The output range of a current output transmitter is 4–20mA. In a previous article, we explained why the conventional current output range is 4–20mA.
When the current output transmitter produces the minimum and maximum current, it respectively represents the minimum and maximum rated output values calibrated for the current transmitter.
Let’s take a current transmitter with a measurement range of 0–100A as an example. For a transmitter with a 4–20mA output, 4mA corresponds to an input of 0A.
Although the circuit debugging and data processing for a transmitter with a 4–20mA output can be quite cumbersome, this type of transmitter is more widely used because it allows for detecting whether there is a fault in the circuit or if the transmitter is damaged.
This is done by checking whether the current is within the normal range (since the minimum value in normal conditions is still 4mA), even when the transmitter circuit is disconnected, short-circuited, or damaged. Additionally, current output transmitters have features such as current limiting, voltage limiting, and reverse connection protection.
Since the voltage across the sampling resistor is not equal to 0 when a 4–20mA transmitter outputs 4mA, the digital value converted directly through the analog-to-digital conversion circuit is also not 0, making it difficult for the microcontroller to use directly, and calculating through a formula is too complicated.
Therefore, the common approach is to use a hardware circuit to eliminate the voltage drop generated by the 4mA across the sampling resistor before performing the A/D conversion.