ECEN3000 Course Project HW debugging Project 7meline Signals of Interest •  Bio-­‐poten)als: electrochemical ionic currents become electron flows at the skin-­‐electrode interface •  Frequency range: 1-­‐~100 Hz •  Time-­‐series voltage amplitudes –  Brainwaves (EEG): –  Eye movement (EOG/Electrooculography): –  Muscular (EMG): 5-­‐50 μV 50-­‐200 μV 20-­‐500 μV Our Enemy: Noise •  Body-­‐antenna effect –  Your body capaci7vely-­‐couples with electrical mains, florescent lights and injects them into the signal path –  Common-­‐mode voltage power in 60Hz band exceeds signal power by over 100x –  Solu)on: Right-­‐leg drive (RLD) •  Mo7on Ar7fact –  Slight movements alter electrode-­‐
skin impedances, resul7ng in false voltage ar7facts •  ∆V = I·∙∆R –  Solu)on: Tight electrode-­‐skin coupling Low Noise Design Considera7ons •  Distance between electrode and amplifier –  The shorter, the be5er –  Wire length ∝ parasi7c capacitance ∝ pre-­‐amplified capaci7vely-­‐coupled EMI •  Electrode-­‐skin impedances –  The lower, the be5er –  Use high conduc7vity materials (Ag, Cu) •  Impedance matching •  Shielding Signal + 60 Hz Noise Ac7ve Noise Suppression •  Body common-­‐mode voltage (BCMV): voltage noise shared across all areas of the body •  “Right-­‐leg Drive” (RLD) circuit –  BCMV à phase inversion & high gain amp. à back to body –  Originated with ECG (electrode was placed on right leg) Opera7onal Amplifiers “In order to measure a voltage accurately, the input impedance of the measurement device should be considerably larger than the impedance at the skin” •  Total Gain:
600x •  Bandpass Filtering:
0.15 Hz to 70 Hz •  Target Electrical Characteris7cs –  Input impedance:
–  Input bias current: –  Voltage noise-­‐density: –  Current noise-­‐density: >1012
<10-­‐12
<10-­‐9 <10-­‐15
–  Electrode-­‐skin impedance: 150 KΩ to 1 MΩ •  Dry electrode Characteris7cs Ω A V/√Hz A/√Hz Larger the difference, the be5er Analog-­‐to-­‐Digital Converter •  Oversampling delta-­‐sigma (∆∑) ADC –  Signal of interest is low frequency –  Oversampling helps smooth out noisy samples •  High resolu7on (16-­‐24 bit) à less amplifica7on à less injected voltage and current noise •  Low sampling rates (250-­‐500 Hz) •  Serial data output reduces circuit complexity Data Transmission to ARM •  Serial Peripheral Interface (SPI) •  Full duplex communica7on •  Beoer suited than I²C for streaming applica7ons –  Higher throughput –  No addressing in data band •  4 wires –  Serial Clock (SCLK) –  Master out slave in (MOSI) –  Master in slave out (MISO) –  Slave Select/Chip Select (SS/CS) AFE PCB Func7on VBATT LMV321 (VDD/2) NCV8570 (VDD) MAX4250 ADS1292 TLC2262 ANALOG DIGITAL Sensors ARM PCB Chip Loca7ons MAX4250 TLC2262 LMV321 ADS1292 NCV8570b Power Supply Components •  NCV8570b (LDO): provide clean, stable VDD power Electrical Characteris)c Value Maximum Input Voltage 5.5 V Output Voltage 3.3 V Power Supply Rejec7on 82 dB Voltage Noise 10 μVRMS Supply Current 75 μA •  LMV321 (op-­‐amp): provide stable virtual ground (VDD/2) Electrical Characteris)c Value Input Offset Voltage 1.7 mV Power Supply Rejec7on 60 dB Voltage Noise 46 nV/√Hz Supply Current 80 μA MAX4250 Op-­‐amp (Signal Buffer) •  Func)on: provide a buffer to the bio-­‐poten7al signal as close to the electrode source as possible •  Gain: 2x •  Filtering: HF-­‐rejec7on Electrical Characteris)c Value Input Impedance 1012 Ω Input Bias Current 0.1 pA Voltage Noise Density 8.9 nV/√Hz Current Noise Density 0.5 fA/√Hz Common-­‐mode Rejec7on 115 dB Supply Current 400 μA TLC2262 Op-­‐amp (Signal Gain) •  Func)on: provide gain and filtering to the bio-­‐
poten7al signal prior to ADC sampling •  Gain: 51x •  Filtering: 0.15 Hz to 70 Hz Electrical Characteris)c Value Input Impedance 1012 Ω Input Bias Current 1 pA Voltage Noise Density 12 nV/√Hz Current Noise Density 0.6 fA/√Hz Common-­‐mode Rejec7on 83 dB Supply Current 850 μA ADS1192/ADS1292 (AFE & ADC) •  Func)on: provide final stage gain, driven-­‐right leg func7onality, and digital sampling of the bio-­‐
poten7al •  Gain:
1-­‐12x (6x for us) Electrical Characteris)c Value Effec7ve Number of Bits 16 (ADS1192), 24 (ADS1292) Input Impedance 109 Ω Signal-­‐to-­‐Noise Ra7o 112 dB Common-­‐mode Rejec7on 115 dB Total Harmonic Distor7on 98 dB Supply Current 909 μA Auxiliary Slide MAX4250 (2) TLC2262 (1) ADS1292/ADS1192 (1)