High precision capacitance bridge for studying lipid bilayer membranes

A simple ac bridge for the measurement of the capacitance of lipid bilayer membranes between 100 Hz and 10 kHz with a nominal accuracy of about 0.1% without corrections is described. The voltage across the unknown need be only 7 mV (rms). Using a simple calibration procedure, the capacitance can be determined with a nominal accuracy of 0.05%. The resolution of the bridge is about 1 part in 20 000. The novel features of the bridge are (1) 100:1 resistance ratio arms and a floating unknown, (2) a single-ended output and a balanced input utilizing a photocoupled isolator, and (3) a detector to monitor voltage across the unknown at balance.