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In this how to we will discuss techniques for reducing electronic noise coupled into sensitive circuits. As in any design, it is necessary to identify potential noise sources, which noise sources are most important, and the effort that is to be put into noise abatement. This how to will be broken down as follows: Section one will present general design guidelines. In section two we present a detailed look at the types of interference and the impact of different types of shielding. The majority of the in formation here is a distillation of the thorough but sometimes frustrating discussion in Henry Ott's book.^[1]

In addition to the techniques described here it is important to have a well designed DC Power Distribution System.

Design Guidelines

Building a circuit with optimal rejection of external noise sources requires a detailed analysis of those noise sources and consideration of the devices which are to be coupled. In this section we present a number of rules which will help to design robust devices and will reduce coupled noise in most cases.

Low Frequency (<10kHz) Interference

The SOPs for eliminating low frequency noise are:

A differential input circuit. R6 is used when the source has a separate ground and R5 when the output drives a circuit with a different ground. If either is omitted, R6 should be left open and R5 should be shorted. The gain is R3/R1. For the circuit to reject the common mode signal, R1 = R2 and R3 = R4. The best performance will be achieved when .1% resistors are employed.

• Sensitive signals should be sent through shielded twisted pair wires with the shield connected to the chassis ground at the receiver end ONLY.
• It is often a good idea to connect the shield to the signal source ground through a 100Ω resistor to ensure that the grounds are at similar levels without allowing significant ground currents to flow.
• The input amplifier should be differential (see figure). In order for the amplifier to have a good CMRR the resistors used should be well matched. We will soon have a pile of 10K and 1K .1% resistors for this purpose.
• If coaxial cable is used, the shield will generally be grounded at the source end. In this case use a differential input stage with a 10Ω resistor to the receiver ground. NOTE: For Coax to shield against capacitive coupling the shield MUST have a low impedance connection to ground at one end.

Without using magnetic materials it is not possible to shield magnetic coupling at low frequencies, typically <lt 10kHz. Consequently, when working with low frequency signals we concentrate on eliminating capacitive coupling, ground loop potential drops, and minimizing the loop area for magnetic pickup. Grounding the shield at one end shields against capacitive coupling and while preventing ground loops. The twisted pair results in a small loop area for magnetic pickup. Using a differential input amplifier rejects remaining noise voltages which have coupled to common modes.

High Frequency Interference

The SOPs for eliminating high frequency interference are:

• If the source is grounded:
  • Use Coax and ensure that the shield has a low impedance connection to ground at EACH end. If DC ground loops are a problem, connect one end to ground through a capacitor with low impedance at relevant frequencies.
  • Use non-isolated BNCs.
• If the source is ungrounded leave it ungrounded. The shield doesn't provide "magnetic shielding" (see below) as no shield current flows, but coax has a very small effective loop area.

Grounds

Even in a perfectly shielded system, noise can be coupled between individual components through the common ground leads. In order to minimize this coupling:

• Keep high and low level ground returns separate.
• Combine grounds at a single point close to the source.
• Keep chassis and signal grounds separate except at that single point.

Understanding Interference

There are three channels through which external electromagnetic fields can couple into a system. The first is capacitive coupling where the circuit acts as one plate in a capacitor with the noise source driving the other. The second is inductive coupling where magnetic flux from the interference source threads some loop in the circuit and generates an EMF around the that loop.

Finally, there is direct electromagnetic coupling where the circuit acts as an antenna. This becomes an important mode of interference only at high frequencies where the dimensions of the circuit approach the wavelength of the radiation. This type of interference will be neglected here except to note that strong RF can seriously degrade Op Amp performance. Consequently, when using an Op Amp to amplify a mixer generated error signal it is a good idea to put a filter network on the input of the amplifier even if all other mixer products are well outside the bandwidth of the amplifier or circuit. See, for example ^[2]

Capacitive Coupling

Capacitive coupling, often referred to as electrostatic coupling, is perhaps the easiest to shield and also the most important at low frequencies.

References

1. ↑ Ott, Henry W. Noise Reduction Techniques in Electronic Systems, 2nd ed. Wiley-Interscience, 1988.
2. ↑ Data sheet for the Linear Technologies LT1167 instrumentation amplifier.