GBM8320 Dispositifs Médicaux Intelligents

Transcription

1 GBM830 Dispositifs Médicaux Intelligents Biopotential amplifiers Part Mohamad Sawan et al. Laboratoire de neurotechnologies Polystim M February 010 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 GBM830 - Dispositifs Médicaux Intelligents 1

2 Electrical interference reduction Shield driver (Guarding with common mode voltage) When a shield is driven with the average of the input signal, the input capacitance for v cm is vanishingly small because there exists no potential difference between shield and inner wires to create signals. Driven right leg circuit Metting van Rijn et al., High-quality recording of bioelectric events, MBEC, v9,1991. Shield driver GBM830 - Dispositifs Médicaux Intelligents 3 Electrical interference reduction Isolation amplifier using transformers and optocpouplers Transformer coupled amplifier using the transformer T. Note that the isolator separates circuit common on the amplifier side from the Earth ground on the output side. Optical isolation using the photodiode D and the photodetector P. The circuit common is also isolated from the earth ground in that case. Webster, The Measurement, Instrumentation and Sensors Handbook, CRC, GBM830 - Dispositifs Médicaux Intelligents 4

3 Intrinsic noise Noise from electronic circuits: Thermal noise Thermal noise: caused by collision of carriers with the atomic lattice; Shot noise: caused by random passage of charge carriers across a potential barrier such as the pn-junctions of a diode or bipolar transistor; Their spectral density V n (f) takes the following shape: I d (f) = g m V g (f) Vn µ V ( f ) Hz 3. Root spectral density V ( ) 3. n f = µ V Hz ( ) I R f = 4kT γg m 0 ( Hz) V R ( f ) 1 = 4kT γ W g m Gate-referred noise voltage GBM830 - Dispositifs Médicaux Intelligents 5 Intrinsic noise 0log Noise from electronic circuits: Flicker noise Flicker or (1/f) noise: Mainly due to trapping of carriers and their subsequent random release. Due to its power distribution is particularly important in low frequency applications; 1/f noise is particularly common in MOS transistors and poly-silicon resistors. K 1 V ( f ) = + 4kT γ ( V n ) 1/f K VR ( f ) = WLC f + ox f (log scale ) V ( ) n f 10 µ V Hz /f noise dominates -10dB/decade 10 R Root spectral density 1/f noise corner White noise dominates WLC f g ( Hz) ox 6 ( ) ( ) V ( f ) = f Total noise PSD (Thermal + 1/f) 6 n GBM830 - Dispositifs Médicaux Intelligents 6 m 3

4 Intrinsic noise Noise from electrodes E he A body-surface electrode is placed against skin, showing the total electrical equivalent circuit obtained in this situation; Each circuit element on the right is at approximately the same level at which the physical process that it represents would be in the left-hand diagram. There is a noisy resistive part R d Electrode Gel Epidermis Dermis and subcutaneous layer C d C e R s E se R e R u R d Sweat glands and ducts C P E P R P Webster, Medical instrumentation: application and design. 3 rd ed, Wiley, GBM830 - Dispositifs Médicaux Intelligents 7 Intrinsic noise Noise from electrodes System intrinsic noise = Electrode noise + Preamplifier noise Main source of electrode noise is thermal noise V ne = 4kTR N Δf R N is noise resistance (real part of probe impedance magnitude). It has a 1 / f α frequency dependence (0.85 < a < 0.95); Δf is recording bandwidth; Example for AP recording: Biological frequency band: 100Hz-10kHz; Extracellular action potentials have amplitude in the range of µV; Thus, total recording channel input-referred noise should be < 0µV rms for a SNR ~> 8 db. GBM830 - Dispositifs Médicaux Intelligents 8 4

