18~24 MARCH 2013 (Week 9)

Tittle of Activity

- Pulse Oximeter .
-Transimpedence  Amplifier.

Objectives

-To make sure to understand the concept of pulse oximeter and the Transimpedence as well .

Content/Procedure

-Pulse oximeter

-The first thing that you need to remember is that if you are going to simply use a premade oximeter and just read the voltages coming out of it .

-The main things you need to remember is that you are looking at two diffrent output voltages .One from a red led and another from an infrared LED.The basic principle of the whole thing is sort of like what you probably did as a kid , taking a flashlight, holding it yo your hand at night and seeing your bones.As you can see at the figure below the two different LEDs will have dufferent voltage levels depending upon the saturation of the hemoglobin with O2.

-For more information about the pulse oximetry you can check out wikipedia website as a reference .

For getting to the LabVIEW
-As you can see at the above figure we are actually looking at two voltages.That means you will simply need to have two separate analog channels measuring the voltages coming from the diffrent LEDs .The pulse will be easy to see because the voltages will be going up and down easily showing the systole and diastole of the heart . You should be able to plot one of the two lines on a waveform graph and see the heartbeat (but we should keep in mind that pulse oximetry is very sensitive to noise artifact).The oxygen Saturation is alittle more difficult to measure and may require some custom scaling. However, what the difference is and how exactly that relates to a percentage. if you get a percentage of less than about 90% then you may want to head straight to the nearest hospital or stop smoking .

Pulse oximetry is a particularly convenient noninvasive measurement method. Typically it utilizes a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip . One LED is red, with wavelength of 660 nm, and the other is infrared, 905, 910, or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form; therefore, the oxy/deoxyhemoglobin ratio can be calculated from the ratio of the absorption of the red and infrared light. The absorbance of oxyhemoglobin and deoxyhemoglobin is the same (isosbestic point) for the wavelengths of 590 and 805 nm; earlier equipment used these wavelengths for correction of hemoglobin concentration.

The monitored signal fluctuates in time with the heart beat because the arterial blood vessels expand and contract with each heartbeat. By examining only the varying part of the absorption spectrum (essentially, subtracting minimum absorption from peak absorption), a monitor can ignore other tissues or nail polish, (though black nail polish tends to distort readings). and discern only the absorption caused by arterial blood. Thus, detecting a pulse is essential to the operation of a pulse oximeter and it will not function if there is none .


- Transimpedence  Amplifier.

-Most engineers know that to design a transimpedance amplifier circuit, they just need a large-enough resistor to convert the input current to a reasonable output voltage range. To stabilize this circuit, a large enough capacitor must be placed in parallel with the feedback resistor. This article will show how to calculate the value for the feedback capacitor to ensure that the design has the largest possible bandwidth, and will still be stable.

-Calculating the feedback factor for an op amp that is set up for current-to-voltage conversion may be a bit of a mystery to many engineers. By deriving the transfer function for a transimpedance amplifier and using a voltage amplifier op amp, the conversion will be easy to understand. Here we use the LMV793 op amp as an example for a transimpedance amplifier design. A basic transimpedance amplifier configuration is shown in Figure 1.

-The figure shows the complete transimpedance amplifier; only the power supply decoupling capacitors are not shown. In most cases, the selection of the photodiode will allow the designer to use the same supply for V_BIAS and +V. Using split supplies keeps the inverting input of the op amp at virtual ground. To derive the feedback factor, it is necessary to examine the equivalent circuit of the photodiode, Figure 2 :

-The diode is an ideal diode in the equivalent circuit. Since the photodiode must be back-biased for proper operation, the ideal diode is not included in the feedback factor calculations. C_J is the capacitance that occurs from the depletion region of the diode and it is included in the photodiode specifications. I_PH is the current that occurs from the photodiode action. The impedance of the current source is the series resistance of the photo diode, shown as R_SH. The series resistance is at least 10MΩ and typically much higher, usually over 100MΩ.

-The feedback factor is simply what is fed back to the input from the output of the op amp. This is calculated by assuming that the node at the input to the op amp is not connected to the input of the op amp, then calculating the ratio of the input voltage to the output voltage, V_IN/V_OUT. Figure below shows the circuit used to calculate the feedback factor.