Bio Instrumentation

ECG, EEG, EMG, Cardiovascular measurements & Venous Pressure

Op ampOperational amplifiers is a very high gain DC differential amplifier that uses one or more external

feedback networks to control its response and characteristics. We can connect external resistors or

capacitors to the op-amp in a number of different ways to form basic “building Block” circuits such as,

Inverting, Non- Inverting, Voltage Follower, Summing, Differential, Integrator and Differentiator type

amplifiers.

Op-amp Symbol

An “ideal” or perfect Operational Amplifier is a device with certain special characteristics such as infinite open-loop gain Ao, infinite input resistance Rin, zero output resistance Rout, infinite bandwidth 0 to ∞and

zero offset (the output is exactly zero when the input is zero).

Operational amplifiers are available in IC packages of either single, dual or quad op-amps within one

single device. The most commonly available and used of all operational amplifiers in basic electronic kits and projects is the industry standard μA-741.

SensorsSensors (often called transducers – the difference is unimportant here) convert the energy of the patient (pressure, for example) into a form that can be used by an instrument. There are two types of sensors/transducers:

Some transducers’ output changes in response to a change in surroundings. The output is a change in resistance, capacitance, or inductance. These variations can then be measured, often using Wheatstone bridge circuitry because the changes may be very small. Common

Examples include: Strain gauges – Change in resistance when some external event occurs. Potentiometers (variable resistors) – Change in resistance when some external event

occurs. Since these devices often have a mechanical knob or lever, they often convert mechanical movement or position into a change in resistance.

Thermistors – Change in resistance when temperature changes.▷ Photoresistors – Change in resistance when light hitsthe device.▶ Some transducers produce a voltage or current in response to a change in environment. Common examples include:▷ Piezoelectric crystals – Produce a voltage as the crystal is compressed (even tiny amounts).▷ LVDTs (linear variable differential transformers) – Convert linear motion (may be very small amounts) into an electrical signal.▷ Thermocouples – Measure temperature differences using dissimilar metals. They require a known reference temperature.Note: There are many types of sensors/transducers. Commonlyseen ones are described here, but engineers create specialized andunique devices to serve particular needs.When blood pressure is measured directly (in an artery), it is commonly measured using strain gauges, piezoelectric crystals, and silicon membranes. The hospital environment is very physically demanding on equipment. Even though transducers are usually used only once (disposable), a good transducer must be very durable.

TransducerThe device that performs the conversion of one form of variable into another is called a transducer.

Two different principles are involved in the process of converting nonelectrical variables into electrical signals.

One of these is energy conversion: Transducers based on this principle is called active transducers.

Second of these is control of an excitation voltage or modulation of a carrier signal. Transducers based on this principle is called a passive transducers.

Magnetic Induction:

if an electrical conductor is moved in a magnetic field in such a way that the magnetic flux through the conductor is changed, a voltage is induced which is proportional to the rate of change of magnetic flux.

Basically two basic configuration are used using this concept: one is linear motion and the other is rotary motion.

The applications are heart sound microphones, pulse transducers and electromagnetic blood flow meters.

Figure Magnetic induction based transducer

The Piezo Electric EffectA non-conductive material get deformation/distortion due to applied pressure the change separation occur in the material and electric voltage can be measured across the material such as crystal of quartz Si02

The natural materials in which this piezoelectric effect can be observed are primarily slices from crystals of quartz (SiO2) or Rochelle salt (sodium potassium tartrate).

The piezoelectric process is reversible. If an electric field is applied to a slab/block of material that has piezoelectric properties, it changes its dimensions.

The piezoelectric effect principle is used in microphones for heart source or other acoustic signals from within the body.

We will see equivalent circuit of the piezoelectric transducer connected to an amplifier.

Fig: Equivalent ckt

The piezoelectric principal is also used in ultrasonic instruments.

Figure: Principle of Piezoelectric effect

The Thermoelectric effect If two wires of dissimilar metals (e.g., iron and copper) are connected so that they form a closed

conductive loop, a voltage can be observed at any point of interruption of the loop which is proportional to the difference in temperature between the two junctions between the metals.

The sensitivity of a thermocouple is small and amounts to only 40 microvolts per degree Celsius. (microV/oC) for copper-constantan.

The use of thermoelectric effect to convert from thermal to electrical energy is called the seeback effect. In reverse direction it is called the Peltier effect.(its used to cool parts of instruments. E.g. a microscopic stage)

The photoelectric Effect: The selenium cell, has been used to measure the intensity of light in photographic exposure meters or the light absorption of chemical solution.

The silicon photoelectric cell (solar cell), has a much higher efficiency than the selenium cell.

Passive Transducer

Passive transducers utilize the principle of controlling a dc excitation voltage or an Ac carrier signal.

There are only three passive circuit elements that can be utilized as a passive transducers. Resistors, capacitors, and Inductors.

Passive Transducers Using Resistive Elements: Special linear potentiometer can be used to convert displacement into a resistance change.

Strain Gauge Transducer

Unbounded strain gauge has a resistance wire stretched between two frames. The rigid pins of

the two frames are insulated. When the wire is stretched due to an applied force, there occurs a relative

motion between the two frames and thus a strain is produced, causing a change in resistance value. This

change

A bonded strain gauge will be either a wire type or a foil type as shown in the figure below. It is connected to a paper or a thick plastic film support. The measuring leads are soldered or welded to the gauge wire. The bonded strain gauge with the paper backing is connected to the elastic member whose strain is to be measured.