5 Intrinsic noise Noise from electrodes Therefore, it is important to keep circuits noise in the recording channel low compared with the noise from the site and electrode itself. GBM830 - Dispositifs Médicaux Intelligents 9 Intrinsic noise Other unwanted effects from electrodes Impedance: Isolation of at least 10 MΩ input impedance is needed in the amplifier to avoid excessive loading of the electrode which would otherwise results in distortion; DC polarization: DC polarization of the electrode must be remove. Mismatch voltages of hundreds of mv are seen across differential electrodes. These offsets can easily saturate the 1 st rank amplifier; Electrode drift: Electrodes can drift by tens of millivolts over time. GBM830 - Dispositifs Médicaux Intelligents 10 5

6 Electrical protection circuits Voltage-limiting devices (a) Current-voltage characteristics of a voltage-limiting device. (b) Parallel silicon-diode voltage-limiting circuit. (c) Back-to-back silicon Zener-diode voltage-limiting circuit. (d) Gas-discharge tube (neon light) voltage-limiting circuit element. Webster, Medical instrumentation: application and design. 3 rd ed., John Wiley, GBM830 - Dispositifs Médicaux Intelligents 11 Electrical protection circuits Electrical protection circuit: resistance R limits the current, reverse-biased diodes D limit the input voltage, and the spark gap S protects against defibrillation pulse-related breakdown of the isolation transformer T. Photocouplers or CMOS isolators can also be used for electrical isolation. GBM830 - Dispositifs Médicaux Intelligents 1 6

7 Electrical protection circuits Simplified representation of the INA118 Each input is protected by two FET transistors that provide a low series resistance under normal signal conditions, preserving excellent noise performance. When excessive voltage is applied, these transistors limit input current to approximately 1.5 to 5mA. GBM830 - Dispositifs Médicaux Intelligents 13 System implementation Example: A portable multi-channel neural data acquisition system Gosselin & Sawan, "A Low power portable multichannel neural," IFESS, 005. GBM830 - Dispositifs Médicaux Intelligents 14 7

8 System implementation Example: A portable multichannel neural data acquisition system 16 channels Sampling frequency per channel: 3 ksps Programmable gains from 60 db to 100 db Low input referred noise (10 nv/ Hz) Bandwidth is from 100 Hz to 10 khz High CMRR (85 db) Input protection (INA118) Battery powered Low-power controller (Xilinx Coolrunner) Low-power consumption (75 mw) Electrical isolation with CMOS isolator Gosselin & Sawan, "A Low power portable multichannel.," IFESS, 005. GBM830 - Dispositifs Médicaux Intelligents 15 Advanced instrumentation amplifiers AC-coupled instrumentation amplifier: alternate topology In (a), R reduces the common mode input impedance which degrades the CMRR; (b) provides a AC coupling network but no DC current flow trough the RC network when a v cm is applied. This imply an infinite Z cm. (a) Typical circuit for balanced AC coupling. (b) Proposed AC-coupling circuit without any grounding resistor. Spinelli et al., AC-coupled front-end for biopotential measurements, TBME, v50, 003. GBM830 - Dispositifs Médicaux Intelligents 16 8

9 Advanced instrumentation amplifiers Current-feedback IA (automatic balancing to enhance CMRR) Current-feedback instrumentation amplifier. Ql,Q, and R1 form a V-to-I converter with transfer 1/R1. Feedback from the output is realized by another V-to-I converter Q3, Q4, R with transfer 1/R. Loop amplifier A adjusts Vout so that Vout/Vin = R/R1. The loop amplifier A keeps the collector currents at a constant value. A disadvantage of this setup is that two accurate V-to-I converters are needed. Indirect current-feedback instrumentation amplifier. Transconductance stage T1 converts the input voltage into a current i1 = Gm1.Vin and the second transconductance stage T does the same with the attenuated output voltage. Loop amplifier A with high gain will make i1-i = 0, and therefore the overall transfer function becomes van den Dool & Huijsing, Indirect current feedback instrumentation, JSSC, v8, GBM830 - Dispositifs Médicaux Intelligents 17 Advanced instrumentation amplifiers Indirect current-feedback IA Instrumentation amplifier with p-n-p V-to-l converters. This method is called indirect current feedback, because the output voltage is not fed back directly to the input of the amplifier but to the input of a second input stage T, and then the output currents of input stages T1 and T are compared and fed back to the loop amplifier. The advantage of the indirect current feedback method over the resistive feedback circuit previously introduced is that the balanced resistor network, connecting the output to the input, has been eliminated. van den Dool & Huijsing, Indirect current feedback instrumentation, JSSC, v8, GBM830 - Dispositifs Médicaux Intelligents 18 9

