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Signal Chain Basics #119: 2-wire 4-20 mA sensor transmitter measurement basics

In The basics of 4-20mA current loop transmitters, I discussed the basics of 4-20 mA transmitters, a popular communication method to and from field elements in factory automation and other industrial control applications. Debugging faulty 2-wire sensor transmitters at the circuit level can be troublesome because of the grounding and isolation requirements inherent to 2-wire transmitters. Today my focus is on avoiding common pitfalls when making measurements on 2-wire sensor transmitter designs, with an example shown in Figure 1 .

Simplified circuit level representation of a 2-wire 4-20 mA sensor transmitter

Simplified circuit level representation of a 2-wire 4-20 mA sensor transmitter

For 2-wire transmitters to function properly, the 2-wire ground (GND) potential must be able to float up and down relative to the loop supply (VLOOP ) GND. If the 2-wire GND potential is inadvertently shorted to the GND potential of the loop supply (VLOOP GND), the transmitter cannot regulate the output current (IOUT ) properly. During a 2-wire transmitter design, this can occur because of sensor or chassis grounding issues or improper isolation when accepting inputs from an external source.

When debugging 2-wire transmitter circuits a similar situation can occur when an earth GND-referenced measurement tool is connected to the 2-wire GND potential. The most common example occurs when an oscilloscope ground is placed on the 2-wire GND while making measurements on the transmitter circuitry. The ground connection of most wall-powered oscilloscopes is directly connected to, or has a weak path to, earth GND for safety reasons. If the lab supply powering VLOOP has an earth GND referenced output, when the oscilloscope GND is connected to the transmitter circuit, the 2-wire GND will be shorted to VLOOP GND and the transmitter will fail to function. This situation makes debugging already malfunctioning transmitter troublesome, see Figure 2 .

Figure 2

Issues caused by shorting the 2-wire GND to VLOOP / earth GND through the oscilloscope GND.

Issues caused by shorting the 2-wire GND to VLOOP / earth GND through the oscilloscope GND.

Using a floating power supply and/or a battery-powered oscilloscope can help you to avoid these issues because there will not be a connection to earth GND on one or both of the two instruments. Another common measurement issue occurs when trying to observe the voltages on the transmitter circuitry relative to the 2-wire GND while simultaneously observing the load voltage (VLOAD ) relative to the VLOOP GND. The issue occurs because the negative connections (GND clips) of the different channels of almost all oscilloscopes are shorted together inside the scope. Therefore, placing one GND clip on a 2-wire GND and another on the VLOOP GND will short the two GND potentials together through the oscilloscope GND clips. Figure 3 illustrates one oscilloscope channel monitoring VLOOP while the other channel monitors VOPA .

Figure 3

Issues caused by shorting the 2-wire GND to VLOOP GND through the oscilloscope GND connections.

Issues caused by shorting the 2-wire GND to VLOOP GND through the oscilloscope GND connections.

In lieu of purchasing additional equipment or probes that allow for isolated / floating measurements, an effective method is available if the oscilloscope has built-in math functions. These math functions allow you to observe the differential voltage between two channels, which can be used to make measurements relative to the 2-wire GND while placing all the oscilloscope GND clips at the VLOOP GND. With all of the GND clips connected to the VLOOP GND, the measurement issues described above will not occur. See Figure 4 where measurements are being made at VLOAD, the output of the op amp (VOPA ), and 2-wire GND. To observe the VOPA voltage relative to the 2-wire GND potential, use the oscilloscope’s MATH functions to subtract the 2-wire GND voltage from the VOPA voltage. Now measurements can be simultaneously made at VOPA (or anywhere else in the transmitter) and VLOAD . Now you can conveniently debug the system.

Figure 4

Making measurements relative to VLOOP GND to prevent measurement GNDing issues.

Making measurements relative to VLOOP GND to prevent measurement GNDing issues.

One challenge when using this method is that you may be trying to observe dynamic / transient signals with amplitudes in the 10s of millivolts that are biased to several DC volts above VLOOP GND, depending on the value of RLOAD and IOUT . This concept is illustrated in Figure 4 where the DC node voltages with a 12 mA transmitter output are included for two different loads: 250 Ω and 10 Ω. Notice that with a 250- Ω load resistor, the VOPA measurements may be almost 4 V above VLOOP GND while being within 1 V with the 10 Ω load.

The measurement results in Figure 5 show the VLOAD  (CH1), 2-wire GND (CH2) and VOPA (CH3) voltages in response to a small signal step input with IOUT originally configured for 12 mA. The math channel is used to subtract the difference between the VOPA and 2-wire GND voltages. Now the designer can debug the transmitter circuitry by simultaneously observing the transmitter circuit voltages relative to the 2-wire GND along with VLOAD .

Figure 5

Making measurements relative to VLOOP GND to prevent measurement GNDing issues

Making measurements relative to VLOOP GND to prevent measurement GNDing issues

Conclusion

Be careful when making oscilloscope measurements on 2-wire transmitter circuits to prevent measurement GNDing issues. Place the GND clips on the VLOOP GND and use the oscilloscope’s math functions to measure the difference between voltage potentials on the 2-wire transmitter circuit.

Stay tuned for the next Signal Chain Basics article with advice on working with data converters, amplifiers, interface or other analog design challenges.

References

1 Wells, Collin. 2-Wire 4-20 mA Sensor Transmitters blog series, Precision Hub, Texas Instruments, 2015

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