While all of the rigs in the lab are built on an antivibration table and placed within a Faraday cage in order to dramatically reduce movement and external electrical noise, respectively, there are many electrical devices within the proximity of the rig that can act as potential sources of noise if not grounded/shielded properly. Your electrode is highly sensitive to sources of noise, as it acts as an antenna by design, picking up any stray electrical radiation and converting such into time-varying currents. These currents will generate observable voltages as they flow through resistance in the circuit, such as the high-impedance junction between your pipette and the cell.

De-noising consists of grounding and/or shielding both sources and receivers of electrical noise. This processes will likely take several rounds of troubleshooting, as removing each major sources of noise will likely begin to reveal smaller sources as well. Moreover, there is also no one answer for “how noisey should my rig be”. This will largely depend on the amplitude of the signal you are trying to measure in your recordings. You could spend days and weeks trying to find even the smallest sources/receivers of noise on your rig, but it might not matter for your S/N ratio in the end.

For a detailed explanation of how each source contributes to possible noise, consider reading Chapter 11 from the Axon Guide:

📎 Axon Guide - Electrophysiology and Biophysics Laboratory Techniques _ Molecular Devices.pdf

Model cell#

Add how to check amplifier/digitizer noise

Tips before getting started#

  • At the very least, you should:

    1. Ensure that all grounding cables meet at a common grounding bus/block on the air table
    2. You can always use the multimeter to double check if two points on your rig are actually connected via the common ground
    3. Ensure that your ground bus/block is connected to the back of your amplifier [signal ground]
    4. The amplifier does not need to be grounded to the rack, as it is connected to the common ground via the BNC cables connected to your NIDAQ board
    5. Ensure that your NIDAQ board is grounded to your rack [DGND]
    6. Ensure that your TRIPP-LITE power strip is grounded to your rack [metal bolt in the back]
    7. The power strips plugged into the wall outlet are where your rig meets the “true ground” of the building.

  • Good cable management is important!

    • Use low resistance, well insulated wires for grounding cables
    • Keep cables short - longer cables increase the likelihood of impedance coupling
    • Do not daisy-chain grounding cables

  • Minimize the number of wires you are using

    • Avoid grounding components that do not contribute to noise or are unlikely to contribute to noise, as this can cause signal contamination (more current flowing).
    • Similarly, avoid creating any “ground loops” by placing multiple grounding cables to the same object.
    • If you ever get stuck and can’t find the source of an observed noise signal, consider undoing some wires to see if they are actually corrupting rather than helping.

  • Make sure to regularly re-chloride (bleach overnight) your electrode and reference wires

Setting up for de-noising#

Typically, noise values are measured in voltage-clamp mode while measuring peak-to-peak current fluctuations (either on the oscilloscope by eye or in matlab).

Since noise currents will flow through resistance in a given circuit, it can be helpful to de-noise using a high-impedance input. Luckily, this can be done by mimicking your actual patching conditions, in which the input resistance between your pipette and the cell is ideally quite high. Do not try to de-noise while your electrode wire is exposed.

  1. Re-chlorided your electrode and reference wires
  2. The more time you spend troubleshooting, the more your wires will oxidize, and the worse your noise will become with time. You may even need to do this multiple times while de-noising.
  3. Fill a patch pipette with the internal you plan to use and lower it into the saline bath.
  4. Either patch a cell or simply suck up some cells/tissue until your pipette is relatively clogged. The goal here is to create a high input resistance, doesn’t particularly matter “how”.
  5. Close the front of your Faraday cage
  6. In voltage-clamp, zoom in on the current channel on your oscilloscope. Depending on how noisey your rig is, you should be looking for:
  7. Any clear oscillations in your signal? 60Hz? 1. Often, but not always, due to electronic devices
  8. What is the amplitude of your signal? 1. Often, but not always, due to antenna receivers
  9. Does the signal go away if you close the Faraday cage? 1. If yes, the noise may be coming from the nearby surroundings (less important) 1. If no, the noise may be coming from a source/receiver within your rig (more important)
  10. Does the signal go away if you put a model cell on? 1. If not, the noise may be due to an issue with your measurement circuit and/or the grounding hub, and not a source/receiver generating/catching the electrical noise

De-noising electronics (sources):#

There are two easy ways for identifying whether a given electronic device is contributing to observed noise:

  1. Ideally, you should strip the rig and remove all electronic devices. Then, add them back one at a time, checking to see whether each device appears to add any noise or not.
  2. Alternatively, systematically unplug each electronic device or unplug everything and systematically plug each electronic device back in. Do not just turn off the device. For devices that appear to contribute to your observed noise, you could consider doing one or more of the following:
  3. Grounding the electronic device with a ground wire 1. Note: if you are having a hard time grounding a specific device, consider looking up the manual. Many manufactures will indicate where the ground is located.
  4. Insulating the electronic device with copper mesh or aluminum foil
  5. Insulating the electronic device’s power cable with a braided sleeve or aluminum foil
  6. Plugging the electronic device into one of the “enhanced noise suppression” slots on your power strip

Common sources of electronic noise include, but are by no means limited to:

  • Power cords and sources
    • Solution: avoid poorly insulated extension cords
    • Solution: avoid older power strips
    • Solution: ensure all power strips are plugged into the same wall socket (common ground)
  • Computer
    • Solution: place near or on rack and ground
  • Electrode headstage
    • Solution: wrap in foil or copper
    • Solution: avoid grounding the micromanipulator, this will actually create a ground loop with your bath ground
  • LEDs
    • Solution: ensure that LED Driver Cubes (LEDDB1) are plugged into a Controller Hub/Power Supply ( KCH301 or KCH601, requires KAP101 adapter plates)
  • Saline pump
    • Solution: add a drip-chamber
  • Heating plate
    • Solution: place saline/heating plate outside of faraday cage,
    • Solution: replace with in-line heater
  • Mercury lamp
    • Solution: insulate power cable
  • Reiser LED Arena
    • Solution: copper mesh around the arena and ground
    • Solution: insulate cables
  • Cameras
    • Solution: wrap in aluminum foil and/or ground
  • ThorLabs Microscope and electronic XYZ Manipulators
    • Solution: ground controller box (MCM3001)
    • Solution: ground objective arm at either the objective switcher or at the base of the arm
      • May need to ground at both spots, for some reason this does not always create a ground loop?
    • Solution: use foil to insulate the cable (left) connecting the Z motor on the epi-illumination arm

De-noising equipment (receivers):#

The easiest way for identifying whether a given piece of equipment is acting as a receiver for electronic signals, thereby contributing to observed noise, is to use a grounded wire with an alligator clip on the free end. Systematically tap or clamp the alligator clip to different pieces of equipment on the rig while checking to see whether the noise on the oscilloscope is reduced at all. Pay close attention to large antennas (e.g. posts, holders, etc.)

Common sources of equipment noise include, but are by no means limited to:

  • Microscope
    • Solution: ground microscope base
  • Faraday cage
    • Solution: ground metal frame, possibly at multiple points
  • Saline inflow
    • Solution: add small metal tube to inflow tubing and ground, or ground at in-line heater if included
  • Saline vacuum
    • Solution: add small metal tube to vacuum tubing and ground
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