GBM8320 Dispositifs Médicaux Intelligents Biopotential amplifiers – Part 1 Mohamad Sawan et al. Laboratoire de neurotechnologies Polystim http://www.cours.polymtl.ca/gbm8320/ [email protected]M5418 4 February 2011 GBM8320 - Dispositifs Médicaux Intelligents 2 Biopotential amplifiers: Course outline • Biopotentials measurement – Principle, requirements, and metrics • Types of biopotential • External Interference and intrinsic noises – Sources and models • Instrumentation amplifiers – Discrete and integrated • Advanced instrumentation amplifiers • Integrated biopotential amplifiers • Examples, and practical implementations GBM8320 - Dispositifs Médicaux Intelligents 3 Principle of biopotential measurement • Function of a biopotential amplifier: to take a weak electric biological signal, increase its amplitude, and extract it from ambient noise so it can be further processed, recorded, or displayed. • Bioamplifiers are also used to isolate the load from the source. In that case, the amplifier may provides only a current gain to drive the load, leaving the voltage amplitude of the input signal unchanged (Example: a voltage follower). • Most biopotential amplifiers are voltage amplifiers, but current or power can also be addressed. • Design considerations include proper amplification and bandwidth, high input impedance, low intrinsic noise, interference rejection, stability against temperature and voltage fluctuations, and safety.
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GBM8320Dispositifs Médicaux Intelligents
Biopotential amplifiers – Part 1Mohamad Sawan et al.
• Function of a biopotential amplifier: to take a weak electric biological signal, increase its amplitude, and extract it from ambient noise so it can be further processed, recorded, or displayed.
• Bioamplifiers are also used to isolate the load from the source. In that case, the amplifier may provides only a current gain to drive the load, leaving the voltage amplitude of the input signal unchanged (Example: a voltage follower).
• Most biopotential amplifiers are voltage amplifiers, but current or power can also be addressed.
• Design considerations include proper amplification and bandwidth, high input impedance, low intrinsic noise, interference rejection, stability against temperature and voltage fluctuations, and safety.
GBM8320 - Dispositifs Médicaux Intelligents 4
Principle of biopotential measurementBlock diagram of a biopotential measurement system
• Biopotentials exhibit small amplitudes (1 µV to 10 mV) and low frequencies (0.1 Hz to 10 kHz).
• Biopotential measurements are corrupted by environmental and biological sources of interference.
• A first rank biopotential amplifier must implement a suitable interface. Dominates the SNR of the whole
recording channel
Gain stageelectrode
GBM8320 - Dispositifs Médicaux Intelligents 5
Principle of biopotential measurementBasic requirements (What is a suitable interface?)• High input impedance: Zin > 10 MΩ for loading the source/electrode
minimally;• Bandwidth: Must let the signal spectrum unchanged and optimize the SNR;• Amplification: A high gain is needed to boost the signal so it becomes
suitable for display, and subsequent processing modules;• Low intrinsic noise: Must provide a suitable SNR (Its input referred-noise
must be a few times smaller than the input signal);• High noise rejection: Must reject ambient noise and interference;• Protection input circuits : Voltage or leaking currents in the input could
affect the signal, damage the circuit or arm the patient’s body;• Output impedance: Low or high. It provides source impedance adaptation
to suit the load;• Calibration: Accurate and exact measurement of amplitude values are
required for physicians.
GBM8320 - Dispositifs Médicaux Intelligents 6
Principle of biopotential measurementImportant bioamplifier metrics and glossary• Bandwidth (in Hz), gain, attenuation (in dB)• Input referred noise (in V/√Hz or Vrms): Vni
2 = Vno2 / Gain2
• Signal-to-noise ratio (SNR in dB): SNR = 10 log (Vs2/Vn
2)• Noise rejection - Common mode rejection ratio (CMRR in dB)• Dynamic range (DR) and Linear range (in dB) – measured for 1% distortion
DR = 10 log (Vsmax2/Vsmin
2)• Signal-to-noise and distortion ratio (SNDR in dB)• Total harmonic distortion (THD in dB or percentage) and Nth-order
harmonic distortion terms (ex.: HD3, HD5)• Inter-modulation distortion (IMD in dB) and Nth-order inter-modulation terms
(ex.: IMD2, IMD3, IMD5)• Figure of merit: Give an indication of how well the design trade-offs are
resolved in an amplifier. Ex.: Noise efficiency factor (NEF), and other FOM
GBM8320 - Dispositifs Médicaux Intelligents 7
Common biopotential signals• Action potential (AP - intra or extracellular): Measurement of the electrical
potential resulting from the cell membrane depolarization;• Local field potential (LFP): The sum of all dendritic synaptic activity within a
volume of tissue measured with a low impedance microelectrode;• Electroneurogram (ENG): Neural activity conveyed by nerves from or to innervated
organs (Mixture of APs);• Electroencephalogram (EEG): Measured on the scalp. Low frequency waves;• Electrocardiogram (ECG): Heart activity measured on the chest;• Electromyogram (EMG): Muscle activity measured on the skin, or in muscles;• Electrocorticogram (ECoG): Measured with electrodes placed directly on the
surface of the brain (under the skull). ECoG is currently considered to be the “gold standard” for defining epileptogenic zones in clinical practice;
• Electrooculogram (EOG): Electric potentials generated as a result of movement of the eyeballs measured on or off the eye.
• Electroretinography (ERN): measures the individual electrical responses of various cell types in the retina, including photoreceptors, inner retinal cells, & ganglion cells.
Gosselin et al., "A Low-Power Integrated Bioamplifier ….," TBioCAS, v1, 2007.http://spikoscope.sourceforge.net/Spikoscope/Extracellular.html
GBM8320 - Dispositifs Médicaux Intelligents 14
Pre-processing of extracellular AP recordings
Buzsáki, “Large-scale recording of neuronal ensembles,” N. Neuroscience, V7, 2004.
