Biomedical Sensors

Parte 1 - Biomedical sensors

Code course: 39168-ENG

Crediti formativi: 6 CFU (Corso Integrato)

Docenti: Valerio Re valerio.re@unibg.it Massimo Manghisoni massimo.manghisoni@unibg.it Matteo Pezzoli matteo.pezzoli@unibg.it

Materiale del corso: Moodle course - “Biomedical sensors, smart sensors and electronic system” PW: bis2122

On Teams Channel in section “File”

Modalità d’esame: Oral exam, covering the subjects discussed during the course.

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Introduction

Sensors and microelectronic systems, also known as electronic technologies, are considered as the key technology of our modern life. They can be recognized as enablers of novel application, impacting on our society. It is possible to assume that this kind of technology will become more and more important during the years thanks to their progression. It is known that different fields of our life are effected by sensors.

Thanks to the use of this kind of technologies, different fields, starting from healthcare to industrial production, are being revolutionized by interconnected microelectronic systems. This one are able to acquire and process all the data that are reached by the sensors. It can be possible summary this kind of market in four fields, which are:

• Smart industries: Presence of automated and intelligent industrial processing for the definition of final products thanks to sensors and electronic systems (articulated arms, robots, automatics probes).
• Smart thing: Considering wearable, like smart watch, or not wearable, like phone, device, which are able to monitor different kind of parameters of the person, like heart rate, blood pressure, oxygen content. Smart thing s are able to monitor both people and environment in which people are living. A benefit of this kind of technology is that all of them can communicate each other, sending information to cloud or archive system.
• Smart home & city: Presence of sensors, whose role is to control the house, like anti-theft sensors, or also optimize the consume of the energy and water.
• Smart driving: Vehicles are full of sensors which permit to point out all the possible problems that the vehicle could have, giving an advertisement to the driver, like lights on the ignition dial or parking sensors. Nowadays it is working on self-driving vehicles.

During the years there has been an evolution trends of electronic technologies, which had started in 1980 and it continues to improved, decreasing the sizes and increasing the number of processors.

At first there was the Moore theory in which object were use for processing, going to define the information age; these era was characterized by the presence of personal computer and laptops. Nowadays we are living in the More that Moore Theory, which is something that goes over the Moore first law. Also known as interaction age, it is based on the concept of sensing due to the combinations of different technologies for data processing device. After this phase it has been predicted the Beyond Moore Theory, which will be considered as the enhancement age. This era will be characterized from some hypothesis that we don0t know if will became true, like telekinesis and robotic servants.

Enabler: abilitatore; 1 Field: campo, settore; 2 Point out: segnalare; 3 Light on the ignition dial: spie sul quadrante di accensione; 4

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Architecture and applications

The kind of architecture that we will analyzed during the course are composed by integrated sensors and microelectronic systems. Considering the image on the right, it stats from sensing front end, which is the part that interface with the real world (microphone, antenna, gyroscope, radio signals). After the acquisition of the real signal by the sensor, this one convert it into digital form, expressing the real input in voltage or current units.

This is called signal processing. When this data is started we can store the data in electronic device or send them in a remote site which work on specific protocols. Different kind of transmission to go to another electrical device called gateway, which collect information coming from different devices in the same local environment. After that the information goes to cloud where data can be store and analyzed.

The possible applications of kind of architecture can be on the four fields that have been defined before. For the industries is the idea is to monitor some industrial machineries to see if there could be some anomalous behavior that require maintenance actions before the onset of problematics thanks to the use of sensors which can detect some anomalous behaves that cannot detected by human. All the data that have been collected have been send to electronic system, which can process them and then offer some possible solutions to limit the damage.

As it has been seen before, the application of sensors on person can be useful for monitoring they health conditions. In this case it is possible to talk about wearable sensors which can be use for detect the conditions of both person and environment. An important thing is that the sensors can augment the person’s perception, so they do not increase only the five senses. This special feature increases the possibility to define an augmented human.

It has also seen how the new digital technologies are having a large impact on buildings and infrastructures. It is possible to control the performance of the structures using sensors and algorithms. These make possible the monitoring of aging and degradation, going to plan ordinary and extraordinary maintenance. So protecting and monitoring the structure of building consists in:

• Use MEMS sensors for pressure and stability real-time monitoring;
• Consideration of chemical sensors for pH monitoring;
• Analysis of energy harvesting from structural vibrations for data transfer;
• Big data analysis with massive numbers aggregation for better understanding and structure collapse prevention.

