Basic Analog Circuits

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Basic Analog Circuits Instrumentation

• Shane Woolwine

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Outline

• About myself
• Electricity
• Basic Circuits
• EKG
• Filters Gain
• Digital World

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About Myself

• BS Computer Engineering at JHU
• MSE Candidate in Computer Science
• Infinite Biomedical Technologies Intern
• SensorTrak group (www.sensortrak.com)
• President Engineers without Borders 07-08
• President IEEE 08-09

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Electricity

• V IR

• Water flowing downhill

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Voltage Source

• Voltage source drives current
• A battery is a voltage source
• It provides the energy to drive your circuit
• Change of potential energy to kinetic or useful
  energy

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Water Source

• Similar to voltage source, water towers provide
  the necessary pressure / energy needed to supply
  (drive) water to houses

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Resistors

• Limit current of electricity
• Similar to how a clogged or diameter of pipe
  affects current or flow of water

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Capacitors

• C Q / V
• Retains charge and disperses over time
• Think of as a water tank with an input and output
  or a well

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Properties of Capacitor

• Impedance / Resistance 1/ jwc
• w angular frequency
• c capacitance
• Becomes a short (closed wire) at high frequency

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Simple RC circuit

• RC circuit or system simply has a resistor and
  capacitor

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Signals Systems

• The world operates on signals
• We live in a would made up of sinusoids
• Sound, speech, vision, etc.
• We analyze these signals as inputs into a system
• We will look at the heart signal and its input
  into an EKG system

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PQRST Curve
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How do we capture this signal?

• The signal is called an EKG or Electrocardiogram
• Not the only signal in the body
• Not the only signal picked up by sensors
• Signal is in millivolt range
• Need amplification and filtering

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Low pass filter

• Using property of a capacitor, can create a low
  frequency filter

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High Pass Filter

• High frequency do not enter the circuit or
  system

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Operational Amplifiers

• Both inputs are same voltage
• No current flows in or out of input

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Active Filters

• Active filters provide high impedance and work
  better to protect and use the flow of current
  of the circuit

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Amplifier

• Using V IR and an op amp one can create an
  amplifier.
• Gain output / input
• 20 Log (Vout/Vin) dB

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Instrumentation Amplifier

• For dealing really small voltages
• milli volt range

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EKG Signal

• Skin Electrode
• 0.5 - 4 mV
• 0.01 - 250 Hz

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EKG

• Need large amplification
• probably a gain of several hundred
• A high pass filter for signals greater than 250
  Hz
• A low pass filter for signals smaller than 0.01 Hz

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EKG
Instrumentation Amplifier
Low pass filter
high pass filter
Gain of 800, frequency response of 0.04 - 150 Hz
or - 3dB
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Digital

• This is all great, but to be useful must be in
  the digital realm
• Can add a comparator to digitize the data
• Known as a form of ADC (analog to digital
  converter)
• Once digital, can use microcontrollers and
  software to analyze the signal

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Electrical Engineering

• Taking the analog world around us and converting
  it into electrical signals we use
• These electrical signals we process using analog
  methods or digitally
• It is a process of capturing and processing
  details of the world around us.

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Contact and Questions

• email shane.woolwine_at_jhu.edu
• www.sensortrak.com/members/shane/
• Thanks to Thakors instrumentation lab and
  Medical Instrumentation and Design
• Questions and Remarks?