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The knowledge of how to design low noise instrumentation systems is an essential skill that any practicing instrumentation engineer should strive to learn because noise and interference affects any measurement system in one way or the other and this in turn affects the precision of measurements and the detectability of the quantity of measurement.
Generally noise in measurement systems can be classified as random noise and coherent interference. Random noise is considered to arise in a circuit or within the measurement system from completely random phenomena whereas coherent interference, typically has its origins in periodic, man-made phenomena such as power line frequency coupled with measurement system by an electric field and or a magnetic field, radio frequency sources e.g. radio & TV station broadcast antennas, poorly shielded computer equipment, spark discharge phenomena such as automotive ignitions and motor brushes and commutators, and inductive switching transients such as SCR motor speed controls and so forth. Coherent noise typically enters a system from without.
Our discussion in this article focuses on coherent noise in measurement systems: key sources and ways that can be used to minimize the coherent interference.
Periodic or coherent interference, unlike random noise, has narrow band power density spectrums, often with harmonic peaks at integral multiples of the fundamental frequency. Usually a serious problem is created when the coherent interference spectrum has power in the signal frequency band.
Generally the main sources of coherent interference in measurement systems include:
As aforementioned above, there are several sources of coherent interference and the means to reduce a given type of interference depends on the physical mechanism(s) by which it is coupled with the measurement system. In the following sections we discuss several techniques that can be utilized in minimizing coherent interference in measurement systems:
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In most cases, the fix for coherent interference lies in preventing it from escaping from a coherent noise source, such as a computer or digital system, an electronic system with a switching power supply or a radio transmitter. Obviously for radio transmitters, there must be electromagnetic energy radiated from an antenna. But, great care must be taken in all the systems referred to above to keep the coherent interference from entering the power mains, where it can affect the other line power instruments.
Power line filters are generally multistage, LC low-pass filters which attenuate frequencies in the range of about 10 kHz-40 MHz. Maximum attenuation at about 30 MHz ranges from -40 to -65 dB, depending on the number of LC stages in the filter.
Power line filters are effective in keeping high frequency coherent noise from entering a system on the power line, as well as keeping the internally generated noise from escaping from the system.
High frequency interference on power lines can be described in two forms:
In CM interference, the unwanted voltage has the same value on both the black and white wires with respect to ground (green wire).
The CM interference voltage can be written as:
Vi(CM) = (ViB + ViW)/2
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The DM interference voltage is given by:
Vi(DM) = (ViB – ViW)/2
High frequency power line filters are designed to attenuate both Vi(CM) and Vi(DM). Power line filters should be used in conjunction with a robust grounded metal instrument case – the metal case of the power line filter must have a solid, a multipoint electrical connection with the grounded instrument case for best results.
One design trade-off in the application of power line low-pass filters is that the capacitors used to attenuate CM noise inject a current into the green wire power line ground. Generally, this current must be kept < 3.5 mA in non-medical applications. Hence, the size of the line-to-ground capacitors and hence, the amount of CM interference attenuation is limited to this ground current limit.
Shielded coaxial cables and twin axial (twinax) cables are extensively used in instrumentation systems to couple sensors to pre-amplifiers and signal conditioning subsystems together. Unfortunately coaxial cables can act as magnetic loop antennas under certain grounding conditions. As illustrated in the figure below, an ac current I1 in adjacent wire creates flux lines which link the coaxial cable and its shield, creating a mutual inductance between the conductor I and the coaxial cable. The figure below illustrates how this problem can be reduced:
It is important to note that, coaxial shields should be grounded at a single point to prevent ground loop interference with the exception of situations like the one discussed above.
Both ends of coaxial cables carrying RF should be grounded to minimize the radiation of magnetic field interference.
To reduce coherent interference in low level analog signal conditioning systems, first ensure you separate low level leads from wires carrying high currents and voltages. Low level leads should be twisted and routed near the grounded case. If twisted and shielded cable (twin axial cable) is used, it should be grounded at one end only and insulated. Alternate grounds and signal leads on ribbon cables and in connectors must be ensured. High impedance, low level leads should be kept as short as possible, this is especially true in wiring op-amps. The summing junction should never have an exposed lead of more than 1.3 cm connecting components to it.
The use of decoupled, low impedance, dc power supplies with proper power ratings is important factor in interference reduction. Dedicated low noise, analog power supplies for signal conditioning system head-stages can assist in reduction of hum and the cross-coupling of interference from other stages.
Transient voltage suppressors are used to prevent the high voltage, spike like transients occurring in the power mains input to an instrument system from causing physical damage to system power supply components and system circuits or causing damage and abnormal results in associated computer equipment.
