Mirage335BiosignalAmp

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Intended to represent the best achievable biosignal amplifier with commercially available components and dry electrodes. Performance is very near theoretical limits. Input referred thermal noise across a 50Hz bandwidth should be about one microvolt or less. Common-mode rejection is subject to real world conditions, but should far exceed existing commercial and open source units.

Brainwave (ElectroEncephaloGraphy), heart (ElectroCardioGraphy), and muscle (ElectroMyoGraphy) detection are possible with this device. Electroencephalography is the most difficult case, with high-impedance low-frequency signals as weak as one microvolt and noise in the tens of millivolts.

Mirage335BiosignalAmplifierHardware.jpg

biosignalAmplifierUSB_Out.png

Contents

[edit] Features

  • USA companies provide all parts, including circuit boards.
  • Safety oriented architecture. Multiple isolation barrier and surge suppressor layers.
  • Active electrodes, wet and dry designs.
  • Future proof. Modular SATA data cable architecture provides smooth upgrade paths.
  • Isolation amplifier. Safe, high-resolution analog output for lab equipment (eg. oscilloscopes), sound cards, microcontrollers, and ADCs.
  • USB support, based on ArduinoDAQ (ATMega32U4 + LTC2440).
  • Efficient, high-order IIR biquad filters notch out 60Hz, 30Hz and high-frequency noise, right at the embedded microcontroller.
  • Wide supply voltage support. +/-6V to +/-15V
  • Resistor programmable gain. 13000x default.

[edit] Repository

Complete schematics, PCBs, BOMs, 3D printable enclosures, firmware, and documentation hosted at github.

[edit] Related

[edit] Getting Started

[edit] Working Unit

HacDC has a working unit, along with detailed usage instructions.

[edit] Assistance

Drop by either HacDC's IRC channel or mirage335's personal IRC channel to reach the developer for guidance.

Additionally, your local hackerspace may also provide assistance.

[edit] Construction

  1. Understand the safety implications thoroughly.
  2. Clone the github repository.
  3. Buy the needed components and circuit boards. See the Cost section.
  4. Solder everything together. Recommend stencil based reflow for quicker assembly, excepting the sensitive amplifier chips.
  5. Test the active electrode surge suppressors. Any voltage above 0.6V should not reach the inputs.
  6. Connect SATA cables, power everything up. Hand-to-hand or across-the-chest electrocardiography (heartbeat) signals are among the most available, strongest, clearest indicators of a working biosignal amplifier.

[edit] Tools Required

  • Fine tip soldering iron or better recommended.
  • Oscilloscope. DSO Nano is adequate.

[edit] Schematics

[edit] Active Probe

activeElectrodeSchematic.png


[edit] Amplifier

amplifierSchematic.png


[edit] Host

hostSchematic.png


[edit] Design

  • Feedback AC Coupling rather than direct filtering removes DC from instrumentation amplifier output without increasing passband noise.
  • Active Electrodes and Driven Right Leg circuits maximize common-mode rejection with difficult electrodes.
  • Driven Shield improves common-mode rejection by reducing electrode cable capacitance.
  • Stray rectification minimized by RF suppression capacitors.

[edit] Innovation

As far as is known, this design represents several firsts:

  • Simultaneous use of feedback ac coupling, active electrodes, driven shield, driven right leg circuit, and RF suppression.
  • Comprehensive study of commercial amplifier chip performance for biosignal applications.
  • The use of SATA data cables to carry shielded analog signals and power simultaneously.
  • Truly modular architecture. Host (isolation), amplifier (13000x instrumentation amplifier), and active electrode (buffer amplifier) systems can be used independently and interchangably with standardized analog SATA ports.
  • Enhanced HTML bill of materials generated from gEDA schematics, complete with distributor and pricing information.

[edit] Comparison

[edit] Open Source

[edit] OpenBCI

Based on TI's ADS1299, perhaps the only other decent EEG system worth using, and definitely a good choice for many-channel, mobile, or casual use. This device may be within half the performance of this Biosignal Amplifier, offering more channels and greater convenience.

For the most challenging biosignals, Mirage335BiosignalAmp should still offer significantly better signal-to-noise performance.

Unfortunately however, the ADS1299 datasheet claims "negligible current noise" without providing an exact specification or upper limit. The ambiguity implies noise with real electrodes may be worse than the datasheet voltage noise specifications (1uVp-p over 70Hz).

