Monday, December 3, 2012

Reducing Measurement Errors with Proper Cabling Part 1

In this two part post we will look at reducing measurement errors with proper cabling and grounding methods. The principles covered in this post can be applied to basic measurement setups, DAQ systems, and automated test systems.

Cable Specifications
A wide variety of general-purpose and custom cables are available. The following factors influence the type of cable that you choose.
  • Signal Requirements – such as voltage, frequency, accuracy, and measurement speed.
  • Interconnection Requirements – such as wire sizes, cable lengths, and cable routing.
  • Maintenance Requirements – such as intermediate connectors, cable terminations, strain relief, cable lengths, and cable routing.
Cables are specified in a variety of ways. Be sure to check the following specifications for the cable type you intend to use.
  • Nominal Impedance (insulation resistance) – Found on cables that are intended for frequencies above DC. It varies with the frequency of the input signal. Check for HI-to-LO, channel-to-channel, and HI- or LO-to-shield. High frequency RF applications have exact requirements for cable impedance.
  • Dielectric Withstand Voltage – Must be high enough for your application.
Warning: To prevent electrical shock or equipment damage, insulate all channels to the highest potential in the system. It is recommended that you use wire with 600 V rated insulation.
  • Cable Resistance – Varies with wire gauge size and cable length. Use the largest gauge wire possible and try to keep the cable lengths as short as possible to minimize the cable resistance. The following table lists typical cable resistance for copper wire of several gauge sizes (the temperature coefficient for copper wire is 0.35% per °C). Using the sense lines on instruments such as DMMs and performance power supply can compensate for cable resistance.
  • Cable Capacitance – Varies with the insulation type, cable length, and cable shielding. Cables should be kept as short as possible to minimize cable capacitance. In some cases, low-capacitance cable can
    be used.
Cabling resistance table (top) and Impedance table (bottom)
Grounding Techniques
One purpose of grounding is to avoid ground loops and minimize noise. Most systems should have at least three separate ground returns.
  1. One ground for signals. You may also want to provide separate signal grounds between high-level signals, low-level signals, and digital signals.
  2. A second ground is used for noisy hardware such as relays, motors, and high-power equipment.
  3. A third ground is used for chassis, racks, and cabinets. The AC power ground should generally be connected to this third ground.
In general, for frequencies below 1 MHz or for low-level signals, use single-point grounding (see below). Parallel grounding is superior but is also more expensive and more difficult to wire. If single-point grounding is adequate, the most critical points (those with the lowest levels and/or the most precise measurement requirements) should be positioned near the primary ground point. For frequencies above 10 MHz, use the separate grounding system. For signals between 1 MHz and 10 MHz, you can use a single-point system if the longest ground return path is kept to less than 1/20 of a wavelength. In all cases, return-path resistance and inductance should be minimized.

Grounding Schemes

For a detailed look at ground loops and noise check the post Ground Loops and Other Spurious Coupling Mechanisms and How to Prevent Them

Shielding Techniques
Shielding against noise must address both capacitive (electrical) and inductive (magnetic) coupling. The addition of a grounded shield around the conductor is highly effective against capacitive coupling. In switching networks, this shielding often takes the form of coaxial cables and connectors. For frequencies above 100 MHz, double-shielded coaxial cable is recommended to maximize shielding effectiveness. Reducing loop area is the most effective method to shield against magnetic coupling. Below a few hundred kilohertz, twisted pairs may be used against magnetic coupling. Use shielded twisted pair for immunity from magnetic and capacitive pickup. For maximum protection below 1 MHz, make sure that the shield is not one of the signal conductors.

Separation of High-Level and Low-Level Signals
Signals whose levels exceed a 20-to-1 ratio should be physically separated as much as possible. The entire signal path should be examined including cabling and adjacent connections. All unused lines should be grounded (or tied to LO) and placed between sensitive signal paths. In DAQ systems or ATE system when making your wiring connections to screw terminals on a connection interface, be sure to wire like functions on adjacent channels.

Stay tuned for part two next week. And as always if you have anything to add use the comments section below and if you have any questions feel free to email me.


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