Onset: insorgenza; 5 energy harvesting: raccolta di energia 6

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Operational amplifier and Laplace transform

An operational amplifier can be defined as a differential amplifier, which structure is generally characterized by the presence of two inputs and one output. In microelectronic system, the main analog signal processing function are performed by circuits based on operational amplifiers, especially in negative feedback configurations. It is possible to talk about two different function:

• The amplification of small signals generated by sensors;
• The attenuation/cancellation of disturbances and interferences with filters.

Schematic image of an operational amplifier.

As we can see in the image above, the operational amplifier is composed by two inputs; these two are named non inverting input (V+) and inverting input (V-) in which it is possible to applied two voltage signals. With regard the output signal, it is seen which is only one and it can be named as V OUT. Making mathematical consideration it is possible to say that:

VOUT = A ⋅ (V+ − V−). VOUT is the multiplication between the gain (A) and the difference value between the non-inverting and inverting input [1] [1] Equations of the value of V OUT.

Consideration about the current (I). At first it can be seen two current: I+ and I-, which the first is related to the non-inverted voltage while the second to the inverted voltage. I+ = I- = 0.

The basic mechanics of operational amplifiers considered the value of the current equal to zero this because on of them is linked to the ground. This can be considered as an ideal condition, because it is possible to have some amount of current but its value is really small. [2] Consideration of the value of the current.

The majority of the operational amplifiers is designed to work with a dual supply voltage, which role is to fed it to improve its work. It is possible to talk about the positive supply voltage (+VDD) and the negative supply voltage (-VSS), which are symmetrical to the ground. The two supply voltages need not necessarily have the same value and it is a characteristics of the modern CMOS microelectronic systems. The versatility of these devices is such that there may be applications where the negative voltage can be set to zero, that is, the component is powered by a single voltage. In dual power supply, the output signal level can range between the two supply voltage values at less than a small margin, which can vary depending on the type of op amp adopted.

Operational amplifier with supply voltages.

CMOS: kind of technology used in digital electronics for the definition of integrated circuits, based on the use of the MOSFET transistor inverter. 7

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Negative feedback and operational amplifiers

So, after this account, we can said that, considering the circuit, it is possible making some consideration.

• I+ = I- = 0
• In the equations [1] representing the relation of VOUT, it has spoken about the gain (A). This one is a very large number, which in the ideal operational amplifiers corresponds to an infinite value.

Kind of circuit: VOUT = A ⋅ (V+ − V−); (V+ − V−) = VOUT / A; considering A → ∞, it is possible to say that (V+ − V−) = 0; V+ = V−.

The yellow arrows represent the currents which pass through the resistances (IR1, IR2).

This circuits represents the inverting amplifier, which uses the inverted input of the operational amplifier as the main input while the non inverted input is grounded. So due to this characteristics it is possible to make some analysis:

• V+ = V− = 0, this because it is connected to the ground;
• IR1 = IR2, this is said considering the Kirchoff’s law on knots.

Considering also the Ohm’ laws it is possible write the same thing in a different way:

VIN / R1 = -VOUT / R2. So from this equation, it is know that: VOUT = - R2 / R1 VIN.

A circuits which presents a transresistance amplifier is used to convert a current signal into a voltage signal. A possible application is for the processing of signals from photodiode, that is a junction did in a reverse region, where the reverse current is proportional to the intensity of the radiation to which the did is exposed.

As it has said before, it is possible to talk also about non-inverting amplifiers, which uses the non-inverted input of the operational amplifiers as the main input while the inverted input is grounded.

Also in this case it is possible to make some considerations.

Positive feedback and operational amplifier.

The equivalence between the currents and the voltages in the operational amplifier, as it has been seen before, can be written as:

• I R1 = I R2; VIN / R1 = (VIN − VOUT) / R2.
• This brings to: VOUT = (1 + R2 / R1) VIN.

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Voltage buffer and differential amplifiers

A variant of non-inverting amplifier is the voltage buffer. Considering the image alongside, the circuit presents a connection from the negative input to the output. Considering an idea amplifier with infinite gain A and the ohm’s and Kirchhoff’s laws, it is possible make some mathematical consideration: V+ = Vin, V- = Vout → Vin = Vout.

Voltage Buffer.

