We need to measure the impulse response of the EEG amplifier we use in our lab. We don't have so much equipment but, luckily, we have a pulse generator for delivering small electrical pulses with a given current amplitude to subjects for performing tests on pain. I was thinking of using this generator for measuring the IR or the amplifier. My plan is to a 100 Ohm load to simulate skin conductance and then use a voltage divider to bring the voltage within the micro Volt range that can be measured with EEG equipment. Is there anything else that I might need to consider?

  • \$\begingroup\$ I’d consider simulating it first and probably believing the sim results without resorting to a test that can damage your amplifier if proper attention isn’t given to the test setup. \$\endgroup\$ – Andy aka Feb 24 '19 at 15:16
  • \$\begingroup\$ We already have a simulation of the RC filter, we want to check whether this is enough. \$\endgroup\$ – Cesare Feb 24 '19 at 18:45

“Impulse” response is hard to measure in practice. It’s more commonly calculated as the derivative of the much more practical step response.

It’s possible, though somewhat risky, to use a nerve stimulus generator to generate the step. After all those things can produce >400V while your EEG amplifier maxes out at <<100mV. Adjusting the current and using a divider to protect the amplifier a ~1mV step should be possible.

But, your stimulator does not generate steps, just pulses. So you would likely have to max out the duration and deal with a bit more complicated math.

It might be much more practical/easier to buy/put together a simple slow square wave generator with a calibrated voltage output.

  • \$\begingroup\$ Our stimulator produces a 1ms long current step. \$\endgroup\$ – Cesare Feb 24 '19 at 18:47
  • \$\begingroup\$ @Cesare EEG sample rates rarely go much higher than 1kS/s. You would need more than 2kS/s to even ensure the detection of such pulse and reduce phase and amplitude uncertainties. \$\endgroup\$ – Edgar Brown Feb 24 '19 at 19:08
  • \$\begingroup\$ We sample at 1024 Hz. We clearly see the pulse as artifact also when recording the EEG. Furthermore, the impulse response is not just the pulse but it is the following response, which we expect to last at least 10 seconds due to the internal amplifier filter. Finally, sample rates are measured in Hz. \$\endgroup\$ – Cesare Feb 25 '19 at 9:08
  • \$\begingroup\$ @Cesare the reason you see the artifact is because the time constants of the body, electrodes, and other involved elements spread it out in time. But you explicitly asked about characterizing a filter I explicitly indicated that at that rate there will be uncertainties. If all you want is a gross estimate, you should have added that to your question. And finally sample RATE is in S/s sample FREQUENCY is in Hz. People confuse the two, carry out this confusion into their fields of study, and then have the nerve to correct those that actually know the difference. \$\endgroup\$ – Edgar Brown Feb 25 '19 at 13:56

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