PROJECT - BIOIMPEDANCE

BY

CHETNA RALHAN

DTU/2K12/ECE/056

2nd YEAR ECE BATCH-C3

GROUP

INTRODUCTION

In biomedical engineering, bioimpedance is the response of a living organism to an externally applied electric current. It is a measure of the opposition to the flow of that electric current through the tissues, the opposite of electrical conductivity. The measurement of the bioimpedance (or bioelectrical impedance) of the humans and animals has proved useful as a non-invasive method for measuring such things as blood flow and body composition like TBW and fat content(known as bioelectrical impedance analysis or simply BIA).Can precisely define changes in body composition during weight loss.

Bioimpedance is about the electrical properties of your body (or other biomaterials), e.g. to what extent you are a good conductor. Bioimpedance is a measure of how well the body impedes electric current flow. Fat has high resistivity, blood lower resistivity. It is popular owing to its ease of use, portability of the equipment and its relatively low cost compared to some of the other methods of body composition analysis.

BIOIMPEDANCE APPLICATIONS :

Skin water content

Impedance imaging (tomography)

Body composition ( nutrition)

Impedance Cardiography (ICG)

Cardiac Output monitoring

Ablation monitoring

Blood volume

Meat quality assessment

Single cell motion microscope

Single cell counting and characterization

Fingerprint sensors

MEASUREMENT OF BIOIMPEDANCE

Impedance is measured by applying a small electric current e.g. via 2 electrodes and picking up the resulting small voltage with another pair of electrodes: The lower the voltage the lower the tissue impedance for a given current.

Figure shows electrodes around a cylinder, for instance skin surface band electrodes around an arm or a leg. The impedance Z is given by:

Z = ρ L / A

where ρ is the resistivity of the tissue, L is the distance between the pick-up electrodes, and A is the cross-sectional area . The equation is only correct if the tissue is homogeneous and the current is applied by band electrodes far away from the pick-up electrodes.

The resistance (R) of the body is a consequence of its length (L) and cross-sectional area (A), and how easily current can pass through the body’s tissue which is a function of the volume of water present in the cylinder (body).

We treat the body as one large cylinder; the conductive length of this cylinder is the length of the body from one electrode to the next.

Fat has very, very little water, while lean tissue has approximately 73% water. It is the lean tissue (actually the water content of lean tissue) in the body that allows the current to pass and that is what we measure with BIA.

Tissue consists of cells and membranes, and membranes are thin but have a high resistivity and do electrically behave as small capacitors.

By using High measuring frequencies the current passes right through these capacitors, and the response is dependent on tissue and liquids both inside and outside the cells.

At Low frequencies, however, the response is dependent only on liquids outside the cells. 

The resistance and the capacitance of tissue are the two basic properties in bioimpedance.

Resistance has the same e ect on a.c. current as d.c. current.ff

Direct current cannot pass through a capacitor. A.c. can pass because of the rapidly reversing flux of charge.

 Methods Of Bioimpedance Analysis :

Single frequency BIA  : Involves using a single frequency at 50kHz which predominately measures the water outside of the cell (extracellular) and about 25% of the water inside the cells (intracellular).

Multi-frequency BIA : Similar to SF-BIA, but multiple frequencies are used and measured. Some researchers report that MF-BIA is better at predicting extracellular fluid while SF-BIA was better at predicting total body water.

Bioelectrical spectroscopy : It uses mathematical modeling and mixture equations to generate relationships between R and body fluid compartments. The published equations seem accurate and reliable in healthy individuals but are wildly disparate in various disease states.

Segmental-BIA : Done by adding two additional electrodes on the wrist and ankle of the opposite side (four electrodes total) or placing the two electrodes in various places to measure just the leg or arm or torso. There are several problems associated with this approach.

We are trying to approximate a cylinder with the body, which is not a perfect cylinder. So there are practical problems associated with this.

PRACTICAL PROBLEMS

PROBLEM 1:  As the cross section of a cylinder increases, the resistance decreases. As a result, the arm and leg contribute a great deal more to resistance than the torso. In fact, the arms and legs contribute from 90% of the resistance while the torso is only 10%, yet the torso represents as much as 50% of the whole body mass.

PROBLEM 2: The torso represents 50% of body weight but only 3-10% of the body’s impedance. This means that impedance is more closely related to changes of the muscle mass of the limbs; changes in the muscle mass of the torso is not adequately represented; even large changes in the fluid volume in the abdominal cavity have only minor influence on the measured impedance.

PROBLEM 3: The equations use the height of the individual instead of the entire length between the ankle and wrist. A person with different ratio of arm/leg to torso relative to the average person used to derive the equation will have a different result.

PROBLEM 4: Use assume a constant hydration of FFM at 73%. Anything that changes that hydration will change the measured resistance. Such factors as dehydration, exercise and diuretics will effect the results.

