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December 2016 TechTip

Hybrid Input Board with DC Coupling

Throughout the evolution of the Digital Lynx system, Neuralynx developed three versions of Input Boards for electrophysiology data acquisition:

  • the DC Coupled Input Board;
  • the AC Coupled Input Board; and
  • the Hybrid Input Board.

All three boards provide the same high performance features:

  • 24-bit A/D converters
  • 1.3 µv rms low noise
  • +/- 132 mv AC input signal range
  • 32 differential input channels

However, the main difference between these three Input Boards is the response of the low frequency signals below 1 Hz.

The DC Coupled Input Board had flat frequency response from DC to 7 KHz, but would saturate/clip when electrode offset voltages exceed 132 millivolts.

The AC Coupled Input Board used the same amplifier and A/D circuitry with a single pole 0.1 Hz High Pass Filter to block electrode input offset voltages with zero response at DC.

The Hybrid Input Board provides a lower noise “hybrid” design of the AC and DC input models using a very unique circuit design that featured:

  • DC coupled inputs – to eliminate the large matched capacitors and lower noise – while offering accurate DC response with an expanded input range of +/-1V DC;
  • AC input signal range of +/- 132 mV AC; and
  • high resolution of both the DC and AC Input Boards.

The only trade-off of the Hybrid Input Board design is lower resolution at DC, which is normally not needed for neural recording.
The Hybrid Input Board offers the same 0.1 Hz HP filter of the AC Input Board with an attenuation limit of 90% at DC, thus giving the AC and DC responses described above.
The frequency response of the Hybrid Input Board is shown below. Notice the same flat response at 1Hz, the 0.1 Hz “knee” of -6 dB, and -18 dB response (~90% reduction) at DC.

Log Frequency versus Decibels Response
Log Frequency versus Decibels Response

The benefits of this unique Hybrid Input Board are:

  • better noise performance in the low frequencies;
  • DC accurate measurements; and
  • no saturation or “amplifier lockup” immediately following stimulation delivery.