An implantable apparatus for sensing biological signals from an animal includes at least two electrodes disposed at locations to sense the biological signals. The electrode locations may be internal or external to the animal. Insulated conductors couple the electrodes via a passive network of filters to an instrumentation amplifier that has an internal voltage reference. Thus a sensed biological signal is filtered and amplified to provide an amplified differential signal. A signal analysis module processes amplified differential signal to determine at least one physiological parameter of the animal. The signal analysis module may include a first derivative zero detector for signal transition detection and feature detection and analysis. The apparatus may also comprise a signal presentation module to display amplified signals and physiological parameters associated with those signals.