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In communication circuits, a current loop is an electrical signaling scheme that uses a current source and a current receiver. Voltage signaling systems use the current-to-voltage converter and a remote current receiver for sending high-speed data over larger distances. A current loop may use different current levels for analog and digital communications. The analog current loop usually uses a 4- to 20-milliampere (mA) signaling level that corresponds to an analog value.
Electrical signaling schemes vary depending on the required rise and fall times of digital signals. Integrated circuits on a printed circuit board use the voltage-driven scheme, which is suitable for very short distances inside a printed circuit board. In a voltage-driven circuit, the receiver senses voltage that is very straightforward since there is minimal distributed inductance and capacitance that may limit signal transfer. When a voltage driver has to send to a remote location, which is more than a few meters away, the signal takes time to reach the receiver and may not even reach the receiver in some cases.
For digital serial communications, the current loop may use any current level within safe limits. The current levels for digital signals due to the nature of wire loops — “mark/space” or “ones and zeros” — are usually polarity reversals. In comparing, serial buses may use differential voltage drives that use a twisted pair to send one bit at a time, and to send a single bit, two wires will be used in a double-ended or balanced output.
The current loop in a similar application will send a current level proportional to the signal voltage and does not send the voltage directly. Any voltage at the transmitter end of a voltage drive will have to deal with several factors. These include the output impedance of the driver circuit, the distributed inductance and capacitance in the wire, and the external electromagnetic noise.
The current driver in a current loop is able to compensate for the wire performance and the external noise. For instance, if a current loop transmitter needs to send 12 mA to the remote receiver to indicate an analog value, the same 12 mA will be available in the receiver even if the wire resistance has changed. This is due to the controlled current source principle wherein the source intends to send 12 mA. If loop characteristics change even fairly fast, the resulting current is still the same since the controlled current source adjusts accordingly.
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