Millivolt (mV) • Volt (VDC) • Current (4-20mA)

Millivolt (mV) Output

Pressure transducers with millivolt output are generally the most economical pressure transducers. They are often called “low-level” transducers because they are unamplified and only contain passive electronics necessary to develop and thermally compensate for the low electrical output of the Wheatstone bridge. This also means they tend to be smaller and lighter than voltage or current output transducers.

The Full-Scale Output (FSO) of a Taber millivolt transducer is typically 30mV, with 10VDC nominal excitation. This is also expressed as an output of 3mV/V, 30mV FSO with 10VDC excitation, or as an output of 3mV per Volt of excitation. The optional FSO range is 0-20mV.

The actual output of these instruments is directly proportional, or ratiometric, to the excitation voltage. This means that if the excitation fluctuates, the output will change proportionally. As a result of this dependence on a steady excitation voltage, regulated power supplies are highly recommended. High-resolution data acquisition systems are also recommended.

Because the output signal is so low, a millivolt output transducer tends to be more affected by an electrically noisy environment (hand radios, switchgear, electric motors, etc.) due to this noise impacting the data acquisition equipment which overshadows the transducer output voltage. The distances between the transducer and the readout instrument should also be kept relatively short.

Transducers with a mV output signal typically have a better Response Time than most high-level output transducers because there is less electronic circuitry and no isolation of the excitation voltage from the output signal.

The basic passive electronics in these low-level transducers can withstand higher and lower temperatures than the active amplifying circuits used in high-level transducers. As a result, millivolt output pressure transducers are popular for use in high heat (+400°F) and cryogenic (-450°F) applications. Additionally, since there are no active components within these transducers, they are not susceptible to radiation and are immune to EMI/EMC events.

Pros	Cons
Economical	Requires a Regulated Power Supply
Less Mass	Can be difficult to use in an electrically noisy environment
Good Temperature Extremes (High/Low)	Short Cables
Functions in Cryogenic Temperature	Signal Conditioning Not Possible
Fast Response Time
Immune to Radiation and EMI/EMC Effects
Able to withstand aggressive shock and vibration environments

Voltage (VDC) Output Pressure Transducer

Voltage output pressure transducers are amplified and add higher-level electronics to the low-level passive circuit discussed above. This permits the integration of noise filtering, voltage regulation, excitation-to-output isolation, and advanced signal conditioning circuitry.

The additional components of high-level output pressure transducers mean they are typically longer and heavier than low-level transducers of the same pressure range due to circuit board size.

A voltage output transducer provides much higher output than a millivolt transducer (normally 0-5VDC) and its output can be isolated or non-isolated from the excitation voltage. Because the output of this transducer is not a direct function of excitation, an unregulated power supply is sufficient, provided that it falls within a specified voltage range.

Due to active electronic components being used within these transducers, they are sensitive to radiation effects unless radiation-hardened components are selected. Additionally, if a voltage output transducer is selected, it can be susceptible to EMI/EMC events if EMI filtering is not incorporated into the electrical circuit.

The compensated temperature ranges of these transducers generally extend from a low of -65°F [-54°C], to a high of +250°F [+121°C]. This is the normal maximum operating temperature range for active electronic circuits.

Pros	Cons
Lower Pressure Ranges	Less Temperature Flexibility
Unusual Pressure Ranges	Larger Mass
Higher Proof Ranges	Higher Cost
Allows EMI Protection	Slower Response Time (For Isolated Output)
Unregulated Input Power	Sensitive to radiation if rad-hard components are not used
Improved Accuracy Options	Sensitive to EMI/EMC events if EMI filtering is not incorporated
Longer Cable Distances	More sensitive to shock and vibration
Better Computer Compatibility
Good Response Time (For Non-Isolated Output)

Current (4 - 20 mA) Output Pressure Transducer

This type of high-level pressure transducer is also known as a pressure transmitter. Since a 4-20mA current signal is least affected by electrical noise and resistance in the signal wires, these transducers are best used when the signal must be transmitted long distances. It is not uncommon to use these transducers in applications where the lead wire might be 1000 feet [305 meters] or more.

The excitation range of a Taber Industries 4-20mA unit is wider (8-36VDC) than that of transducers with voltage output, and elaborate EMI protection electronics are not necessary due to the nature of the current loop signal arrangement. Because of this, there is less electronic circuitry and the Response Time is on a par with 0-5VDC non-isolated units.

Pros	Cons
Best Inherent Immunity To EMI/EMC Events	Narrowest Compensated Temperature Ranges
Longest Cable Distances	Max: -40°F To +250°F (-40°C To +121°C)
Comparable To A 0-5VDC Non-Isolated Output Transducer	Medium Cost