[edit] ModularEEG (aka. OpenEEG)

Not designed for, and inconvenient with, active electrodes. Does not use a driven shield or feedback ac coupling. High flicker noise TLC277P operational amplifiers. Higher noise INA114 instrumentation amplifier.

[edit] OpenMind

No mention of active electrodes, driven right leg circuit, or driven shield. Excessively noisy. The INA155 has high voltage noise and limited voltage swing. Unsafe, lacks electrical isolation.

[edit] OpenEXG

No mention of active electrodes. Relatively high flicker noise and poorly documented AD8605 instrumentation amplifier with limited voltage swing.

[edit] MonolithicEEG

No mention of active electrodes, or driven shield. High impedance internal amplifier coupling. Relatively high flicker noise and poorly documented AD8606 operational amplifier with limited voltage swing. Noisy. INA118 has high current noise in the flicker noise region.

[edit] Commercial

Many consumer electronics electroencephalograph systems make dubious performance claims. Many are likely dominated by muscle (electromyograph) noise in normal use.

As far as is known, active electrodes are still uncommon in clinical EEG systems. Further, these systems tend to be rather expensive.

[edit] Improvements

CAD models for cases will be released soon, including an improved headset using saline sponge electrodes.

The newly released ADS1299 may form the basis for a multi-channel system.

Additional software in progress.

[edit] Testing

Biosignal amplifier has been completely assembled, and is correctly processing real-world inputs. Several electrocardiograms on multiple participants were taken Saturday (2013-03-16) night. Electroencephalography is also working, occipital lobe alpha rythms are reliably detected on users with closed eyes.

Vrtually any biosignal including ElectroEncephaloGraphy, ElectroMyoGraphy and hand-to-hand ElectroCardioGraphy is readable on this amplifier. Noise seems less than 1uV across 3kHz. Safety and isolation systems are working. Electrode surge suppressor circuit was specifically tested before complete assembly.

[edit] Alpha Waves

08-AlphaWavesLowRes.jpg

[edit] Heartbeat Sample

02-heartbeat.jpg

[edit] Electrode Surge Clamping

03-clamped.jpg

[edit] Credit

Design is by mirage335. Credit to HacDC and its biohacking community for electronics testing and construction equipment. Additionally, special thanks to the following for enthusiasm and prototype funding:

  • Shawn Nock
  • Sharad Satsangi
  • Stuart Washington
  • The Real Plato
  • Logan Scheel

[edit] Cost

Parts cost of the full system is $186.96 in parts ordered from mouser and two from digikey.

For those wanting professionally made circuit boards, total board cost for one order is approximately $134.10 from oshpark. Made in the USA.

Board Name 3 Board Batch Approximate Cost Per Board Approximate Cost
Amplifier Board $81.30 $27.10
Active Electrode $5.70 $1.90
Host $47.10 $15.70

More detailed cost analysis available in CostTable spreadsheet.

REMINDER: At least TWO active electrode boards are required.

[edit] Safety

Reasonable safety features have been included. If possible, these should be non-destructively tested.

Nearby lightning strikes will penetrate the isolation amplifier. A low-impedance earth connection to -ONLY- the non-isolated Vgnd may divert these faults. Alternatively, some 3-prong surge protectors may help, but they must clamp fast and well below 1.5kV.

High frequency radio transmissions will penetrate the isolation amplifier. Amateur radio operators should be careful.

Keep saline and other conductive materials out of the active electrodes and/or seal their circuit boards with hot glue.

No claim of liability is made by anyone. Your accident is your accident. Use common sense. If you don't understand the safety features and limitations, don't use this circuit.

[edit] History

  • BiosignalAmplifier Early prototype, used far lower-quality components and relied on external surge protection.
  • Project was named after its first developer and maintainer as that is the only guaranteed single durable distinguishing characteristic. There are already too many similar sounding biosignal amplifier names (eg. ModularEEG, OpenEEG, OpenEXG, MonolithicEEG). Linux is named for similar reasons.

[edit] Mirage335BiosignalAmp Copyleft

Mirage335BiosignalAmp is Copyright (C) 2012 mirage335 under the FSF GPLv3. The schematic images are part of Mirage335BiosignalAmp.

See license.txt for Mirage335BiosignalAmp license conditions.

gplv3-127x51.png

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