There are several sources of power line spikes. Some spikes are periodic, occurring once or more every power frequency cycle. These spikes are generally of a biphasic nature and their voltages may be as much as 30-50 V above the instantaneous line voltage. They are typically several μs in duration and arise from SCR or triac switching of inductive loads attached to the power line, such as the motor speed controls. While these SCR spikes typically don’t pose safety issues, they do constitute a key source of coherent interference, if allowed to get into the signal ground path or the power supply outputs. SCR spikes are normally attenuated by line filters, but in some cases, they may be large enough after filtering to pose a problem. Other power line spikes occur randomly as the result of lightning strikes on power lines or infrequent switching of inductive loads attached to the power line. The large singular transient spikes are best reduced by the use of nonlinear circuit elements such as varistors or Zener diodes. Metal oxide varistors (MOV) are best utilized in applications where the current rise times (from the power line) are longer than 0.5 μs.
The following illustration shows a simple 6-outlet, power bar or line monitor power condition, normally used with PCs and related equipment to protect them from line spike transients. One MOV-type varistor is used to protect each pair of line outlets. The power bar also has a switch, a neon pilot light and an low cost circuit breaker for 15 A.
At high frequencies, ground wires appear as series R-L circuits, whose impedances increases with frequency. If a circuit card contains both digital circuitry and low level analog amplifiers, and has a single ground connections path, it is almost certain that analog system will pick up the coherent interference from the ground current flowing through the common impedance of the digital circuits. Separate ground paths will help fix this problem as illustrated in the figure below:
Figure 1.4 above demonstrates good grounding and electrostatic shielding architecture. Notice that there is a common tie point for all grounds at the power supply terminal. This terminal may or may not be tied to the metal instrument case, which is always tied to the green wire (power line earth ground). It is important to keep in mind that separate grounds are used for low level analog, high level analog, digital and inductive (arcing) circuits such as motors, solenoids and relays. The ground wire itself should be of heavy gauge; if the + and -15 V dc supplies a circuit card use #18 wire, then the ground should be #14 stranded wire.
Photo-Optic Couplers
These are nonlinear devices primarily used in digital interfaces where it is desired to have complete ground and signal isolation between digital circuits. A typical optocoupler is made up of an LED on a chip in optical, but not electrically conductive, in proximity with an output circuit which consists of a phototransistor or photodiode, which conducts when illuminated by the input LED. The principal use of optocouplers is isolated, high speed, serial digital data transmission. The diagram below demonstrates a simple TTL logic circuit coupled with a photo-optic coupler.
Isolation Transformers
Isolation transformers are extra apparatus which can enhance measurement of the system signal to noise (SNR) when used between analog signal conditioning subsystems.
A typical isolation transform used with 50 𝝮 coaxial cables has terminating and source impedance of 50 𝝮 and a frequency response of 25 Hz-20 MHz. There can be several thousand megaohms impedance between the primary and secondary coaxial shields and thus, the common mode EMF has little effect on the signal.
A longitudinal choke is used to reduce coherent interference caused by high frequency, common-mode (CM) interference picked up by wires connecting sensors to signal conditioning modules or connecting subsystems in an instrumentation system.
The figure below illustrates the use of a toroidal ferrite magnetic core to form a longitudinal choke:
The longitudinal choke may be used with pairs of current carrying wires, twisted pairs of wires or coaxial cables. More than one pair of wires of coaxial cable may be wound on the same core without the circuits cross-talking. In a not complex form, the wires are passed through the center of a cylindrical ferrite core to form a one-turn longitudinal choke. This arrangement is often seen on accessory cables used in PCs.
When extremely low-level signals are being measured, or when very high common mode (CM) coherent noise is present, as in the case of certain biomedical applications, strain gauge bridge applications or thermocouple measurements, the use of a guard shield can effectively reduce the input capacitance to ground of the amplifier that is giving rise to coherent interference.
A typical application using an isolation amplifier in conjunction with a guard shield and external shield is illustrated below:
Here the guard shield is tied to the amplifier ground and is made to float with respect to the “true ground” and as a result of the RF power supply of the isolation amplifier. The current in the ground loop and thus, the magnitude of difference mode (DM) interference voltage, is reduced at least twenty-fold because the capacitance from either amplifier input to ground is now reduced by the external shield and is of the order of 1 pF.
Unwanted coherent interference on a signal and power lines at frequencies above 1 MHz can be attenuated effectively using one or more (not often more than three) ferromagnetic beads strung on the wire carrying the aberrant interference.
Ferrite beads act as small chokes, making the wire upon which they are strung appear to be a series R-L circuit. Due to their internal losses, some ferrite beads cause the wire to have a nearly constant (resistive) impedance of about 30 𝝮 to 75 𝝮 and a frequency < 1-100 MHz. Other beads are made to appear more inductive and their impedance (Z) increases linearly with frequency, going from 20 𝝮 at 5 MHz to 110 𝝮 at 100 MHz.
Ferrite beads make effective, well-damped, second order low-pass filters when combined with suitable bypass capacitors to ground. One extensively used bypass capacitor is the feed through capacitor, used to decouple RF noisy sources from the power supplies. Typical feed through capacitors have cylindrical, threaded metal bodies which are mounted in the metal case or shield wall and the terminals at both ends permit the dc current to pass through. The capacitance from the center through the conductor to the case can range from 0.5 nF to 5 nF, depending on the application.
Just to sum up everything we have discussed above with regard to coherent noise minimization, we can say that to reduce coherent interference/noise:
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