A differential amplifier is described that provides a high common mode rejection ration (CMRR) without requiring the use of precisely matched components. One variation employs a method of noise reduction to increase the SNR of the device. The differential amplifier may be used in an apparatus for measuring biopotentials of a patient, such as an electrode for measuring brain activity. The electrodes can communicate the measured biopotentials with a remote system for further processing, while providing electrical isolation to the patient.
