1.1 INPUT ISSUES
• Analog signals are more complicated to deal with than digital signals. This is primarily because electrical noise will degrade the quality of the signal quickly.
• To deal with this there are a number of measures to be taken,
shielding - shielding is used to reduce the effects of electromagnetic interference.
single/double ended inputs - shared or isolated reference voltages (commons).
• A simple shielding example is shown below. A shielded cable has a metal sheath. This sheath needs to be connected to the measuring device to allow induced currents to be drained. This prevents electromagnetic waves to induce voltages in the signal wires.
• The common voltage for each analog signal may be the same but more often it is different. If the commons are tied together we have a single ended system. If each signal is given its own common the connections are double ended. Most analog input cards allow a choice between one or the other.
• Signals from transducers are normally small and cannot be directly input into an analog input. To make these signals more usable Signal Conditioners are used.
• Signal conditioners are normally amplifiers to increase the signal strength, but some will also change the signal (eg. conversion from current to voltage).
• An example of a simple single ended amplifier is given below,
• An example of a differential amplifier with a current input is given below. Note that Rc converts a current to a voltage.
• An example of a differential to single ended amplifier is given below.
• The circuit below can be used to amplify the output of a resistive device.