We can say that there is a unity going circuit (A=1) that works as an impedance adapter; it is placed between a circuit that has an high output impedance and a circuit that has a low input impedance. This kind of structure t is common used some application where it is not necessary have a change between an input and output signal, like when we have to drive a load. So this circuit gives the opportunity to don’t have effect.

Circuits that present differential amplifiers are really important due to their function. In many cases, when we have to acquire information from sensors, it is necessary making a measure in a differential way like analyzed the difference value between a quantity that we are detecting and its reference value i.g. measure of temperature.

The sensor provides two inputs: one is related to the reference quantity and the second one to the quantity that has change. In order to measure this difference, it can be possible use this kind of amplifier, that will give an output signal proportional to the difference of the inputs signals with a gain factor.

Differential amplifier.

Considering the Ohm’s and Kirchhoff’s law, it is possible make some considerations. If R1 = R3 and R2 = R4, it is possible to say that Vout = R2 / R1 (V1 − V2).

It can be also possible to talk about the instrumentation amplifier, which is an high-performance version of this simple differential amplifier thanks to the presence of three differential amplifiers. i.e. measurement of the ECG signal.

The logic conforms that ideal and real operational amplifiers have a different behavior. We have seen that ideal condition supposes that everything is perfect but this terms cannot be applied in real world. i.e. The gain (A) in real amplifier cannot be equal to an infinite value because it depends on frequency of the signal applied on circuit. Another i.e. can be the value of the current: in ideal case it can be equal to zero but in reality is has always a small value. Also the CMRR value: in ideal operational amplifiers the output has given by the gain multiple by the difference between V+ and V-. According to this we aspect that Vout will be equal to zero but in reality internal circuits works even if the two input voltage have the same value, defining a Vout’s value near to zero.

CMRR: parameter used to evaluate the performance od a circuit when it has to amplify the difference between two signals. 8

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CMRR and common mode rejection ratio

It is possible deepen the CMRR concept. Also know as a Common Mode Rejection Ratio, it is a parameter which defines how well te operational amplifier approximates its ideal behavior as an high-gain differential amplifier.

In mathematical way it can be written as: CMRR = |Ad / ACM|.

Where: Ad = vout / vin, which represents differential gain in which: vin+ = v / 2, vin− = -v / 2, with: vin+ − vin− = vin.

While: ACM = vout / vin, which represents the common-mode gain where: vin+ = vin− = vin.

Remember: the differential gain gives informations about the capability of the circuit to extend the difference between two inputs voltage signals. Instead, the common mode gain can be calculated when the voltage input has the same value or the same sign of the voltage output. In a good operational amplifier, the CMRR’s value can be around 80-100dB. In addition it is possible to say that CMRR’s value can’t be equal to zero: this because it is impossibile that two resistors can have the same value due to factory feature.

Charge sensitive amplifier

The circuit that has been reported on the side can be considered similar to the trans-resistance amplifier. This because there is a presence of a negative feedback operational amplifier but the feedback element is a capacitor instead a resistor. In this case we have to considered the relationship between the voltage current with the capacitor. Considering the Kirchhoff’s law, it is possible assume that the output voltage is related to the input voltage. This one, before arriving to the output section passes through the capacitor and ideally the input currents, which flow in the capacitor, have to be equal to the current that is absorbed to the generator that goes out of the node.

iIN = C d Vout / dt → d Vout / dt = iin / C.

Considering the circuit and the mathematical relation, it is possible to say that: So, this electric charge gives rise to a current signal, whose integral corresponds to the total charge generated in the sensor.

Considerino the integration current signals, it is possible writing: Vout(t) = 1 / C ∫0t iin(x) dx.

So it is possible to say that output voltage signal has to be proportional to the integration of the input signal.

In this kind of circuit, a voltage signal is generated at the output, whose amplitude is proportional to the charge generated in the sensors. In some possible applications, like in radiation sensors, the relevant information is associated with the amount of electric charge generated in the sensor, which is proportionate to the intensity of the radiation to which the sensors itself has been exposed. The electric charge gives a measure of the intensity of radiation that hits the sensor. So due to the physical properties of the semiconductor material, whose the sensor is made by, when the radiation hits the sensors, it releases energy and this generates an electric charge, which will give a current signal. We are talking about a charge sensitive amplifier where the integral of the current correspond to the duration of the current itself, that will be equal to the total charge delivered by the sensors when it is expose to radiation.

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In a real contest, the structure is not achievable due to the non ideal behavior of the operational amplifier, especially for the value of the current that we have already seen that can’t be equal to zero. This because even if we considered o t