Features of Analog Signal Transmission

Analog voltage or current signals are hardwired between the transmitter and the receiver. These analog signals can be relatively be slow to settle compared to digital signals, due time constant of the lead capacitance, inductance and resistance.

Analog signals can lose accuracy if the signal lines are long with high resistance. These signals can be susceptible to ground offset, ground loops, EMI noise from transmitters & motors and radio frequency pick-up.

To reduce these problems, the following precautions should be taken:

• The ground connection is used only for the signal return path.
• The dc supply to the transmitter is generated from the AC line via an isolation transformer and voltage regulators, to minimize noise from the power supply.
• The signal and ground return leads are a screen twisted pair, with the screen grounded at one end only as illustrated below:

The Radio Frequency (RF) and Electromagnetic interference (EMI) noise pickup are reduced by the screen and the induced noise in both lines is greatly reduced. Because variations, in the supply voltages can produce changes in the offset voltage and the gain of the sensor/transmitter, the supply must be regulated.

The reduced pickup affects both signals by the same amount and will cancel in the differential receiver in the controller. Differential signals are not normally affected by ground offsets.

Related: Sources of Noise in Measurement Systems

Voltage Signals

The voltage signals are usually standardized in the voltage ranges 0 to 5 V, 0 to 10 V, or 0 to 12 V, with the 0 to 5 V being the most common.

The requirements of the transmitter are a low output impedance to enable the amplifier to drive a wide range of loads without a change in the output voltage. Other transmitter’s requirements are low temperature drift, low offset drift, and low noise.

Related: 4-20 mA Transmitters (2-wire, 3-wire and 4-wire Types)

Current Signals

Current signals are standardized in the ranges i.e. 4-20 mA and 10 to 50 mA where 0 mA is a fault condition. The latter range was the preferred standard but it was dropped, and the 4-20 mA is the accepted standard. The requirements of the transmitter are high output impedance, so that the output current does not vary with load, low temperature, offset drift, and low electrical noise.

The main disadvantage of the current signal is the longer settling time due to the high output impedance of the driver, which limits the current available to charge up the line capacitance.

The signal current at the controller after the line capacitance is charged is the same as the signal current at the transmitter and is not affected by normal changes in lead resistance. The internal resistance of the controller is low for current signals e.g. a few hundred ohms, which helps to lower the time constant of the transmission line.