In the world most signals are voltage signals i.e. that the information obtained from the sensor is proportional to voltage.The following are the disadvantages of sending voltage signals in cables.
Disadvantages of voltage signal transmission:
- susceptibility to electrical noise which is usually in the form of voltage spikes.
- The voltage will drop at the resistance of the signal wire.
For short distances, wires can be shielded from noise, voltage drop is negligible, but for long distance transmission these will place a significant role. The below figure shows that the voltage signal is diminished by the cable resistance.
The perfect solution for the above problems of noise and signal attenuation is to use current signal transmission instead of voltage signal transmission to convey the information. The current signal transmission is effective because unlike voltage, the current is not susceptible to noise and does not drop when it goes through through a resistance. As we know according to Kirchhoff’s current law, the current input to the branch is equal to the current output to the branch. This can be understand by the following figure. It can be noticed that the current signal from the transmitter is fed to the receiver and it is loop backed to the transmitter with out any drop. The receiver will senses the current signal, it does not siphon.The resistance of the signal wire cannot alter any form the current signal.
Opamps can be used to implement both the transmitter and receiver in current loop. First we will see about the transmitter which is voltage to current converter.
Voltage to current conversion:
Figure shows the Opamp voltage to current converter (transmitter) circuit . It converts the input voltage signal Vin into an output current Iout, which is proportional to the input voltage. The advantage of opamp circuit is that it causes the output current to be independent of any resistance in the line. It accomplishes this by automatically increasing or decreasing its output voltage(Vout) in response to any increasing or decreasing line resistance.
As we know that opamp works on the principle of virtual ground, with that we can assume that the V1 is virtually same as V2; So the voltage across the resistance (R) must be Vin. So the current flowing in the resistance R is
IR = Vin/R
Where IR is the value of the current in resistor R. The IR is the loop current and is dependent only on the input signal voltage (Vin ) and the resistor, not by any line or load resistance.
So the input voltage gets converted to loop output current at the output. This current is should be reconverted in to voltage at the receiver for conveying the information. This is done by the current to voltage converter.
Current to Voltage Converter:
A resistor is the most predominant direct way of converting current signals in to voltage signals. But the output of a voltage to current converter is looped current where the current is fed back to the input. So we have to detect the voltage across the load resistor, without knowing the reference ground voltage.
Consider the figure complete current loop shown above, the output current is floating in the branch with neither end is grounded. we cannot ground the bottom of the Rrec, because the output current escapes from the loop and total concept will diminish. The problem is effectively solved by using differential amplifier at the receiver side which detects the difference in voltage across the resistor. The differential amplifier senses and amplifies only the voltage difference across the resistor caused by the output current. The differential amplifier should have high input resistance to prevent the hurdles to the current flow in the loop. The output of the differential amplifier which is a difference of input voltage is a single ended that can be referenced with the ground.
Voltage across the receiver resistor is Vrec = Iout * Rrec
Where Iout is a loop current
Rrec = receiver resistance.
Output voltage of the voltage receiver is Vout = AVVrec = AVIRrec
Where AV is the differential amplifier gain.
The current loop technique is one of the industry standard methods of connecting sensors with industry control system. The standard system of current signal is 4 to 20 mA. corresponds to maximum signal. This is analogous to the voltage. For example if a sensor voltage is 0-5V, then 0V is analogous to the 4mA current signal and 5V is analogous to 20mA current signal.