10 Advanced instrumentation amplifiers A CMOS low-power low-noise monolithic IA The circuit has two feedback loops, one realizing the gain and the other the low cutoff frequency. The first feedback loop is based on the current feedback technique, where the structure has been transformed, so that only a single-stage OTA is obtained. Further, in this IA circuit, a low cutoff frequency has been realized with an integrator structure (OTA- Int and Cext ) and an equivalent resistor made by OTA-Gm (R = 1/g m ), whereby only single-stage OTA s are used. for and Steyaert and Sansen, A micropower low-noise monolithic instrumentation, JSSC, v, GBM830 - Dispositifs Médicaux Intelligents 19 Advanced instrumentation amplifiers Current-mode IA Op-amps OA1 and OA both act as high input impedance voltagefollowers for the noninverting and inverting inputs V1 and V, respectively. The potential across R1 is V V1 and this defines the current in the feedback path of OA1. This current is sensed by the current mirrors CM1 and CM. The outputs of the two current mirrors are recombined at the current sensing node of the output stage comprising OA3 and R. Toumazou & Lidgey, Novel current-mode instrumentation, Elec. Letters, v5, GBM830 - Dispositifs Médicaux Intelligents 0 10

11 Advanced instrumentation amplifiers CMOS Instrumentation Amplifier Using Chopper Modulation Low pass filtered Noise in MOSFETs Nielsen & Bruun, A CMOS Low-Noise Instrumentation., AICSP, v4, 005. GBM830 - Dispositifs Médicaux Intelligents 1 Advanced instrumentation amplifiers CMOS Instrumentation Amplifier Using Chopper Modulation Fabricated CMOS 0.18-µm chip (a) Measured performances (b) Measured output PSD (c) (a) (c) Nielsen & Bruun, A CMOS Low-Noise Instrumentation., AICSP, v4, 005. (b) GBM830 - Dispositifs Médicaux Intelligents 11

12 ENG Cuff recording Tripolar cuff electrode recording The amplitude of the ENG signal recorded using this method depends to some extent on the dimensions of the nerve cuff but is typically on the order of 1-µV rms with a broad flat power spectral density (PSD) centered at about 1 khz; The signal is embedded in noise generated by various mechanisms, notably white noise from the interstitial fluid and from the electrode tissue interface; By short-circuiting the end-contacts, a terminal is created that gives the average potential of the two end-contacts, as long as the two contact impedances are equal. The same average potential is also recorded by the central contact (under a few conditions). Differential recording between the center contact and the connected end-contacts will then cancel the potential difference between the cuff-ends. Rieger et al., Design of a low-noise preamplifier., JSSC, v38, 003. GBM830 - Dispositifs Médicaux Intelligents 3 ENG Cuff recording Voltage and frequency ranges of some common biopotential signals Power spectral densities of ENG, EMG, and background noise recorded from a cuff implanted around the digital nerve in a human hand [source: Dr M. Haughland, SMI, Aalborg, Denmark]. Rieger et al., Design of a low-noise preamplifier., JSSC, v38, 003. GBM830 - Dispositifs Médicaux Intelligents 4 1

