EXAMPLE 2 – PHOTODIODE A photodiode is a semiconductor device that converts light into current. The current is generated when photons are absorbed in the photodiode As for LED – Lights are emitted as a result of electrons recombine with holes. Circuit symbol for photodiode Operation: A photon (light) of sufficient energy strikes the diode, it excites electrons – creating an electron-hole pairs. If this process occurs near the depletion region, the electrons will be swept towards the negative terminal producing current flow Photodiode is connected similar to reverse-biasing of a diode

I-V Characteristic of Photodiode Dark current refers to the leakage current produced by the diode although there is no light Photoconductive mode Diode is reverse biased Photovoltaic mode; no biased

The detector is reverse biased to produce a linear response to the applied input light. The amount of photocurrent generated is based upon the incident light and wavelength and can be viewed on an oscilloscope by attaching a load resistance on the output. The function of the RC filter is to filter any high-frequency noise from the input supply that may contribute to a noisy output. Application of Photodiode

Basic Transimpedance Amplifier V out = - I x R F (for low frequency signal - DC Also known as current to voltage converter – used when current has a more linear response compared to the voltage The feedback capacitor, C f is used to improve the stability of the circuit.

Analogue and Digital Sensors Analogue Sensors produce a continuous output signal or voltage which is generally proportional to the quantity being measured. Examples: Temperature, Speed, Pressure, Displacement, Strain Characteristic: Continuous in nature For example, the temperature of a liquid can be measured using a thermometer or thermocouple which continuously responds to temperature changes as the liquid is heated up or cooled down. Very small signal (µV to mV) range Amplification Analogue to Digital conversions

Digital Sensors produce a discrete digital output signals or voltages that are a digital representation of the quantity being measured. Digital sensors produce a Binary output signal in the form of a logic “1” or a logic “0”, (“ON” or “OFF”). For example, digital temperature sensor, DS1620 provides temperature of device with 9-bit temperature readings. thermostat with its three thermal alarm outputs. temperature of device is greater than or equal to user defined temperature, T H then T HIGH is driven high. temperature of the device is less than or equal to user defined temperature, T L then the T LOW is driven high. If the temperature of the device exceeds T H and remains high until it falls below that of T L, then the T COM is driven high.

SIGNAL CONDITIONING Noisy

Key Functions of Signal Conditioning:  Amplification  Filter  Isolation  Attenuation  Linearization

AMPLIFICATION Many sensors develop extremely low-level output signals Two common examples of low-level sensors are thermocouples and strain-gauge (less than 50 mV) Requires amplification Using Operational Amplifier (Op-Amp) Inverting Amplifier Non-Inverting Amplifier Differential Amplifier Instrumentation Amplifier

NOTE: The maximum input signal that the amplifier can handle without damage is usually about 2 V less than the supply voltage. For example, when the supply is ±15 V DC, the input signal should not exceed ±13 V DC. INVERTING AND NON-INVERTING AMPLIFIERS

Gain = -10 Vo = - 5 V Gain = 11 Vo = 5.5 V

DIFFERENTIAL AMPLIFIERS Differential-input amplifiers offer some advantages over inverting and non- inverting amplifiers. The output signal of the differential input amplifier responds only to the differential voltage that exists between the two input terminals. Example to use differential amplifier is when thermocouple is used. In this figure, the gain is unity. But, we can make it the gain to be equal to 10 by making R f to be 10 times larger than R i But both R f must be equal and both R i must also be equal

For a gain of 10, where: R f = 100 kΩ and R i = 10 kΩ i.Prove that the output equation, V o = 10 (V 1 – V 2 ) ii.What is the output voltage if the voltage difference is 50 mV

INSTRUMENTATION AMPLIFIERS ideal for measuring low-level signals in noisy environments without error, and amplifying small signals the gain increases even more than just using differential amplifier Using a potentiometer can vary the gain VAVA VBVB = R 3 / R 2 ( V B – V A ) DIFFERENTIAL AMPLIFIER I I I V out = (V 2 - V 1 ) [2(R 1 / R gain ) + 1 )](R 3 / R 2 )

FILTERING The three most common types of filters are called Butterworth fairly flat response in the pass-band a steep attenuation rate, a non-linear phase response Chebyshev steeper rate of attenuation, develop some ripple in the pass band. The phase response is much more non-linear than the Butterworth. Bessel have the best step response and phase linearity. But requires a number of stages or orders

BESSEL BUTTERWORTH CHEBYSHEV Sharper frequency cutoff Better step response in time domain, less ripple Step response in time domain shows the characteristic of the system for the output to go from 0 – 1 at the point of the cutoff frequency

LOW-PASS FILTER A low-pass filter allows for easy passage of low-frequency signals from source to load, and difficult passage of high-frequency signals. The cutoff frequency for a low-pass filter is that frequency at which the output (load) voltage equals 70.7% of the input (source) voltage. Above the cutoff frequency, the output voltage is lower than 70.7% of the input, and vice versa. First order low pass filter

HIGH-PASS FILTER A high-pass filter allows for easy passage of high-frequency signals from source to load, and difficult passage of low-frequency signals. The cutoff frequency for a high-pass filter is that frequency at which the output (load) voltage equals 70.7% of the input (source) voltage. Above the cutoff frequency, the output voltage is greater than 70.7% of the input, and vice versa. First order high pass filter

BAND-PASS FILTER By connecting or “cascading” together a single Low Pass Filter circuit with a High Pass Filter circuit, we can produce another type of passive RC filter that passes a selected range or “band” of frequencies that can be either narrow or wide while attenuating all those outside of this range. known commonly as a Band Pass Filter To set the first band pass frequency To set the second band pass frequency

EXAMPLE 1 A second-order band pass filter is to be constructed using RC components that will only allow a range of frequencies to pass above 1kHz (1,000Hz) and below 30kHz (30,000Hz). Assuming that both the resistors have values of 10kΩ´s, calculate the values of the two capacitors required Answers: C 1 = 530 pF C 2 = 15.8 nF

BAND-STOP FILTER combine the low and high pass filter to produce another kind of RC filter network that can block or at least severely attenuate a band of frequencies within these two cut-off frequency points.

Band-pass filters are constructed by combining a low pass filter in series with a high pass filter Band stop filters are created by combining together the low pass and high pass filter sections in a “parallel” type configuration as shown. Cutoff frequency is 200Hz Cutoff frequency is 800Hz

The band stop filter can be configured using 2 op-amps as non-inverting amplifiers (voltage followers) and their outputs are then connected using the summing amplifier configuration QUESTION: Can this configuration be used to create a band-pass filter instead?