High-density recording of unit activity in the somatosensory cortex of the rat
(a) Placement of an eight-shank silicon probe in layer 5 (8 sites per probe);
(a) A short epoch of raw recording, illustrating both field and unit activity (1–5 kHz);
(c) Two-dimensional views of unit clusters (out of 28 possible views from an eight-site probe) from one shank.
GBM8320 - Dispositifs Médicaux Intelligents 15
Analysis of extracellular AP recordings
• Brown et al. “Multiple neural spike train data analysis…,” Nat Neurosci, v7, 2004.• Nicolelis et al., “Reconstructing the engram: simultaneous, …” Neuron, v18, 1997.
Cross-correlogram
Cross-coherence
function(frequency domain)
Joint peri-stimulus time
histogram (JPSTH)
Parametric fitting
(assume a probability
model)
GBM8320 - Dispositifs Médicaux Intelligents 16
Recording APs for use in prosthetic devicesPopulation encoding of movements
Georgopoulos et al., J. Neurosci, 1982.
Population vectors: Vector contributions of each of the 241 directionally tuned cells Length of vector is proportional to the % change in firing rate from the mean firing rate.
Cells vary firing rates with direction of movement. Almost all cells show some modulation with each direction
•The right side shows a one-op-amp differential amplifier, but it has low input impedance;•The left side provides high input impedance and additional gain;•We have
• Acm ideally ≈ unity in the first stage and = 0 in the differential amp;• However, if R3’ ≠ R3 and R4 ≠ R4’, the output of the instrumentation amplifier
is
or,
Instrumentation amplifiers
34
'3'4'3
434
43 RRv
RRR
RRRvvo −
+
+=
+
+
++
−
+
+⋅=
−
+
+⋅=
34
'3'4'3
434
34
'3'4'3
4341
34
'3'4'3
4341 21
RR
RRR
RRRv
RR
RRR
RRRvA
RRv
RRR
RRRvAv
IdIcm
o
Acm AdIddIcmcmo vAvAv +=
GBM8320 - Dispositifs Médicaux Intelligents 20
Instrumentation amplifiersTunable instrumentation amplifier with a bandpass characteristic
Webster, The Measurement, Instrumentation and Sensors Handbook, CRC, 1998.
• The gain of the first stage (amplifiers A1 and A2) is 1+2·R2/R1, the second stage (amplifier A3) is -R4/R3, and the third stage (amplifier A4) is 1+R7/R6.
• The lower corner frequency is 1/(2πR5C1) and the upper corner frequency is 1/(2πR7C2).
• The variable resistor R4 is adjusted to maximize the CMRR. E1 and E2 are the recording electrode terminals while E3 is the reference or the ground electrode terminal.
The INA118 is a low power, general purpose instrumentation amplifier offering excellent accuracy. Its versatile 3-op amp design and small size make it ideal for a wide range of applications. The INA118 is laser trimmed for very low offset voltage (50 µV), drift (0.5 µV/°C) and high common-mode rejection (110dB at G = 1000).
GBM8320 - Dispositifs Médicaux Intelligents 22
Instrumentation amplifiersHigh-input impedance AC coupled instrumentation amplifier
A highpass cutoff frequency is obtained by connecting a Miller integrator from the amplifier output to its reference terminal.
GBM8320 - Dispositifs Médicaux Intelligents 23
Electrical and biological interferenceExamples: External interference in ECG recordings
Webster, The Measurement, Instrumentation and Sensors Handbook, CRC, 1998.
(a) Baseline changes and motion artifacts;
(b) Muscle signal interference;
(c) Electromagnetic interference (60 Hz power line and RF);
GBM8320 - Dispositifs Médicaux Intelligents 24
Electrical and biological interferenceExample : External interference in ECG recordings
Webster, The Measurement, Instrumentation and Sensors Handbook, CRC, 1998.
(d) Respiration
GBM8320 - Dispositifs Médicaux Intelligents 25
Electrical interference and noiseElectric-field pickup in an electrocardiograph
•C1-C3 are coupling capacitances;
•This causes displacement currents to flow through skin-electrode impedances on its way to ground (Z1, Z2 > ZG)
Typically, id ≅ 6 nA for a 9-m cable, and skin-electrode impedances may differ by as much as 20 kΩ. Hence, we can calculate the resulting differential voltage pick-up at the amplifier inputs:
This interference can be minimized by shielding the leads and grounding each at the electrocardiograph and/or lowering the skin-electrode contact impedances.
vA − vB = (6 nA)(20 kΩ) = 120 µV
)( 211 ZZivv dBA −=−
GBM8320 - Dispositifs Médicaux Intelligents 27
Electrical interferenceCommon-mode voltage
everywhere on the body
• A displacement current flows from the power line through the body and ground impedance• This creates a common-mode voltage everywhere on the body. • Zin is not only resistive but, as a result of AC couplings or RF bypass capacitors at the amplifier input, has a reactive component as well.
Determine vcm on the patient when the driven-right-leg circuit is used. Choose appropriate values for the resistances in the circuit so that vcm is minimal. What is vcm for a displacement current of id = 0.2 µA.
According to Eq. (4), the effective resistance between the right leg and ground is
Ro should be large to limit current. Values as high as 5 MΩ are used. The voltage vcm is made small by providing a low-resistance path to ground (by making RRL small). Thus, Rfmust be large and Ra small. Rf can equal Ro, and Ra typically equals 25 kΩ. For an electrode with resistance RRL = 100 kΩ, the effective resistance between the right leg and ground would then be
For id = 0.2 µA, the vcm is , compared to 10 mV without driven-right-leg circuit.