13 ENG Cuff recording Tripolar cuff electrode recording Tripolar ENG amplifier configurations. (a) Quasi-tripole (QT). (b) True-tripole (TT). EMG rejection by the quasi-tripole relies on perfect symmetry in cuff geometry and tissue resistivity, which will only be an approximation, at least due to manufacturing tolerances. The main benefits of the true-tripole system are: 1) the amplitude of the ENG signal recorded is about twice that of the quasi-tripole and ) the gains of the input amplifiers can be adjusted independently to compensate for any imbalance. Demosthenous & Triantis, An adaptive ENG amplifier for tripolar cuff.., JSSC, v40, 005. GBM830 - Dispositifs Médicaux Intelligents 5 ENG Cuff recording Tripolar cuff electrode recording: true-tripole arrangement Lumped-impedance model of the cuff and idealized ENG and EMG potentials inside the cuff. Typical impedance values: Z to = 00 Ω, Z t1, = 1:5 kω, Z e1,,3 = 1 kω. (See previous page for amplifiers configuration) Demosthenous & Triantis, An adaptive ENG amplifier for tripolar cuff.., JSSC, v40, 005. GBM830 - Dispositifs Médicaux Intelligents 6 13

14 ENG Cuff recording Tripolar cuff electrode recording: Low noise preamplifier BiCMOS diff-pair Av = g m1, r o = (I C /ktq) r o // r ds3 (I C /ktq) V E /I C = V E /ktq CMOS diff-pair Av = g m1, r o = g m1, r ds // r ds4 Input-referred noise Vno ( f ) = ( gm 1ro ) Vn 1( f ) + ( gm3ro ) Vn3( f ) V ( f ) = V ni no = V ( f ) /( g n1 r ) m1 o g ( f ) + g m3 m1 Vn3( f ) Bipolar and pmos input devices are used they generate less 1/f noise Rieger et al., Design of a low-noise preamplifier., JSSC, v38, 003. See D. Johns et K. Martin, Analog Integrated Circuit Design, Wiley, GBM830 - Dispositifs Médicaux Intelligents 7 ENG Cuff recording Tripolar cuff electrode recording: Low noise preamplifier An operational transconductance amplifier (OTA) terminated in a load resistor (R1), a low-pass filter (R and C) to restrict the bandwidth to about 15 khz, and an output buffer. BiCMOS and CMOS preamplifier candidates. Simulated input-referred noise voltage PSDs of three candidate OTAs. The BiCMOS OTA has lower 1/f input-referred noise than its CMOS counter parts. Rieger et al., Design of a low-noise preamplifier., JSSC, v38, 003. GBM830 - Dispositifs Médicaux Intelligents 8 14

15 ENG Cuff recording Tripolar cuff electrode recording Final BiCMOS preamplifier schematic diagram. Circuitry was included to cancel the base currents of Q1 and Q. This is very important, as significant current flowing into the tissue cannot be permitted. Transistor Q8 generates a replica of the base currents of Q1 and Q, which is fed into the pmos current mirror M4, M5, M6. The common base transistor Q9 level shifts the current from Q8 and ensures that its dc conditions match those of Q1 and Q as far as possible. The pmos mirror transistors M5 and M6 feed the bases of M1 and M, respectively. M3 implements a source follower. Rieger et al., Design of a low-noise preamplifier., JSSC, v38, 003. GBM830 - Dispositifs Médicaux Intelligents 9 ENG Cuff recording Adaptive Instrumentation amplifiers for tripolar cuff electrode recording Adaptive-tripolar architecture : ENG amplifier developed to automatically compensate for cuff imbalance, for improving the quality of the recorded ENG. The system consists of two voltage preamplifiers, each with a fixed gain A, providing a very low-noise interface with the cuff electrodes. The preamplifiers are followed by two operational transconductance amplifiers (OTAs) with variable gains G m1 and G m, controlled by the differential feedback currents I f1 and I f. The control stage operates by first obtaining the moduli of the currents at the output of the variable-gain OTAs and applying them to a current comparator to establish which is the largest. The comparator voltage output is subsequently applied to a large time- constant integrator which generates I f1 and I f. The variable-gain OTAs counterbalance cuff imbalance, by equalizing the amplitudes of the EMG signals at their outputs. As a result, when the output signals of the OTAs are summed at the input of the output-stage amplifier (gain G o ), the equal and anti-phase EMG signals from the two channels are cancelled, and the in-phase ENG signals are added and further amplified. Demosthenous & Triantis, An adaptive ENG amplifier for tripolar cuff.., JSSC, v40, 005. GBM830 - Dispositifs Médicaux Intelligents 30 15