Signal-to-Noise Ratio (SNR)

An SNR of 3 to 4 is the working floor for a sortable unit and 5 to 6 is a good recording. Below 3 a spike is a threshold crossing that could be noise, and sorting fails first.

Both halves of the ratio are set at the electrode. The signal is set by distance — a spike is tens to a few hundred microvolts within a few tens of micrometers of the cell body and gone at about a hundred — so a site's SNR for a given neuron is mostly where the site is. The noise floor is set by the site's impedance (thermal noise rises with the square root of resistance), by the amplifier, and by what the reference fails to cancel: mains, muscle, the reference's own signal. A small site trades a higher noise floor for a more local signal; a coating lowers the floor without giving up the site size. The practical levers are in order: a clean, low-impedance site; a reference in quiet tissue; the amplifier at the head; shielding and grounding of the rig — the sequence in Noise Reduction and Troubleshooting. SNR should be known during the session, not after it: a site that reads low while the animal is on the rig can be moved or excluded, and Allego reports per-channel signal quality live for that reason. Radiens computes its SNR as the mean spike peak over a robust estimate of the noise floor; How Radiens Calculates Signal Metrics gives the formula.