Brands     Vishay Measurements Group     Interactive Guide     Tech Notes     TN-501

Noise Control in Strain Gage Measurements

Electrostatic Noise Reduction

Shielding
The simplest and most effective barrier against electrostatic noise pickup is a conductive shield, sometimes referred to as a Faraday cage. It functions by capturing the charges that would otherwise reach the signal wiring. Once collected, these charges must be drained off to a satisfactory ground (or reference potential). If not provided with a low-resistance drainage path, the charges can be coupled into the signal conductors through the shield-to-cable capacitance (Fig. 501.3).



Fig. 501.3 - Electrostatic shielding.

The two most popular types of cable shields are braided wire and conductive foil. The braided-shield construction provides about 95 percent coverage of the cable, and is characteristically low in resistance. Although commonly higher in resistance, foil shields give 100 percent cable coverage, and are also easier to terminate. Following are commercially available examples of the two types of shielded cable: When long reaches of multiple conductors are run adjacent to each other, problems with crosstalk between conductors can be encountered. With runs of 50 feet (15 m) or more, significant levels of noise can be induced into sensitive conductors through both magnetic and electrostatic coupling. Even though bridge-excitation conductors may carry only a millivolt of noise, there can be significant coupling to signal conductors to produce potentially troublesome microvolt-level noise in those conductors. The noise transfer can be minimized by employing an instrumentation cable composed of individually shielded pairs - one pair for excitation, and one pair for the signal. This type of construction is embodied in Micro-Measurements Type 422-DSV cable. When using such cable (those having separate shields), both shields should be grounded at the instrument end of the cable. Electromagnetic coupling between excitation and signal pairs can be reduced somewhat by using a cable that has its conductor pairs twisted on separate axes. Belden No. 8730 cable has the conductor pairs separately twisted, including one pair shielded with foil.

The shield-to-conductor capacitance can also become significant for long runs, since the capacitance is proportional to the cable length. Therefore, a significant portion of the residual noise can be coupled from even a well-grounded shield to the sensitive conductors. To minimize this effect, some strain gage instruments (for example, Vishay Measurements Group Instruments Division's 2300 System ) incorporate a feature called a driven guard. A driven guard (also known as a driven shield) functions by maintaining the shield at a voltage equal to the average signal, or common-mode voltage. Since, with this arrangement, the voltage difference between the conductors and shield is essentially zero, the effective capacitance is decreased, and there is minimal noise transfer. The result is a very quiet shield. It is important to note that, for proper operation, the driven shield is connected at only one end to the driven-guard pin on the instrument input connector. The driven shield is ordinarily surrounded by a second shield, which should be grounded at one end.

In a fully guarded amplifier system (for example, Vishay Measurements Group Instruments Division's Model 2200 System ), the common-mode voltage of the bridge excitation supply and the signal input terminals "float" to the level on the guard shield. Connecting the shield to the test structure or source of common-mode voltage at the gage installation site can provide very effective noise reduction since the voltage between signal conductors and the shield is minimized.

Leakage to Ground
Another often-overlooked source of noise is leakage-to-ground through the strain gage and/or the cabling. If excessive, this leakage can cause noise transfer from the specimen to the gage circuit, since even supposedly well-grounded specimens may carry some noise. It is not uncommon to have strain gages installed on nominally grounded test objects which, in fact, have noise levels expressible in volts. And, of course, any strain gage installation on a conductive specimen forms a classic capacitor which can couple noise from the specimen to the gage. In light of these considerations, it is always a good practice to make certain that the specimen is properly grounded and that leakage between the gage circuit and the specimen is well within bounds.

Prior to connecting leadwires to the strain gage, the insulation resistance from the gage to the specimen should be measured with a megohm meter such as the Instruments Division's Model 1300 Gage Installation Tester . A reading of 10 000 megohms is normally considered a minimum for satisfactory system operation. Readings below this level are indicative of a possibly troublesome gage installation which can deteriorate with time and strain. It should also be kept in mind, for gage installations which will operate at elevated temperatures, that leakage resistance tends to decrease as the temperature increases.

After cable placement and connection at the gage-end of the cable, the following resistance measurements should be made, preferably from the instrument-end of the cable: conductor-to-ground, shield-to-ground, and conductor-to-shield. Because of distributed leakage, these resistances may be somewhat lower than the gage-to-specimen resistance; but cables with significantly lower resistances should be investigated, and the excessive leakage eliminated to avoid potential noise problems.



Page 4 of 8