Introduction: Thoracic Impedance

Picture of Thoracic Impedance

Thoracic Impedance is a measure of the electrical activity in the chest that varies with changes in body size and composition, fluid volume, ventilation status, and heart characteristics such as cardiac output and stroke volume.

Our goal was to measure respiration rate. During inspiration, there is an increase in the gas volume of the chest in relation to the fluid volume; this increase causes conductivity to decrease. The length of the conductance paths also increases because of expansion.

Step 1: Components

Picture of Components

1 x AD5933

1 x AD5933 adapter

2 x TL072 op amp

3 x 1kΩ resistor

2 x 10kΩ resistor

1 x 2kΩ resistor

2 x 1MΩ resistor

1 x 560Ω resistor

1 x 2.2nF capacitor

1 x 8.2nF capacitor

1 x INA128 Instrumentation amplifier

1 x Arduino Uno

1 x Circuit board

Plenty of wires

Note: The AD5833 needs to be soldered onto the AD5933 adapter, which needs to have long pins.

Step 2: Circuit Diagram

Picture of Circuit Diagram

The circuit is composed of:

  • The AD5933 which outputs 5V at a frequency of 50 kHz at Vout and measures the output voltage at Vin
  • A high-pass filter that attenuates low-frequency noise from the output
  • A current source comprised of a voltage follower and resistor
  • A transconductance amplifier that has a protective resistor and load resistor in parallel
  • An instrumentation amplifier that outputs the voltage difference between the non-inverting and inverting inputs.
  • The Vref generator creates a 2.5V offset to ensure the signal remains positive, as the AD5933 cannot read negative voltages.
  • The Arduino Uno is interfaced with the AD5933

Step 3: Tissue Equivalent Model

Picture of Tissue Equivalent Model

A human body was represented with an RC circuit.

2 resistors: representing the extra- and intracellular space

1 capacitor: representing the cell membrane

A potentiometer is used to alter the resistance of one of the resistors and mimic breathing activity.

Step 4: Calibration

Picture of Calibration

AD5933 requires calibration before using to determine the Gain Factor.

Calibration Steps:

1.Place a known resistor(R) at the position for body.

2.Measure the magnitude of voltage output(V) from Arduino.

3.Calculate the gain factor(GF)

Step 5: Arduino Code


TheThinker (author)2016-04-20

Excellent! How close is your design to a commercial equivalent? Also is this useful for multiple purposes with the right DSP?

DIY Hacks and How Tos (author)2016-04-20

Cool circuit project

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