Download band-pass filter

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
Photovoltaic mode; no biased
Photoconductive
mode
Diode is reverse
biased
Application of Photodiode
• 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.
Basic Transimpedance Amplifier
Also known as current to voltage converter – used when current has a more
linear response compared to the voltage
The feedback capacitor, Cf is used to improve the stability of the circuit.
Vout = - I x RF
(for low frequency signal - DC
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, TH then THIGH is driven high.
•
temperature of the device is less than or equal to user defined
temperature, TL then the TLOW is driven high.
•
If the temperature of the device exceeds TH and remains high until it
falls below that of TL, then the TCOM 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
INVERTING AND NON-INVERTING AMPLIFIERS
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 VDC, the input signal should not exceed ±13 VDC.
Gain = -10
Vo = - 5 V
Gain = 11
Vo = 5.5 V
DIFFERENTIAL AMPLIFIERS
• Differential-input amplifiers offer some advantages over inverting and noninverting 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 Rf to be 10
times larger than Ri
But both Rf must be equal and
both Ri must also be equal
For a gain of 10, where:
Rf = 100 kΩ and Ri = 10 kΩ
i. Prove that the output equation, Vo = 10 (V1 – V2)
ii. What is the output voltage if the voltage difference is 50 mV
INSTRUMENTATION AMPLIFIERS
VA
I
= R3 / R2 ( VB – VA)
I
Vout = (V2 - V1) [2(R1 / Rgain ) + 1 )](R3 / R2)
I
VB
DIFFERENTIAL AMPLIFIER
• 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
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
Better step
response in
time domain,
less ripple
BUTTERWORTH
Sharper
frequency
cutoff
CHEBYSHEV
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:
C1 = 530 pF
C2 = 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?