2-Wire 4-20 mA Sensor Transmitters: Understanding the 2-wire Transmitter GND (Part 2)

Part 1 of this blog series provided an overview of the operation and uses of 2-wire 4-20 mA field sensor transmitters and provided a basic example system. The post also introduced the current transmitter compliance voltage, which when violated prevents the transmitter from properly regulating the current loop.

Today we’ll discuss the circuitry that regulates the output current of a 2-wire transmitter by deriving the basic transfer function. Using the transfer function we will then explain another common application issue caused by connecting the transmitter return (I_RET) to the loop supply ground (GND) or a fixed potential relative to the loop supply GND, as shown in Figure 1. The transmitter can’t regulate the output current (I_O­) if I_RET­ is connected to GND or another potential. It’s a very common question with a fairly simple solution. 

Figure 1: 2-wire transmitter with I_RET incorrectly shorted to V_LOOP GND

To understand why these issues occur, let’s use Figure 2 to take a closer look at the operation of the XTR116 with a focus on the I_RET pin. As marked below, the XTR116 I_RET pin serves as the local, or 2-wire transmitter GND, for the sensor excitation and conditioning circuitry. The output voltages of V_REG and V_REF are referenced to I_RET. The circuitry they power, including the bridge, instrumentation amplifier (INA), and internal op amp (U1), all return current to the I_RET pin as defined in Equation 1. The I_RET currents are marked in blue in Figure 2.

Figure 2: XTR116 4-20mA transmitter showing current flow 

The I_RET current sums with the bipolar transistor (BJT) current (I_BJT), shown in orange, and flows through the 25 Ω sense resistor. The input current (I_IN), shown in green, flows through the 2475 Ω sense resistor and then sums with I_RET and I_BJT to form the final 4-20 mA output current (I_O), shown in red. The voltage drop across the 25 Ω resistor (V_IRET­) connects to the op amp inverting input and drop across the 2475 Ω resistor (V_IIN) connects to the non-inverting input. Therefore, the op amp will regulate I_BJT such that V_IRET and V_IIN equal. Equation 2 uses Ohm’s law to determine the ratio of the currents flowing through the two resistors. 

Since I_O equals the sum of I_IN, I_RET and I_BJT, we can simplify Equation 2 as shown in Equation 3, which is the final transfer function. 

We can now define the voltage at I_RET relative to the V_LOOP GND. Figure 3 shows that V_IRET depends on the 25 Ω internal resistor, the 250 Ω load resistor (R_LOAD) and I_O.

Figure 3: Determining the V_IRET voltage relative to V_LOOP GND

V_IRET is defined in Equation 4.

V_IRET is based on I_O and therefore will change as the output current changes. This is shown in Figure 4 by measuring the V_IRET voltage over the 4-20 mA I_O span. 

Figure 4: Measuring V_IRET while sweeping I_O 

Connecting I_RET to V_LOOP GND, as shown in Figure 1, forces V_IRET to 0 V. Therefore, the transmitter can’t regulate the output current because the V_IRET voltage is not allowed to move up and down with the output current.   

It is common to inadvertently connect I_RET to V_LOOP GND during testing or calibration/programming of the system. For example, making measurements with an oscilloscope on the transmitter side referenced to I_RET, while making measurements on the loop side referenced to V_LOOP GND, will short I_RET and V_LOOP GND together. This happens because the negative probe connections of the oscilloscope channels are internally connected, and also usually connected to earth GND.

The preferred solution is to reference all oscilloscope probes to V_LOOP GND and use an additional oscilloscope channel to monitor the voltage at I_RET. For measurements on the transmitter side, simply subtract the I_RET voltage using the oscilloscope MATH function to obtain the measurement without shorting the I_RET and V_LOOP GND node.

In my next blog post we’ll talk about the current consumption limitations that affect the sensor and transmitter circuitry powered from V_REG and V_REF.

See more posts in our 2-wire transmitters series.

Related TI Designs for 2-wire transmitters:

• Bridge Sensor Signal Conditioner with Current Loop Output, EMC Protection
• Low Cost Loop-Powered 4-20mA Transmitter EMC/EMI Tested Reference Design

Related 3-wire blog posts from Kevin Duke:

• See an overview of analog outputs and architectures.
• Read about the evolution of 3-wire analog outputs.