Oscillation killer
A potential problem with fast capacitance compensation is that in current clamp mode, overcompensation causes the amplifier to become unstable, and the resulting high-frequency oscillation is an excellent way to kill a cell. This is a general problem of the fast capacitance compensation technique, rather than being specific to the Optopatch, but we can now offer a specific solution to it.
We have incorporated a simple accessory circuit which detects a series of fast voltage transients in current clamp mode, of the sort that occurs when an oscillation is building up. This circuit then disconnects the compensation signal for about half a second, which it indicates by illuminating the overload LED, and will keep disconnecting it if the problem persists. The voltage threshold for operation is set on the headstage driver board, and the feature is normally supplied enabled at its maximum sensitivity, but it can be disabled or the sensitivity modified if required.
When this facility was tested, we were delighted to find that even the small (and normally very fragile in this respect!) granule cells in brain slices were protected by it, so it was extended it to operate for series resistance compensation as well.
