Formal Report Instrument Baby

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TITLE

Active Filters Signal Conditioning

INTRODUCTION

Digital Filters are used primarily when transfer-function requirements have no

counterpart in the analog world, or when a Digital Signal Processing (DSP) already resides on

the circuit board to perform other functions. The three most common types of filters are called

Butterworth, Chebyshev, and Bessel. Each type has unique characteristics that make it more

suitable for one application than another. All may be used for high-pass, low-pass, band-pass,

and band-reject applications, but they have different response profiles. They may be used in

passive or active filter networks. The Butterworth filter has a fairly flat response in the pass-band

for which it is intended and a steep attenuation rate. It works quite well for a step function, but

shows a non-linear phase response. Chebyshev filters have a steeper attenuation than

Butterworth, but develop some ripple in the pass band and ring with a step response. The phase

response is much more non-linear than the Butterworth. Finally, Bessel filters have the best step

response and phase linearity. But to be most useful, Bessel filters need to have a high order

(number of sections) to compensate for their slower rate of attenuation beyond the cut-off

frequency.

Signal conditioning modules, SCMs, used for measuring process control variables such

as temperature, pressure, strain, position, speed, level, etc. are always subject to externally

induced noise signals. Electrically and magnetically induced noise voltages/currents are

inevitable. Field sensors with output voltages in the millivolt range are certainly degraded by

induced noise levels on the order of volts. Consequently, signal conditioning modules must

provide filtering to eliminate induced noise components.



OBJECTIVE

1. Introduce a practical application of operational amplifier in signal conditioning

2. Give an exposure to student in practical circuits which capable of selectively filtering

range of frequencies out of a mix of different frequencies.

THEORY

Filter circuits are used in a wide variety of applications. In the field of communication,

band-pass filters are used in audio frequency range (0 KHz to 20 KHz) for modem and speech

processing. High-frequency band-pass filters are used for channel selection in telephone central

offices. Data acquisition systems usually require anti-aliasing low-pass filters as well as low-pass

noise filters in their preceding signal conditioning stages. A filter is a circuit that removes

selected frequencies from the signal. Filtering is most often performed to remove unwanted

signals and noise from the data. The most common form of filtering is low-pass filtering, which

limits the bandwidth of the data by eliminating signals and noise above the corner frequency of

the filter. The importance of low-pass filtering is apparent when measuring a signal that has

source of noise above the frequency range of the signals of interest.

High-pass filtering is required when the main source of noise is below the frequency

range of the signals of interests.



MATERIALS AND EQUIPMENT

1. Resistors ( 10k x 3, 1k x 1)

2. Capacitor 0.01 µF

3. Signal function generator

4. Oscilloscope

PROCEDURES

EXPERIMENT 1 LOW PASS FILTER

1. RC network was used and tested build an active low-pass filter. The circuit was

completed as follows.



2. The frequency response was measured and the resulting gain (Vout/Vin) was plotted as a

function of the operating frequency.

EXPERIMENT 2 HIGHPASS FILTER

1. An active high-pass filter was build. The high-pass filter is formed by interchanging the

resistor R and capacitor C in the low-pass filter that you made.

2. The frequency response was measured and the resulting gain (Vout/Vin) was plotted as a

function of the operating frequency.



EXPERIMENTAL RESULTS

(1) LOW PASS FILTER

Table 1 Low-pass filter gain versus frequency

Frequency

[kHz]

Vin

[V]

Vout

[V]

Gain

0.2 336 692 6.275

0.5 337 697 6.312

1 338 698 6.299

2 341 693 6.160

5 349 692 5.946

10 363 690 5.579

20 391 694 4.984

50 390 691 4.986

100 384 681 4.976

200 419 658 3.920

500 393 480 1.737

1000 384 267 -3.156

SAMPLE CALCULATION

For frequency = 0.2 kHz

= 6.275 [dB]



DATA ANALYSIS

1. Determine the cut off frequency from your graph and compare it to the theoretical value

if given that

From the graph the cut off frequency is 20 kHz and the theoretical value is 15.92kHz.

The value from experiment more higher compare the theoretical value. The error bar

regions of the theoretical and experimental result is close. Percentage error is between

experiment and theoretical value is 25.6%.

2. What is the pass band gain? Compare it to the theoretical gain, 20 log ( 1+Rf/R1)

Pass band filters allow transmission of a range of frequencies between a lower and upper

cutoff limit. Pass band filter transfer functions can be rearranged to function as ³notch´

filters, which eliminate frequencies between a lower and upper cutoff limit. From the

graph the pass band gain is 6.11 dB and the theoretical gain is 6.03 dB. The experiment

value more higher compare the theoretical value. Percentage error is between experiment

and theoretical value is 1.33%.

