It is well known that building a setup for recording neural spikes is not trivial. Many older electrophysiological systems are bulky, expensive and difficult to use. Their system components require elaborate shielding and grounding to reduce the electromagnetic interference. Recently, the technology advances have been quite rapid. It has become possible to build the millimeter-scale electrophysiology amplifiers from commercial off-the-shelf components. Here, I would like to share an amplifier design that is capable of intracellular and extracellular recording, as well as LFP, EMG, and EEG. It is small, easy to build and extremely cheap. The system uses differential mode of recording, thus eliminating the need of extensive shielding from environmental noise. The circuit had been tested on freely-moving animal exposed to their regular environment and was able to record neural spikes with a good signal-to-noise ratio (SNR).
The amplifier is divided into two stages. The first stage is an Instrumentation Amplifier (IA) with the differential input. The input signals have DC removed (with a high-pass filter) with highly tuned RC components. It is important to have very closely matched components to have high Common Mode Rejection Ratio (CMRR). The capacitors Cpf and Cnf are for power supply regulation with typical value of 0.1µF. The gain of the instrumentation amplifier is typically set in the range from 100 to 1000. The second stage is a second order low-pass filter (sellen-key). The gain of this filter should be maintained about 10; resulting in a total gain of the amplifier of ~10,000.
Following are the example values for the filter amplifier:
Chp1 = CHR = 100nF, RH1 = RHR = 4.68MΩ; Low Cut-off frequency = 0.34Hz;
R1= 10kΩ, R2= 150kΩ, C1= 1nF, C2= 1nF, R3= 10kΩ, R4= 100kΩ; High Cut-off frequency = 4,109Hz;
NOTE: For multiple channels build the same circuit and use them in parallel. For single reference electrode just short all the inverting terminals (-) of all instrumentation amplifiers and use only one reference high-pass filter (CHR, RHR) instead.