Neural measurement

An implantable device for measuring a neural response to a stimulus, the device comprising:

• a plurality of electrodes including one or more nominal stimulus electrodes and one or more nominal sense electrodes;
• a stimulus source for providing a stimulus to be delivered from the one or more stimulus electrodes to neural tissue in order to evoke a neural response;
• a measurement amplifier for amplifying a neural response signal sensed at the one or more sense electrodes, wherein an impedance between the sense electrodes is sufficiently large that a voltage arising on the sense electrode tissue interface in response to the stimulus is constrained to a level which permits assessment of the neural response voltage seen at the sense electrode,
• wherein an input impedance to the measurement amplifier (ZIN) is limited by:
• ZIN>A×ZC(Vs1−Vs2)/VE
• where
• A is a scalar provided to give sufficient margin of VE over (Vs1−Vs2),
• ZC is the constant phase element impedance of the or each sense electrode,
• Vs1−Vs2 is the differential voltage arising on the sense electrode tissue interface in response to the stimulus, and
• VE is the neural response voltage seen at the sense electrode;
• and
• a control unit configured to control application of a stimulus to the neural tissue and measurement of an evoked neural response, the control unit configured to apply an electrical stimulus from the stimulus electrodes to neural tissue, the control unit further configured to impose a delay during which the stimulus electrodes are open circuited, and the control unit further configured to, during the delay, measure a neural response signal present at the sense electrodes with the measurement amplifier.