3. What is the roll-off rate (in dB/decade) in the stop band?

The roll off rate in the stop band is -12.5 dB/decade.



(2) HIGH PASS FILTER

Table 2 High-pass filter gain versus frequency

Frequency

[kHz]

Vin

[V]

Vout

[V]

Gain

0.2 338 79.2 -12.604

0.5 338 89.4 -11.552

1 337 104 -10.212

2 339 134 -8.062

5 347 232 -3.497

10 344 356 0.2978

20 349 522 3.497

50 348 621 5.0302

100 354 644 5.1977

200 366 628 4.6896

500 363 468 2.2068

1000 345 265 -2.2915

SAMPLE CALCULATION

For frequency = 0.2 kHz

= -12.604 [dB]



1. Determine the cut off frequency from the graph and compare it to the theoretical value if

given that

From the graph the cut off frequency is 20 kHz and the theoretical value is 15.92kHz.

The value from experiment more higher compare the theoretical value. The error bar

regions of the theoretical and experimental result is close. Percentage error is between

experiment and theoretical value is 25.6%.

2. What is the pass band gain? Compare it to the theoretical gain, 20 log (1 + RF/R1)

The pass-band for a high-pass digital filter is limited to the maximum bandwidth,

sampling rate, and word length that the filter order allows. Pass band filters allow

transmission of a range of frequencies between a lower and upper cutoff limit. Pass band

filter transfer functions can be rearranged to function as ³notch´ filters, which eliminate

frequencies between a lower and upper cutoff limit. From the graph the pass band gain is

4.73 dB and the theoretical gain is 6.03 dB. The theoretical value more higher comparethe experiment value. Percentage error different between experiment and theoretical

value is 21.56%.

3. What is the roll-off rate (in dB/decade) in the stop band?

The roll of rate in the stop band is 10.5 dB/decade which is smaller than the roll of rate

for the low pass filter. Above the cutoff frequency , attenuation rapidly decrease to

nothing and all higher frequency pass with case.



CONCLUSION

After doing this experiment, we can summarize that we had achieved the objectives.

From this experiment, we can also determine the pass band and roll of rate in the stop band. We

learn about low pass and high pass filter. An electrical low pass filter and higher pass filter was

tested with input sinusoidal signals of different frequencies. At frequencies below the break

frequency the input and output signals had essentially by the same amplitude. As the input

frequency was increased above the break frequency, the output signal amplitude began to

decrease significantly and vise versa for high pass value. The theoretical formula that predicts the

magnitude response gave results very similar to the experimental ones. This experiment was

successful in demonstrating the validity of the theoretical formula for low pass filter and higher

pass filter constructed from a single resistor and capacitor. We learn the general principles of manual and automated tuning procedures for active filter controller and application of amplifier

in signal conditioning. Then we learn the circuits which capable of selectively filtering range of

frequencies out of mix of different frequencies.



QUESTIONS

1. Explain briefly about low-pass filter and high-pass filter?

Low pass filters allow transmission of only a range of low frequencies from Direct Current to a

higher cutoff frequency. Low-Pass filtering is required in most industrial signal conditioning

modules to eliminate induced 60 cycle harmonics and randomly induced high frequency

transient noise. Low pass filter allows low frequencies to pass while blocking high frequency

signals. The low pass filter is constructed from only two passive components : a resistor and a

capacitor. A high-pass filter is an filter that passes high frequencies well but attenuates

frequencies lower than the cut off. The actual amount of attenuation for each frequency is a

design parameter of the filter.

2. State the application of low-pass filter and high-pass filter in engineering

Rumble filters are high pass filters applied to the removal of unwanted sounds below or near to

the lower end of the audible range. For example , noise( e.g. footsteps, or motor noises from

record players and tape desks) may be removed because they are undesired or may overload the

RIAA equalization circuit of the preamp. Low pass filter for eliminating induced industrial noise.

High-frequency band-pass filters are used for channel selection in telephone central offices. High

pass and low pass filters also used in digital image processing to perform transformation in the

spatial frequency domain. High pass filters are also used for AC coupling at the input and output

of amplifiers.



3. Sketch a schematic diagram for op-amplifiers, LM741 used in this experiment



APPENDIX

FIGURE TOOL NAME

1)

Power supply

2)

Signal function generator

3)

Oscilloscope



4)

Circuit

5)

Capacitor 0.01 µF

6)

Resistors ( 10k x 3, 1k x 1)

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Formal Report Instrument Baby