Mar 292012
 

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.

  16 Responses to “Build your own amplifier for electrophysiological recording @ US$10 per channel!”

  1. Hi, very nice. Is there a pcb design file for building this amplifier? What type of OP-amp and IA were used? What is RGIA and the supply voltage (+Vbb, -Vbb)?
    Thanks !!

  2. Selection of Instrumentation Amplifier (IA) is very crucial here. The best performance was found with INA116, but if you have size limitation you can also use AD8220. For OP-amp any low offset, low noise precision amplifier such as OPA177 or OPA188 can be used. The supply voltage can be as high as ±18v depending on amplifier selection. But it is preferred to you higher voltage to give the system larger span for input signal. ±9v is used in the example circuit.

    RGIA is the gain resistor for Instrumentation Amplifier. For INA116 use 100ohm Resistor for 500 Gain.

    Thanks.

  3. Thanks so much!

  4. Hi there, how do we alter the circuit so that the low cutoff frequency is 10 Hz instead of 0.34 Hz on the instrumentation amplifier?

    • Use, Chp1 = CHR = 10nF, RH1 = RHR = 1.5MΩ; Low Cut-off frequency = 10.61Hz

      Please remember to use very high precision capacitors and resistor for better matching (IA stage).

      -Monzurul.

  5. Excellent circuit; I built 3 amplifiers based on this circuit. I used INA110 as a first cascade; it has the possibility for 4 different fixed gains. Thank you, Monzurul!

  6. Thank you Anton. I’m happy to know that my design came in some use for others as well. I’ve also made a head-mountable amplifier (8 channel) for neural signal recording from Microelectrode array (MEA) using this design. You can try the same. It works as good as any expensive commercial amplifiers such as Plexon, TDI and AM-systems.

    • Ho Monzurul,
      do you have a pcb design for that amplifier? The 8 channel system would be nice to build and I have never generated such design myself.
      Thank You so much, Monzurul

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  8. Hi! Any reason to not use the INA121 instead of the INA116? It is a bit cheaper (nice for more channels) and its datasheet mentions EMG/EEG etc. The common mode looks good and the input offset is almost at good as the INA116. Thoughts?

  9. INA121 should be OK also; look, for example: Land et al, J Neurosci Methods 106 (2001): 47 – 55

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  11. Hi Monzurul,
    Thanks a lot for posting this design, it has been very helpful. I have been trying to design an EMG system and this helps. However, I have a few questions. The scope of my project requires that I eliminate the need for the reference electrode in an active surface electrode design. In your design, you seem to have done something similar. Could you please explain to me the use of the “common terminal” as the signal reference for the instrumentation amplifier?
    Do you perhaps use an opamp circuit as a voltage buffer to provide a reference voltage?

  12. Timothy,
    You can short the Reference In and Common terminals together. In this case you will get the single-ended front stage.
    You can also use a couple of voltage followers (TL072 or similar), one connected between the electrode and the Signal In; and another connected to the Reference In, with its input shorted to ground (if you want the single-ended amplification). I tried all of the above with this particular circuit; everything work excellent.

  13. Hi Monzurul,

    Thanks so mush for your excellent post. I’m now planning to try your design. I have one question here. Did you use coaxial cables when you choose INA116 as your instrumentation amplifier.?

  14. Hi, has anyone built this circuit in ngspice? i’m getting a convergence error and it seems to have been caused by the spice model for AD620. did anyone else get it as well?

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