Techniques to enhance op amp signal integrity in low-level sensor applications (Part 2 of 4)

Jerry Freeman, Applications Engineer, National Semiconductor Corp. -December 11, 2008

Editor's Note: This lengthy and insightful article is presented in four parts:

Part 1: click here
Part 2: below
Part 3: to be posted December 16, 2008
Part 4: to be posted December 18, 2008

Shielding, grounding and ground loops
Successful shield grounding cannot be divorced from the issue of good signal grounding; they go hand-in-hand. Thus both subjects will be treated as a combined topic in Part 2 of this series.

Twisted-pair cable is quite effective in reducing interference at low frequencies, even without a shield. However, the most demanding applications require that the cable be fully shielded. Wires shielded with copper, aluminum, and tin will reduce most interference, particularly electric-field interference, and for very long cable lengths, in industrial sensor/data acquisition instrumentation, they are an absolute must. However, this type of shielding material does not work well with low frequency magnetic field-induced pick up, owing to the long cable runs, where copper and aluminum are not effective. To shield against magnetic interference, the least expensive solution is still that of twisting the signal wires to eliminate the current loop, and thus the pickup.

Grounding and ground loops
Probably nowhere does the designer confront challenges of such difficulty as those presented by protecting sensitive analog input circuits from ground-coupled interference. The circuits can be fairly well protected using an isolated ground plane (isolated from the signal source), but the problem often includes interconnection of wires and cabling to sensors and common (un-isolated) grounds that present many opportunities for ground-loops. The simplest solution is to employ instrumentation with electrically isolated inputs. However, these are typically twice the price of a non-isolated measuring device, and are only justified in process-critical situations in industrial plants. The non-isolated approach is a more appealing and cost-effective solution in less critical applications, although it does require a good understanding of grounds and ground-loops.

Most engineers know that good grounding is important to maintaining high-quality signals throughout the sensor/measurement system. Proper grounding helps ensure that interference noise currents are drained to the system reference level, instead of circulating in the signal path. But just what is a ground, and what are the rules for making connections to it? Put very succinctly, a ground is a return path for current. Its most important job is to close the current loop between the signal source and the signal load.

Except in only a very few cases (i.e., safety), ground does not need to be connected to the actual earth (think of an airplane's avionics, or the host of electronic systems on-board a space craft). When we talk of grounding for RFI/EMI purposes, we are most concerned with limiting ground-loops. Ground-loops produce common-mode currents, and common-mode currents are the source of 90% of all RFI effects in electrical and electronic systems.

What are ground loops?
Whenever more than one current path exists between two points, a ground-loop is set up, and if the two points are at different potentials (which is nearly every case), circulating ground currents will flow. Basically, ground-loops cause problems by adding and subtracting current or voltage from the signal source. As a result, the receiving device can't differentiate between the wanted and the unwanted signals.

The biggest source of ground-loop interference, and why two points, reputed to be ground, can be at different potentials, is ground inductance. Inductance becomes the common coupling mechanism that forces all, or a portion, of the signal return current to share a return path with the interference source. The interference source can be due to noise generated internally, within the instrumentation enclosure, or by external interference sources.

The long ground loops associated with industrial sensor applications can conduct noise current because of external magnetic fields that pass through the loop. The noise currents produce a common-mode voltage across the ground inductance, which pollutes the clean signals from the sensor. If ground inductance were zero, all ground-loop problems would vanish. Failing this, the ground loop(s) can be broken by supplying separate ground paths. These concepts are examined in Figure 4 and Figure 5.

Grounding (the wrong way)
In some applications, you must connect the sensor to a local ground for safety purposes. From a safety perspective, you can't have too many grounds. However, from the perspective of the measurement system, two grounds are one too many.

As seen in Figure 4, improper grounding has resulted in two circulating ground-loop currents. The circuit shows the effects of both the shield ground and signal ground connections.

Figure 4:Multiple ground-loop current
(Click on image to enlarge)

Figure 4A shows that a potential difference exists between gnd1 and gnd2 (system grounds). As a result, two ground loop paths are generated when the shield sensor return, and sensor shield are all grounded to gnd1, while at the other end of the shield, the sensor return, the common amplifier terminal, the amplifier inverting terminal, and the amplifier PCB common (power ground) are connected to gnd2. One circulating current path is through the shield, and the other runs through the signal return path (common) wire.

The potential difference, VCM, driving these currents is generated by the gnd2 current flowing through the ground impedance, ZGND. Thus, the ground reference at the sensor is different from that at the instrumentation end. Hence, the actual voltage appearing at the amplifier input terminals is shown in Figure 4B, and is the sum of the signal voltage, eS, and common-mode noise voltage, VCM. Note that VCM increases with frequency because of ZGND.

The sum of these currents, times the ground impedance Zgnd, is added to eS and appears at the input terminals of the amplifier as:

In addition, a noise current due to shield capacitance, CSS, is forced to flow into the amplifier inputs. Ideally, the shield would represent an equipotential surface (voltage is the same anywhere on its surface) with respect to ground, and directing all shield noise currents to ground.

However, because the shield is connected to ground at two points, a difference potential across the shield, dvs, will charge and discharge CSS, given by:

thus coupling E-field noise into the amplifier by capacitive coupling (cross-talk).

Grounding the right way (breaking up sensor/instrumentation ground loops)
The best grounding arrangement is one that minimizes the possibility of having current flow in one part of a circuit create IR drops which wind up at the inputs to other parts of the circuit. If the ground-loop is broken, then the common-path from the aggressor circuit and the victim will be lost. Figure 5 shows how to eliminate the ground-loop current through the shield when its ground connection is removed from gnd2. Similarly, the loop current in the signal return path is eliminated when the ground connection is removed from the inverting terminal of the amplifier.

The measurement voltage, Vm, appearing at the amplifier inputs is now

which is the uncorrupted sensor signal.

Figure 5: By converting the amplifier from single-ended to full-differential and breaking two ground connections, we have eliminated the previously troublesome ground currents.
(Click on image to enlarge)

(to be continued. . . . )

About the author
Jerry Freeman is an amplifier applications engineer at National Semiconductor Corp., Santa Clara, CA. He received a BSEE from Heald College of Engineering in 1961.

Editor's note: If this article was of interest to you, also check out:
"Understanding noise optimization in sensor signal-conditioning circuits (Part 1a of 4 parts)",
by Reza Moghimi, click here; note that Parts 1b, 2a, and 2b are linked.

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