Design tradeoffs for loop-powered transmitters
Loop-powered transmitters have progressed from purely analog signal conditioners to highly flexible smart transmitters, but the chosen design approach still depends on a system’s performance, functionality and cost requirements. This article presents three difference bench-tested transmitter designs.
In loop-powered designs, the 4-20mA loop provides both power and data, so the system must operate on less than 4mA loop current. In fact, 3.6mA or lower is a more typical target as this represents a low alarm current on the loop. Other key considerations for a design are the target performance, functionality, size, and cost.
The first circuit we’ll discuss (Figure 1) uses a purely analog signal chain.
Figure 1. Analog 4-20mA loop-powered transmitter (reference to Circuits from the Lab® Reference Design)
This circuit measures a resistive bridge pressure sensor, which is excited by a 5V reference. An instrumentation amplifier gains up the sensor signal. Its voltage output is converted to a current by R1, and is summed with an offset current generated through R2. This current flows through R3 and is amplified, via the op amp configuration, through R4 to form the 4-20mA output. As the current consumed by the entire transmitter returns through R4, it is included in the regulated 4-20mA current, making the circuit loop powered.
Using 0.1% resistors this circuit can achieve better than 1% max accuracy at 25°C. Calibration would greatly increase the accuracy; allowing adjustment of R2 and R1 would cater for offset and gain calibration respectively. However, the accuracy is still limited by sensor performance and component drift over temperature, as the circuit does not easily allow for calibration over temperature or sensor linearization.
This circuit consumes less than 1.9mA (excluding sensor excitation), which is well below the 4mA target.
In summary, this purely analog transmitter allows for a simple, low-cost solution, but the sensor cannot be linearized, it does not offer calibration over temperature, it provides no diagnostics, and any changes to the sensor or output range would require hardware changes.