Hunting for Electrical Noise on an Electrophysiology Rig – Part 2/3

In Part 1 of the 3-part blog series on noise troubleshooting, we looked at noise theory and some general principles of causes of electrical noise. This second chapter focuses on general good practice for low-noise recordings. Make sure to also dig into Part 3, with some tool recommendations and further hacks for sorting out electrical noise on your patch rig:

1. Good hunting ethics – what a low-noise rig looks like

Remember, your primary weapons for sorting the noise are repositioning, swapping or shielding cables and equipment. But first consider some good practices for setting up a low-noise rig:

1.1. The hunting grounds – your lab space

Awareness of where you set up your rig is critically important. I’ve been in labs where the rig was set up on the other side of a wall attached to a hospital’s main transformer and electric breakout box. Plus, positioning directly under air conditioners, or near fridges or laser power supplies is common error.

1.2. Setting up camp – Shielded space

You want to have the environment as hassle-free as possible. If other factors are playing along, you can get away without using a Faraday cage. But the moment you have issues, you’ll wish you had a cage… It is better to invest in one from the start, in my opinion. You can do a self-build (see future blog post for an explainer) or order a custom and cost-effective solution (ask us about where to go for this – we can direct you).

Faraday cages perform optimally when it is made from a continual high conductivity material. Therefore, any cable ports or openings should be kept to a minimum as the length of any opening will determine the cut-off frequency for the cage. Also try to keep doors shut during recording. If you can’t close the doors during the experiment, it may be worthwhile investing in a shielding screen positioned between the recording area and the operator. Additionally, a computer monitor/screen EMF shield might be useful in some instances.

1.3. Everything in its place – Equipment positioning

The more complicated your setup, the more aware you’ll have to be of where to position your devices. Of course, you’ll need to consider available space and ergonomics, but do think about where your amplifier and DAQ are relative to other devices.

Try to position these devices away from typical noise culprits like computers, monitors, supplies for illumination sources and other noisy power supplies. It will be even better if you could install it in a separate rack. If you don’t have enough space, try to position them as far apart as possible or get a thick metal shielding plate between the potential source and the receiver. For more complicated rigs, you’ll need plenty of sockets for all the devices around your rig. Ensure your socket outlets have protective earth contacts. It’s good practice to not use a power strip for your amp and DAQ, and to plug them into an individual socket set separate from one another and other devices it could impact on.

1.4. Connected to earth – Grounding configuration

Use a central grounding point. Most people prefer a grounding bus (conductive metal block, often copper, with holes to plug grounding cables into). You can buy something more sophisticated from NPI Electronic.

The grounding bus is then connected to the breadboard of the antivibration table (acting as a common ground) and, if available, to the “signal ground” socket on the amplifier (Molecular Devices/Axon amps and some other models have this on the back). We’ll discuss grounding in greater detail in the troubleshooting section below. Only ground devices and components when necessary.

1.5. Your primary weapons – Cable management

With some setups looking like proper birds’ nests, the many cables around your setup could very easily become sources (generating electric or magnetic fields) or receivers. Cables can also act as antennae, picking up signals from nearby radio stations, mobile phones or computers or wireless devices in the 2-5GHz range (even short cables can pick up noise from these). Some general comments about cable management:

• Keep cables short – longer cables invite issues. They’re often coiled up (which invite magnetic field generation or pick-up) and longer cables increase likelihood of impedance coupling. When you must use long cables, make sure to use (or add) good quality, heavy gauge shielding.

• BNC cables – buy BNC cables with consistent impedance. 50 Ohm cables (instead of 75 Ohm) are recommended.

• When signals of more than 100kHz are problematic, you may want to use some ferrite toroids or beads around the cables (if not already integrated by the manufacturer)

• Extension leads and power strips should not be draped near power supplies and should be kept as far away from your workstation components as possible

• Keep signal/data cables and power cables far apart, as running them in parallel and close to each other increase chances of noise pick-up. If you can’t keep them apart, try to position them at right angles relative to each other at the point where they cross.

• Run headstage/preamplifier cables in bundles – this may help with noise voltages cancelling each other and keeps them separate from power cables. Try to keep as much of the headstage cables inside the Faraday cage.

• Also (rookie error, we know), make sure you have only one bath reference electrode when you have multiple headstages.

• Bundling AC power cords – as far as possible, bundle these together. This may help with averaging electrostatic and magnetic fields and reduces net radiation as a result.

• And then, a more general principle – having cable placement as a critical component in noise reduction is not a good strategy – it may work for competing sources of hum to cancel each other out, but in a working lab it’s just a matter of time before something in the setup changes, a cable gets moved, which may cost you a lot of time in troubleshooting. Develop a robust strategy which won’t develop constant issues.

 

1.6. The watering hole – around the chamber

Give special attention to the area around the recording chamber – use the shortest pipette and grounding electrode possible (extending 5mm beyond the cap of the electrode holder will be enough). Remember, longer cables invite trouble. Related to this, a short-barrel electrode holder is good practice, if it doesn’t restrict access too much.

Position your perfusion inlet/outlet in a way which will most likely give you laminar perfusion. Turbulent perfusion is more likely to dislodge a slice and could potentially cause electrical issues.

 

1.7. Avoid the booby traps – on ground loops

When adding grounds to your setup, you want to drain the noise away in one direction towards the ground (which, in this case, will be the ground bus on your table).

A ground loop is when you have a return loop connected to the origin of the ground cable. This can act like an antenna or source – a changing magnetic field around the loop causes emitting fields to build up around the loop. Ground loops are a common cause of cycle noise and can be picked up despite the cables being shielded.

Some pointers for avoiding this:

• Only use grounding cables when necessary

• Avoid having more than one grounding bus. Maintain as best as possible a “star” ground configuration for all equipment. In this configuration, there is one central reference point (one single grounding bus) and all common connections go to that point. It makes it easier to spot errors

• Never daisy-chain your grounding cables

If you were to remove the orange cables depicted in the figure below, you won’t have ground loops in the setup anymore: