ME 365 EXPERIMENT 7
SIGNAL CONDITIONING AND LOADING
Objectives:
To familiarize the student with the concepts of signal conditioning. At the end of the lab, the student should be able to:
Understand the basic characteristics of op-amps: what they do and their limitations.
Construct simple active and passive filters, and determine their frequency response characteristics.
Explain how loading occurs and how to minimize its effects.
Understand the information contained in a Bode plot, and how it relates to the measurements that are being made.
Introduction:
In making measurements it is often necessary to modify the signal obtained from a transducer before it is displayed or stored. This modification is often referred to as signal conditioning and consists of changing the signal amplitude and/or frequency content.
This is usually done with amplifiers and filters.
The amplifiers used for signal conditioning, often called instrumentation amplifiers, primarily serve the following functions by providing:
(1) Gain: Typically from 1 to 1000.
(2) High input impedances: typically around 10
8
.
(3) Low output impedance: typically .1 (output current from 10 to 100 mA).
(4) High common mode rejection ratio: typically 100 dB.
There are many other important specifications which will not be discussed here.
Modification of the frequency content of a signal is done with a filter. Filters can be divided into four general classes in which their names are indicative of their function.
There are low pass, high pass, band pass and band stop (notch) filters. Ideally, frequencies in the pass band would not be affected by the filter while those outside the pass band are eliminated. Filters can also be classified as active or passive. Active filters contain “active” elements such as operational amplifiers while passive filters consist of capacitors, resistors, and/or inductors. Depending on circuit configuration and the selection of component values, the form of frequency response function for the filter can be significantly changed.

Laboratory 7 Signal Conditioning and Loading ME365
In this experiment you will look at some of the characteristics of amplifiers and filters.
Amplifiers studied in this experiment will be operational amplifiers rather than instrumentation amplifiers. Some op-amps are often used in instrumentation, however, their performance will be much poorer than that of an instrumentation amplifier, which combines several op-amps to reduce noise and improve performance.
Equipment:
1.
Active Filter patch board
2.
Function generator
3.
Oscilloscope
4.
Digital multimeter (DMM)
5.
Filter patch board
6.
Resistors (2-1kΩ, 3-10kΩ, 1-100kΩ, 2-1MΩ)
7.
Capacitors (2-.1μf, 1μf)
Reference:
The class notes chapter on Filters, Loading and Op-Amps
NOTE: Throughout this experiment measured values of resistances should be used when determining the “theoretical” gain of an Op-Amp circuit. The gain of the opamp circuit is the absolute value of
A. CHECKING OUT THE OP-AMP
The op-amp is an active device and therefore requires external power, in this case provided by the 15 V Op-Amp power supply. Turn on the power supply to the active filter patch board. In the remainder of this lab reference to the amplifier refers to the Op-Amp and its associated circuitry.
1.
Hook up the circuit below. Connect the amplifier input to ground and measure the output voltage using the DMM.
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Laboratory 7 Signal Conditioning and Loading ME365
R
2
R
1
+
V in
Figure 1: Simple amplifier. R
1
10k and R
2
100k
B. USE OF AN OP-AMP AS AN AMPLIFIER.
1. Connect the oscillator to the amplifier in the circuit shown above.
2. Set the oscillator output to a square wave of very low frequency of 2 volts
peak-peak ( 1 Volt). (This is to simulate a DC signal.) Use the DMM to
measure the input and the output. Make sure that the offset on the oscillator is
zero.
I. What is the theoretical gain and how does it compare with the measured
gain?
What is the input impedance of this amplifier*?”
C. SATURATION IN OP-AMPS
Switch the oscillator to a sinusoidal output and set its frequency to 100 Hz.
Connect the input and the output to channel 1 and channel 2 of the oscilloscope, respectively. Increase the amplitude of the input until saturation of the output is observed.
II. At what output voltage does the amplifier saturate in the positive and the
negative directions?
Sketch the output voltage waveform just after it starts to saturate.
Continue to increase the input to its maximum value.
________________
* Determine the input impedance theoretically – do not measure it.
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Laboratory 7 Signal Conditioning and Loading ME365
III. What does the output of the amplifier look like?
D. INTRODUCTION TO LOW PASS FILTERS
1. Set up the circuit shown below:
R
2
R
1
C
+
V o
(active filter)
Figure 2: Active low pass filter circuit. R
1
10k , R
2
10k and C 0.1 f
2.
Set the oscillator to a 1 V peak-peak sine wave (±0.5 V). Observe the waveform of the input and output on the oscilloscope as the input frequency is varied over the range from 1 Hz to 1000 Hz. Repeat observing the
Lissajous figure.
3.
Now run Bode_Manual.vi so that you can generate and print out the frequency response (magnitude and phase) of the circuit between 1 and 1000
Hz.
4.
Construct a passive low pass filter as shown below:
R
3
V o
(passive
C
2 filter)
~
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Laboratory 7 Signal Conditioning and Loading ME365
Figure 3: Passive low pass filter circuit. R
3
10k and C
2
0.1 f
Apply a 1 V peak to peak sine wave (±0.5 V) to the passive filter and determine the frequency response (magnitude and phase) between the input and output of the passive filter by using Bode_Manual.vi, in the frequency range from 1 to 1000
Hz.
IV. Derive expressions for the frequency response functions for both filters.
Compare the theoretical and measured frequency response functions.
What are the theoretical and experimental cut-off frequencies (where
magnitude is down by 3 dB)?
Well above the cut-off frequencies, at what rate (dB/decade) does the
output fall off?
E.
LOADING EFFECTS ON PASSIVE VS. ACTIVE ELEMENT FILTERS
For both the active and passive filters, connect a 1kΩ resistance between the output and ground (to simulate an added load).
Again run Bode_Manual.vi to generate the frequency response functions of these loaded circuits.
V. What is observed when the load resistances are added? Explain.
In Figure 3, if R and C were 1k and 1 f respectively would the theoretical frequency response of the filter be different?
VI. Explain.
F.
CASCADED FILTERS
Let one passive low pass filter formed with R = 10 kΩ and C = .1 μf be called stage A and another passive filter formed with R = 1 kΩ and C = 1 μf be called stage B.
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Laboratory 7 Signal Conditioning and Loading ME365
Calculate the frequency response of a two stage filter (stage A followed by stage
B) assuming that the filters do not interact (i.e., ignore loading effects). Calculate frequency response magnitudes for 100 Hz, 160 Hz, and 200 Hz. Connect the two filters in series with stage A before stage B and obtain measured frequency response plots by using Bode_Manual.vi. Now reverse the order of the two stages and repeat.
VII. Compare the calculated magnitudes of the measured frequency response
plots for the two filter configurations above.
Which filter is closest to the theoretical “no-loading” frequency response?
Explain why the theoretical results differ from the experimental results.
G.
ACTIVE ELEMENT ISOLATION IN CASCADED FILTERS
Set up the circuit shown below:
1MΩ
1kΩ 1MΩ 10kΩ
.1μf
1μf
~
Figure 4: Active circuit used to reduce loading effects
Obtain frequency response plots by using Bode_Manual.vi.
VIII. Compare this frequency response to those obtained in part F. Explain
results.
When well above the cut-off frequency, at what rate (dB/decade) does the
frequency response magnitude fall off?
Using the above circuit, set the input to a square wave at the cut-off frequency.
Observe the output.
IX. What does the output look like? Explain the result.
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Laboratory 7 Signal Conditioning and Loading ME365
H.
HIGH PASS FILTERS
1.
Hook up the circuit shown below:
1 f
V o
Figure 5: Passive high pass filter
2.
Observe the output as a sine wave input of different frequencies is applied to the input of the filter. Experimentally determine the cut-off frequency
(where magnitude is -3 dB of its value at high frequencies).
3.
Run Bode_Manual.vi to obtain the frequency response of this high pass filter. On the printout, mark the cut-off frequency on both the magnitude and phase plots.
X. What is observed in the output as the frequency of the input is changed?
What is the cut-off frequency of the filter in rads/s and in Hz?
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