What’s a Current Loop?

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Current loops use a current source and receiver for electrical signaling, while voltage reporting systems use a current-to-voltage converter. Current loops are suitable for analog and digital communications, compensating for cable performance and external noise, while voltage schemes are better for short distances. Current loops use different levels of current for safe digital signaling, while voltage schemes use differential voltage drives. Current loops send current levels proportional to signal voltage, compensating for various factors, while voltage schemes need to address output impedance, distributed inductance and capacitance, and external electromagnetic noise.

In communication circuits, a current loop is an electrical signaling scheme that uses a current source and a current receiver. Voltage reporting systems use the current-to-voltage converter and a remote current receiver to send high-speed data over greater distances. A current loop can use different levels of current for analog and digital communications. Analog current loop typically uses a signal level of 4 to 20 milliamperes (mA) which 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 scheme, which is suitable for very short distances within a printed circuit board. In a voltage driven circuit, the receiver senses the voltage which is very easy as the distributed inductance and capacitance are minimal which can limit the signal transfer. When a voltage driver needs to send to a remote location, which is more than a few meters away, the signal takes some time to reach the receiver and in some cases it may not even reach the receiver.

For digital serial communications, the current loop can use any level of current within safe limits. Current levels for digital signals due to the nature of wire loops – “sign/space” or “ones and zeros” – are usually polarity reversals. In comparison, serial buses can use differential voltage drives that use a twisted pair to send one bit at a time, and to send a single bit, two wires in a double-ended or balanced output will be used.

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 converter will need to address several factors. These include the output impedance of the driver circuit, distributed inductance and capacitance in the wire, and external electromagnetic noise.

The current driver in a current loop is able to compensate for cable performance and external noise. For example, if a current loop transmitter needs to send 12mA to the remote receiver to indicate an analog value, the same 12mA will be available at the receiver even if the wire resistance is changed. This is due to the controlled current source principle where the source intends to send 12mA. If the characteristics of the loop change even fast enough, the resulting current is always the same as the controlled current source adjusts accordingly.

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