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March 2016 TechTip

Noise Debug 101: Electrostatic Noise Conduction

Electrostatic Noise occurs when AC noise is coupled into the circuit via parasitic capacitance through the air/space. The noise source is any varying voltage and is independent of current flow. For example, an open energized power cord generates just as much noise as when it is powering a device. The amount of coupling is directly related to the frequency (higher frequencies will present more induced noise), distance (inversely), and voltage of the noise source (120/220 VAC). The most common sources are fluorescent lights, switching power supplies, or other common electronics in the lab.

The electronic model of the noise coupling consists of: 1) a parasitic capacitor (any two metallic areas separated by space) between the noise source to your circuit; and 2) the impedance of the circuit (amplifier input or electrode wire) shown as a resistor between your circuit conductor (the electrode, tether wire or PC Board trace) and ground. As mentioned before, current always flows in a loop because electrons must get back to the source. Therefore the conduction loop will usually be from the source, through the capacitance (air) and through your circuit (impedance), to ground and then back to the source (assuming the noise device is AC powered or connected to ground).

Schematic of Electrostatic-Noise Coupling
Schematic of Electrostatic-Noise Coupling

capacitor is usually about .5 picofarads

impedance of the capacitor: Xc = 1/(2 π F C)

Xc ~= 50 Giga-ohms (or more): 50 Gohm resulting noise into amp: 500 µv

Electronic Model of Noise Conduction
Electronic Model of Noise Conduction

In this noise example, the induced noise into the recording system will be about 500 microvolts but will vary based on distances, frequency, and impedances; it will still be in the range of 50 µv to 50 millivolts. Very consistent with what you will observe in the lab!

Electrostatic coupling is the one situation where the “aluminum foil” shields actually work!

Electronic Engineering Model of How Shielding Works
Electronic Engineering Model of How Shielding Works

The shielding usually takes the form of an outer metal sheath around wires in the tether cable. This shield MUST be connected to ground to return the noise current back to the power source. The same amount of coupling capacitance is present between the noise source and the shield, but the shield is connected to ground. The voltage on the shield will still be about 500 µv, but is connected to ground for the return path for the current to return back to the AC power source.

Note: You now have a new parasitic capacitor between the shield and the tether wires, but because the shield only has 500 µv on it the amount of noise induced into the tether cable wires is very minimal – a few nanovolts – which results in a noise free signal measured by the recording system.
Practical Applications of Electrostatic Shielding As stated in Part 1, you first need to identify the noise source and then identify which of the four types of coupling is causing the noise to get into your recording system. Identifying the noise source is usually as simple as turning off lights or other devices to see if the noise immediately stops or changes. You may have multiple noise sources and each one must be dealt with separately.

Electrostatic noise coupling is usually the one encountered in electrophysiology recording because electrode impedances are so high (50Kohm to 2Mohm) and neural signal amplitudes are so small (20 µv to 5 mv). Usually the test for electrostatic coupling is the “aluminum foil shielding” test:
When you are performing this experiment, it is best to monitor your signals with an audio output. Your ears are the best “instrument” for detecting changes in a noise signal without having to continuously monitor a display or oscilloscope.
If you determine that electrostatic coupling is the cause of your contaminated signals, you may have to change cables to shielded cables, grounding the shields. If this is not possible, as in a Patch Clamp experiment, you may have to resort to a fully shielded Faraday Cage Enclosure, making sure to ground the cage.

  • put a grounded metallic surface between the noise source and your low level signals (electrodes, tethers, and microdrives)
  • always connect the foil to ground (with a wire) to maximize the results of the test
  • also place the foil around your subject

When you are performing this experiment, it is best to monitor your signals with an audio output. Your ears are the best “instrument” for detecting changes in a noise signal without having to continuously monitor a display or oscilloscope.

If you determine that electrostatic coupling is the cause of your contaminated signals, you may have to change cables to shielded cables, grounding the shields. If this is not possible, as in a Patch Clamp experiment, you may have to resort to a fully shielded Faraday Cage Enclosure, making sure to ground the cage.