EQUATIONS FOR BIOIMPEDANCE ANALYSIS

Several equations can be used to predict fat-free mass. The following equations have a low standard error for predicting fat-free body mass and is appropriate for the general population.

Females:            Fat-Free Mass (kg) = 0.475 × [(ht2 × (cm2) / R (ohms)] + 0.295 × wt (kg) + 5.49

Males:              Fat-Free Mass (kg) = 0.485 × [(ht2 × (cm2) / R (ohms)] + 0.338 × wt (kg) + 3.52

Note that the equation estimates fat-free mass which can be used to estimate percent fat.

MAXIMIZING ACCURACY

To maximize accuracy of readings, certain parameters must be maintained :

Food – consumption of food and beverage can decrease impedance by 4-15 ohms over a 2-4 hour period representing an error < 3%.

Exercise will decrease R by ~3% and Xc by ~8% immediately after and returns to normal in 1 hour.

Lying down for 60 minutes has resulted in a 3% increase in R. This can result in errors of predicting TBW of 1-1.5 liters due to gravity pulling the water down.

NO interference with pacemakers or defibrillators. No alcohol intake for 12 hours prior to the study. The electrode sites may need to be cleaned with alcohol, particularly if the

skin is dry or covered with lotion. Height and weight should be accurately measured and recorded. No interference with pacemakers or defibrillators. No alcohol intake for 12 hours prior to the study. Attach the electrodes and patient cables as shown in the illustration.

ANALOG-TO-DIGITAL AND DIGITAL-TO-ANALOG CONVERTERS

INTRODUCTION

Signals in the real world are typically analog or continuously varying signals. It represents an exact value.

Eg: Temperature,pressure,light and sound intensity,speed,etc.

In order to use the power of digital electronics, one must convert from analog to digital form on the experimental measurement end by using analog-to-digital converter(ADC) and convert from digital to analog form on the control or output end of a laboratory system by using digital-to-analog converter(DAC).

The physical variables are first converted into electrical signals using transducers and these electrical analog signals are converted into digital signal using analog-to-digital converter(ADCs) .

These digital signals are processed by digital computer and the output of digital computer is converted into analog signals using digital-to-analog converters(DACs).

DIGITAL TO ANALOG CONVERTER

It is a device that converts a value represented in digital code (usually binary) into an analog signal (current, voltage, or electric charge) which is proportional to the digital value .

DAC SPECIFICATIONS

1. Resolution (Step Size)2. Accuracy3. Settling Time4. Offset Voltage5. Monotonicity

1. RESOLUTION

• Defined as the smallest change that can occur in the analog output when digital input changes

• Resolution can be expressed in two cases, either the voltage or Ampere and also percentages.

• Resolution is usually referred to the step size since it was a total change in Vout when the digital input changes from one step to the next step.

Block diagram of 8-bit D/A converter

Resolution Percentages (%)

2. ACCURACY Manufacturer of digital to analog converter has a several ways to define accuracy.Two of them are often referred to Linearity Error and Full-scale error

Full Scale Error• The maximum deviation from the ideal DAC output value.

Linearity Error• The maximum deviation of the step size from ideal step size.

Resolution = Step Size = Input bit for LSB

Number of Step = 2n – 1

Where;

n = Number of input bits

% Resolusi = 1 x 100% Number Of Step

= 1 x 100% 2n - 1

3. SETTLING TIME

• The speed of digital to analog converter is usually referred to the settling time, which is the time required by a digital to analog converter output for change from zero to full-scale during binary input change from all zero to all one.

• Actually, this settling time measured at the time of digital to analog converter output was completed in the range of 1/2 step size full-scale.

• Usually the settling time for  current digital to analog converter  is shorter than the settling time voltage digital to analog converter.

• Examples : If the digital to analog converter has 10mV resolution. The settling time is measured at  fixed output time at  5mV full-scale range.

4. OFFSET VOLTAGE

Digital to analog converter ideal output is 0V when the binary input is all '0 '. In practical there is usually a small voltage value at this time called offset voltage. Most of DAC has external offset adjustment that will adjust to 0V as required.

5. MONOTONICITY

Digital to Analog Converter is monotonic if the output either increases or same if binary input increases from one values to other values.

Monotonic is important in closed-loop system to avoid oscillation. Example:

TYPES OF DAC

1. R-2R LADDER TYPE DAC :

It is the most popular DAC. It uses a ladder network containing series-parallel combination of two resistors R and 2R .

• The circuit is different from the DAC circuit weighted-resistor type DAC because it only uses two resistor values, R and 2R.

• Disadvantage of weighted resistor type DAC is we can see on the circuit was too much of the resistor to be provided. For example, if 12-bit DAC with resistor value MSB (most significant bit) is 1KW then LSB resistor will exceed 2MW. By fabrication technology circuit, it is difficult to produce a large resistance range values with small values of current and can set the exact ratio in the range of temperatures.

This is why R/2R DAC circuits are frequently used to obtain high accuracy and precision.

