Transmitter accurately transfers voltage input

-May 15, 2003

When you connect remote sensors to a central process controller, a simple, robust, and commonly used interface is the 4- to 20-mA loop. The advantages of this current loop include the simplicity of just two twisted wires that share both power and signal, the loop's high noise immunity in harsh environments, and the de facto loop standard within the process-control industry. Within this interface scheme, a typical 24V battery (loop power source) connects through a twisted-wire pair to a transconductance amplifier (voltage-to-current converter) that converts a sensor's—for example, a pressure transducer's—output voltage into a current that then gets transmitted back through the twisted-wire pair to a receiver (load resistor) at the process-control computer. An important criterion in this interface is that the current transfer of sensor information must be accurate. Figure 1 shows the transmitter with one voltage-controlled current source and one fixed-current source.

The fixed-current source is 4 mA, a current that constitutes the power source for the transmitter and the sensor electronics. This fixed current source must have an output impedance greater than 20 MΩ to keep the loop current independent of loop-supply variations. Similarly, current needs to be independent of temperature—that is, greater than 100 ppm/°C—because the transmitter and the sensor can be in harsh environments. The voltage-controlled current source has the same requirements as the fixed-current source and needs to convert the input voltage signal linearly into a 0- to 16-mA current. Thus, it produces an ideal transconductance as the two-port network representation of a voltage-controlled current source. The total loop-current equation is: Loop current=4 mA+(gm)VIN (for VIN=1.2V, gm=13.3 mmho).

The transconductance circuit in Figure 2 allows you to transmit current (4- to 20-mA loop) with less than 1% total error from –40 to +85°C and over a 3.2 to 40V loop-voltage range. Many IC realizations of a current transmitter have existed for years, but none operates at the loop voltage of 3.2V. Also, these ICs are becoming sensor-specific, whereas you can modify and optimize the circuit in Figure 2 for any sensor electronics or loop-current variation (for example, a 1 to 5A loop) at low loop voltage. The total loop current is as follows: Loop current=1.225V(R11/R10)/R3+VIN/R2. The circuit discussion starts with the realization of the fixed current source, IS. The fixed 4-mA current all flows through R3. The servo circuitry, including IC2 and IC3, senses the 44-mV voltage drop across R3 and keeps it fixed. Note that the ground current of all ICs also flows through R3; thus, the 4-mA fixed-current setting includes ground-current errors. The dual op amp, IC2, is both an inverting gain stage and an integrator stage. R10 and R11 set the inverting gain to –27.8V/V. The noninverting-integrator components C1, C2, R5, and R6 provide a comparison of the –44 mV across R3 (gained up to 1.225V) to the shunt-reference voltage of IC3. The output of IC2A adjusts the sum of the current though R4 and any ground current from IC2, IC3, and IC4 to a value of 4 mA. IC4 acts as an analog power-on-reset circuit that holds off the start-up of servo action until all the ICs have sufficient supply voltage. With the divider ratio of R8 and R9 and the 2.32V option of IC4, the start-up voltage equates to: VSTART-UP=2.32V(R8+R9)/R9=2.7V.

This start-up value is higher than the rated supplies of IC1 and IC2 and lower than IC5's regulated output of 3V. R4 level-shifts the output of IC2A up from zero. R7 biases IC3 into its specification range to guarantee 0.1% tolerance and 50-ppm/°C temperature coefficient over the –40 to +85°C range. The circuit discussion continues with the realization of the voltage-controlled current-source. IC1, Q1, and R2 are configured as a voltage-to-current converter. Thus, for a full-scale range of 20 mA, 16 mA comes from the voltage-to-current converter. With the maximum VIN at 1.2V, R2 must be 75Ω to produce 16 mA. IC1 must have a common- mode input range that goes beyond its negative supply (less than –44 mV). R1 is optional and prevents an open circuit on IC1's input. You can remove R1, depending on the output impedance of any input-sensor electronics. Note that R1 directly introduces an error in the full- scale loop-current.

At the heart of the circuit discussion is the realization of an output impedance, ROUT, greater than 20 MΩ in Figure 1. The low-dropout regulator, IC5, accomplishes this task by subregulating the supply to IC1 and IC2. The good line regulation of IC5 keeps the 3V output within 30 mV over the input range of 3.2 to 40V. Additionally, IC5 requires as little as 200 mV of overhead to properly regulate, and it can withstand more than 40V. This current-transmitter circuit is useful for both low-loop-voltage designs, and it's backward-compatible with higher loop-voltage implementations. Furthermore, IC5 has reverse-supply and surge protection. Therefore, this circuit does not require an additional diode within the loop, a common need with other ICs to prevent accidental reverse-wiring damage. The TO-252-package option simplifies the thermal-design considerations. With a 1-in.-sq-area pad for heat sinking, the worst-case power dissipation calculation would keep the junction temperature within its rated 150°C: TJ=85ºC+(20 mA)(40V)×50ºC/W=125ºC.

You could increase the VIN range of the current-transmitter by scaling R2, as long as you don't violate the common-mode input range of IC1. IC1's VCM includes its positive rail. So, to obtain a higher VCM, you can increase the voltage option of IC5. For example, use the LM2936-5V and R2 equal to 312.5Ω for a 0 to 5V input range. This configuration would also require that the loop supply be at least 5.2V. Note that any sensor and other electronics can and should use the 3V subrails that IC5 creates as long as the current they demand does not exceed 3 mA. The 4-mA fixed-current circuitry adjusts for the current demand. Figure 3 shows the total error on prototype units over temperature.

The total error includes the offset (4-mA) and full-scale (20-mA) effects on the ideal loop current. The tolerances of R2, R3, R7, and R8 should be within 0.1% with 50-ppm/ºC temperature coefficients. With IC5 subregulating the rails of IC1, IC2, and IC3, the power-supply-rejection-ratio error of these ICs does not generate a significant error. In like manner, IC2's CMRR (common-mode-rejection-ratio) error does not generate a significant error. IC3's CMRR error and Q1's base-current error both influence the best nonlinearity attainable: less than 0.01%. IC2's offset error is in series with the 44-mV voltage across R3, producing an offset error of 4 mA. Adjust R3 if you need to null this error. Adjust R2 to fine-tune the full-scale range of 20 mA. The op amp's offset-temperature-coefficient error is small compared with the Δ1% temperature-range budget.

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