The first hits from a Google search of the term “guard circuit” produce a series of references to the National Guard on some security circuit. Deep in the list is a printed circuit board company that touts that they design guard rings on critical circuits. So just what are they?
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Guard circuit
Analog Devices references guard shields around their op amps as well as the printed circuit traces [1]. These traces are called guard rings; they circle and shield critical circuits. Another well-known reference on electromagnetic interference (EMI) discusses guard shields in the early edition [2]. The use of op amp shields, together with shielded pairs, and grounded so as to eliminate differential input noise. This is accomplished by connecting the cable shield to the op amp shield. Another section discusses guarded meters.
In this example, the recommended connection should be made so as not to cause current flow through any measuring leads. The term “guard shield” is missing from the author’s subsequent book on the same topic [3].
High-power active devices can use guard shields, in the form of a thin conductive strip placed between two electrical insulating yet thermal conductive gaskets, used to mount the device to a heat sink [4]. The guard shield is returned to the circuit common. This results in lower leakage capacitance between the device case and the heat sink, and lower parasitic currents.
Active circuit guard wiring techniques
Guarding can be done using active circuit devices such as an operational amplifier, as shown in Figure 1. The amplifier is wired as a coupler or isolator; the feedback is between the output and the positive input. The coaxial shield is connected to that output, which is the active shield, a low impedance source equal to the input voltage. A large leakage resistor is shown to complete the Spice simulation. The center wire is connected to the measured devices or circuit.
Figure 1 An active circuit guarding with op amps wired as a coupler or isolator and the feedback is between the output and positive input.
Guard circuit applications
Another possible application for the guard technique is interfacing a pulse signal. A pulse signal’s Fourier transform has a fundamental and odd harmonics. For high-frequency signal transmission, twisted pairs such as Cat 5 are frequently used. The source and load impedance should be equal to prevent reflections. But what if this is not the case? If a guarded circuit is used, the source is connected to the operational amplifier input, which has a high input impedance, and the wire is guarded from the return path.
An example where this circuit could be employed is interfacing industrial or process fluid flow meters. A variety of meters, such as positive displacement, which uses oval gears, and a pickup circuit to count revolutions. This includes turbine meters, which have blades internal to the meter and rotate proportionally to the flow rate.
The vortex flow meter is based on the Von Karman effect. As the fluid flows around a fixed body or blunt object, vorticity is shed alternately. The frequency of this vortex shedding is proportional to the fluid velocity. This signal can be sensed in several ways and is a pulse signal.
The Coriolis mass flow meters make use of two vibrating tubes. Flow through the tubes causes Coriolis forces to twist the tubes, resulting in a phase shift. The time difference between the waves is measured and is directly proportional to the mass flow rate.
All these meters have a calibration factor or K, which is a constant relating to the calibration, for example, K= 800 pulses per gallon. The pulses, electrical circuits, and internal resistances can vary depending on the meter. There are a variety of signal levels as well as input and output resistances between these meters and the input circuit cards.
A frequent application for these meters is to charge a known fluid volume in a tank. An accurate method is to count up or down pulses in an industrial controller. It is more accurate to measure the signal as a pulse, adding interface circuitry such as an analog flow rate signal, and integrating that signal will be subject to circuit inaccuracies and, assuming the operation is done in an industrial controller, be subject to scan sampling errors.
Figure 2 Active circuit guarding, pulse interface circuit based on 200 feet RG-58 coax cable with distributed capacitance and resistance.
Test circuit
This proposed circuit was tested based on a pulse waveform based on a typical meter as discussed. The pulse assumed is 1-ms wide with a 3-ms period. The pulse is generated by a LMC555 wired in astable operation with a 1-kΩ pull up load to a 5-V supply.
The isolation operational amplifier is 1/4 LM324 wired such that the output is a non inverting unity amplifier. The guard circuit is a 40 foot RG-58 coaxial cable. The amplifier is powered by its own 9-V battery. The only connection between both supplies is the single conductor wire parallel to the coax.
The results are shown in Figure 3, the circuit was able to provide an output the same as the input, and able to interface with any input impedance.
Figure 3 Pulse waveforms where yellow is the output and green is the input.
These waveforms agreed with the Spice simulation. The output closely followed the input.
Note the output waveform when expanded time scale when rising. The rapid increase followed by a ramp to the steady state is because the op amp has a very high gain, and is charging based on its supply voltage. However when the outer coax is charged to a point below the steady state output, the RC equivalent circuit is still charging expecting that the steady state at supply voltage. However when input difference is zero, the ramp ceases.
Figure 4 The pulse waveforms where yellow is the output and green is the input. The time scale 1/100 the previous figure (Figure 3).
Because almost all these flow signal transmitters have isolated electronics, the third wire, signal common, may be the same wire as the power supply return. This supply power is typically supplied from the pulse sensing electronics.
If so, that conductive path or reference is already available, usually in the same pair as the supply wire, in the form of a twisted, shielded cable. This provides magnetic and electric field EMI protection. The user only needs to provide the coaxial cable to the flow meter.
More than a shield
A guard shield is more than just a shield, either a solid conductive surface or braided cylinder, it is in concert with thoughtful wiring techniques to both active and passive components that result in mitigating EMI.
Related Content
- Power Supply Guard Circuit
- Telephone Guard Circuit
- Understanding grounding, shielding, and guarding in high-impedance applications
- Analog layout: Why wells, taps, and guard rings are crucial
References
- Sheingold, Daniel H., Transducer Interfacing Handbook, Analog Devices, Inc., Norwood, MA., 1980.
- Ott, H. W., Noise Reduction Techniques in Electronic Systems, John Wiley & Sons, New York, New York, 1988.
- Ott, H. W., Electromagnetic Compatibility Engineering, John Wiley & Sons, New York, New York, 2009.
- Morrison, R., Grounding and Shielding Circuits and Interference, fifth edition, IEEE Press, John Wiley & Sons, New York, New York, 2007.
Bob Heider worked as an electrical and controls engineer for a large chemical company for over 30 years. This was followed by several years in academic and research roles with Washington University, St. Louis, MO. He is continuing to work part-time as well as mentor some student groups.
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