BinBin

Vout

a and b is Monotonic but c is not Monotonic

Bin

VoutVout

b ca

• Advantages

– Only two resistor values (R and 2R)

– Does not require high precision resistors

• Disadvantage

– Lower conversion speed than binary weighted DAC

2.WEIGHTED-RESISTOR TYPE DAC :

• The Op-amp is used to produce a weighted sum of the digital inputs ,where the weights are proportional to the weights of the bit position inputs.

• Operational amplifier is used to produce a weighted sum of digital inputs.

• Weighted resistors are used to distinguish each bit from the most significant to the least significant.

• Op-amp connected as inverting amplifier .

• Advantages

– Simple Construction/Analysis

– Fast Conversion

• Disadvantages

– Requires large range of resistors with necessary high precision for low resistors

– Can be expensive. Therefore, usually limited to 8-bit resolution.

I-+R2R4R2n-1RRfVoutVrefV1V2V3Vn

V out=−IR f=−R f (V 1

R+

V 2

2R+

V 3

4 R+⋯

V n

2n-1 R )

ANALOG TO DIGITAL CONVERTER

• Produces the digital output that is proportional to the value of input analog signal.

• When analog signal is processed by a dgital system, an ADC is used to convert the analog value to digital form suitable for processing by digital system.

• The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output.

TYPES OF A/D CONVERTER

1.Counter Type

2. Tracking Type

3. Flash Type

4. Dual slope Type

5. SUCCESSIVE-APPROXIMATION TYPE

1. COUNTER TYPE ADC

• Simplest type of A/D converter.• Also known as digital ramp ADC as waveform at o/p of DAC is step-by-

step ramp(staircase).• Analog signal applied to non-inverting terminal of Op-amp comparator and

the output of the DAC is applied to the other terminal of the comparator.• Control logic initializes the system, sets counter to 0 and turns on clock

sending regular pulses to the counter.• As the counter counts, its output to the D A C generates a staircase ramp to

the comparator.

DISADVANTAGE

• Conversion time depends on magnitude of analog input• Larger the input , more will be the no. of clock pulses that must pass to reach

the proper count, hence larger will be the conversion time• For each count counter has to start from reset only• Considered quite slow in comparison with other types.

2. TRACKING TYPE ADC

• Instead of a regular "up" counter driving the DAC, this circuit uses an up/down counter.

• The counter is continuously clocked, and the up/down control line is driven by the output of the comparator. So, when the analog input signal exceeds the DAC output, the counter goes into the "count up" mode.

• When the DAC output exceeds the analog input, the counter switches into the "count down" mode.

• Either way, the DAC output always counts in the proper direction to track the input signal.

• Advantage– Faster than the counter-type ADC as counter is not reset after each

sample • Disadvantage

– If analog input remains constant, the counter keeps on changing from up-down-to-up continuously and therefore the output of ADC keeps on oscillating about the constant analog input.

3. FLASH TYPE ADC

• Fastest type of ADC

• Also called the parallel A/D converter

• It is formed of a series of comparators, each one comparing the input signal to a unique reference voltage

• The comparator outputs connect to the inputs of a priority encoder circuit, which then produces a binary output

• If very high speed conversions are needed, e.g. video conversions, the most commonly used converter is a Flash Converter.

• Advantage– Most efficient in terms of speed, very fast – Simplest in terms of operational theory

• Disadvantage– Expensive– Most component-intensive for any given number of output bits.

Three-bit flash ADC requires seven comparators. A four-bit version would require 15 comparators. With each additional output bit, the number of required comparators doubles.

4. DUAL SLOPE TYPE ADC

• The sampled signal charges a capacitor for a fixed amount of time• By integrating over time, noise integrates out of the conversion• Then the ADC discharges the capacitor at a fixed rate with the counter

counts the ADC’s output bits. A longer discharge time results in a higher count

• Advantage– Input signal is averaged– Greater noise immunity than other ADC types– High accuracy

• Disadvantage– Slow– High precision external components required to achieve accuracy

5. SUCCESSIVE-APPROXIMATION TYPE A/D CONVERTER

• A Successive Approximation Register (SAR) is added to the circuit

• Instead of counting up in binary sequence, this register counts by trying all values of bits starting with the MSB and finishing at the LSB.

• The register monitors the comparators output to see if the binary count is greater or less than the analog signal input and adjusts the bits accordingly

ADVANTAGE

• Capable of high speed and reliable• Medium accuracy compared to other ADC types• Good tradeoff between speed and cost• Capable of outputting the binary number in serial (one bit

at a time) format.

DISADVANTAGE

• Higher resolution successive approximation ADC’s will be slower

• Speed limited to ~5Msps

ADC TYPES COMPARISON

Type Speed (relative)

Cost (relative)

Dual Slope Slow Med

Flash Very Fast High

Successive -Approx

Medium – Fast

Low

Counter Slow Low