Reasons for 4-20mA signal in LVDT Applications

Nov. 14, 2012
To reduce noise and voltage drops in analog voltage signals, the market place has adopted an analog current (4-20mA) output as its sensor signaling standard. A very robust signal, 4-20mA current output allows a user to run longer cables from a sensor to the control system without the worry of EMI or noise being induced on the signal.

To reduce noise and voltage drops in analog voltage signals, the market place has adopted an analog current (4-20 mA) output as its sensor signaling standard. A very robust signal, 4-20 mA current output allows a user to run longer cables from a sensor to the control system without the worry of EMI or noise being induced on the signal. Recalibration to correct for voltage drop due to cabling impedance is also unnecessary.

Note the three elements in the below figure:
(1) is the LVDT Position Sensor used to monitor the valve position.
(2) is the scaled 4-20 mA signal outputs sent to the control system.
(3) is the means with which RS-485 communicates with computer ... via USB/Serial connection over Graphical User Interface (GUI) Software.


In a typical 4-20 mA current loop circuit, a sensor and its electronics provide the output based on the parameter being measured (1). This signal, then, provides feedback or display, to control states or outputs of dynamic systems (2).

New digital signal conditioners incorporate advanced ASIC electronics, improved firmware, and push button zero and span controls, thereby, eliminating the repetitive null and span potentiometer adjustments associated with conventional analog based signal conditioners. Plus, with the added benefits of RS-485 communication and a Graphical User Interface (GUI), units can now be hot swapped, set up remotely and monitored from a computer (3).

State-of-the art digital LVDT signal conditioners achieve faster and easier set ups, while surpassing the demanding needs of the process control market.

For multiple channel applications, modules can be connected together in master/slave mode to synchronize their excitation oscillator frequency, eliminating effects of heterodyning, spurious beat frequency signals, cross talk, and intermodulation effects.

For more information, visit www.te.com, or visit their sensor product line here

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