Cut noise from high-resistance measurements
Dale Cigoy, Applications Engineer, Keithley Instruments -- Test & Measurement World, 4/1/2006 2:00:00 AM
Noise in circuits reduces measurement accuracy, which reduces the effective resolution of an instrument as well. Electrostatic coupling is the most common source of noise in resistance measurements above 10 megaohms.
Electrostatic interference occurs when an electrically charged object comes close to an uncharged object. At low-impedance levels, the effects of the interference are negligible, because the charge dissipates rapidly. High-resistance materials, however, don't let the charge decay quickly, and the interference may result in unstable measurements. Delays could be in the range of tens of seconds to several minutes, depending on the application. To reduce the effects of electrostatic interference, it's important that you use both shielding and guarding when making high-resistance measurements.
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| A metal shield and guard circuits reduce noise in high-impedance measurements. |
Shielding a circuit with a metallic enclosure prevents electrostatic interference from affecting a high-impedance circuit (figure). Noise current generated by the electrostatic voltage source and the coupling capacitance flows through the shield to ground rather than through the signal conductors.
The easiest shield to make is a simple metal box or mesh screen that encloses the test circuit. Connect the shield to the low-impedance point on the measurement instrument. If this point floats above ground, you should observe special safety precautions to prevent any shock hazard. You should also use shielded cables to minimize noise.
Guarding adds a low-impedance conductor, maintained at the same potential as the high-impedance circuit, to intercept any interfering voltage or current and divert it to ground. Connect the guard to the instrument's guard terminal, if available, or use an external buffer amplifier. A guard does not necessarily provide electrostatic shielding, but it will reduce noise from other sources.
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