ENGR 104: Data Acquisition Lecturers: Dr. Binh Tran Dr. Otto Wilson Jr. © The Catholic University of America Dept of Biomedical Engineering

Class Objectives Introduction to Analog and Digital Signal Introduction to A/D conversion Introduction to LABVIEW Develop LabView Program to Measure Heart Rate (fr. Function Generator) Design Project: Develop a Velocity Measurement System

ECG System & Heart Rate Counter Differential OpAmp Electrodes Filters Gain OpAmp Scope Use PC analyze ECG signal Develop a Heart Rate Counter: Read ECG signal via A/D board Count the number of peaks Calculate the heart rate Fxn Gen.

Analog vs. Digital Continuous Time Signal Represented by function of time X(t), where t is continuous Computer can not deal with continuous-time signal Discrete Time Signal n Represented by indexed sequence of numbers X(nT), where n is integer index & T is fix time interval n X(nT) quantized into finite number of bits (ex: 8 bits CD player) n Computers deal with digital signal through bit word

Analog-to-Digital Process Sampling X(nT) Analog Signal X(t) A/D Converter Bit/Sample Analog Signal (Continuous) Digital Signal (Discrete)

Nyquist Sampling Rate A sample is taken at every fixed time T, X(nT) Time T has to satisfy Nyquist Criterion, which states that the sampling rate (1/T) has to be at least twice the frequency (f), 1/T > 2f or T < 1/2f Terminology: – T: sampling time (second) – 1/T: sampling rate (samples/second) – f: signal frequency (Hz, cycles/second)  X(5T)=0.7 V X(6T)=0.8 V  V  V Time (sec) Voltage (V)

A/D Converter Every sample X(nT) is quantized into a finite number of bit words (Binary bit: a system of numeration having 2 as its base, Ex: 001=1, 101= 5) Quantization level (Q) depends on the number of bit (B), number of level (Q) = 2 B (Ex. Q = 8-bits = 2 8 = 256 bits) The range of the analog data (ex. +10 V) divided by Q = the Resolution of the A/D converter, Resolution = (Max – Min) / Q – Example: For 8-bit A/D, Q= 256 bits Input Range of +10 V = 20 V peak-to-peak Resolution = 20 V / 256 bits = volts / bit

3 Bit Quantization Level 8 Level 7 Level 6 Level 5 Level 4 Level 3 Level 2 Level Quantization Level 3 Bit Word X(5T)=0.7V=110 X(6T)=0.8V = 111 Max = 1Volt Min = -1 Volt

3 Bit Quantization 3 Bit Word Level 8 Level 7 Level 6 Level 5 Level 4 Level 3 Level 2 Level Quantization Level X(5T)=0.7V=110 X(6T)=0.8V = 111 Max = 1Volt Min = -1 Volt Res=2/8=.25 volts

Labview Windows Diagram Window Panel Window Diagram Window is where you “write the program”. Panel Window is the “front panel” or “user interface.”

Labview Project #1: Build a HR Counter 1.Question: How would you compute heart rate? 2.Connect signal function generator (simulate ECG waveform) to pins of the A/D box. 3.Open LabView 8.6 program (Start Menu) 4.Add a waveform display of the ECG signal 5.Create user controls: 1.Sampling Rate (Samples/Second) 2.Number of Samples (Samples) 6.Time (sec) = # Samples / Sampling rate 7.Count the peaks of the signal and display the results 8.Calculate the heart rate (=peaks/Time = cycles/sec). Convert to “cycles (i.e. beats)/minute” and display the results in BPM.

For Next Week: Project #2 Velocity Measurement System Built Last Week Voltage Divider Circuit Scope Use PC analyze Light Interrupter signal Develop Velocity Measurement System: Read Waveform signal via A/D board Create input constants: Sample rate, # samples Create outputs: waveform graphs Measure Duration (i.e. time) Calculate the velocity (ball dia / duration) Light/Photosensor (Function Gen.)