TAs: Deniz Aktas, Emine Bardakci

Experiment #3 Filters and Amplifiers

Objective:

This lab introduces students to electrical filters and amplifiers by using automated systems board with NI ELVIS III. Then, students can build filtering and amplifier systems on the protoboard, figure out input-output relations of such systems with active electronic elements.

1. Theory Overview

Filter circuits play an important role in many electronics designs. They are primarily used to pass desired signals while blocking unwanted signals, based on their frequency characteristics. Filters behave differently with signals of different frequencies and can affect the different components of our signal. For example, if our measurement is made up of a 10mV 1kHz signal wave combined with a 5mV 60Hz noise component, a filter could attenuate only one of those components and leave the other.

A filter can be designed to implement the behavior that we want, and we will see later how these designs can be enacted.

The first property of a filter that we will discuss:

The cutoff frequency: the frequency at which the filter begins having a noticeable attenuation effect.

Ideal Filters

In an ideal filter, frequencies past the desired cutoff frequency would be completely attenuated and desired frequencies wouldn’t have any attenuation. You can see an example of this in the image below.

Figure 1. An ideal filter

High Pass Filter

A high pass filter passes signals of a higher frequency than the cutoff and attenuates signals lower than the cutoff frequency. Some of the most common uses of a high pass filter include:

Blocking DC from circuits sensitive to non-zero voltages
Blocking DC from circuits sensitive to radio frequencies

To calculate the cutoff frequency of a high pass filter for an RC circuit, the following formula should be applied:

This is an example of a high pass filter within a circuit:

Figure 2. High Pass Filter Circuit Diagram

Low Pass Filter

A low pass filter passes signals of a lower frequency than the cutoff and attenuates signals with a higher frequency. It is important to note that low pass filters are complementary to high pass filters. Low pass filters come in many forms such as:

Anti-alias filters
Digital filters
Acoustic barriers

To calculate the cutoff frequency of a low pass divider, the following formula should be used:

An example of a low-pass RC filter is below:

Figure 3. Low Pass Filter Circuit Diagram

Introduction: Amplifiers

Whether it is your mobile phone, computer speakers or you fit bit, amplifiers can be found in almost every electronic device. When it comes to capturing a signal emitted from a sensor, amplifiers are especially important.

Changes in a physical phenomenon sometimes occur at such a minute level. While sensors are able to detect those changes, their corresponding emitted signal has a very low amplitude (see figure below).

.

Figure 4. The above is a signal emitted from a strain gage fitted onto a bridge. The signal amplitude changes on the order of millivolts.

Such amplitude changes can not be captured accurately. Computer limitations can easily downgrade this signal by the time it is acquired. Below is the same signal above but after it was converted into a digital signal and read into the computer (see the figure below).

Figure 5. The above is the signal from the strain gage after it was digitally converted and read into a computer.

Amplifiers increase the power of the signal directly from its sensor, and certain configuration can minimize common-mode noise. Once a signal is amplified and its characteristics preserved, small changes in the signal are magnified to a scale large enough to be captured by an ADC (Analog to Digital Converter) into a computer for analysis.

In the following, you will experiment with various amplifiers. Your Automated Measurements Board is equipped with multiple types of amplifiers. Let’s experiment with them one by one. But before that, let’s take a look at Op Amps (Operational Amplifiers).

Operational Amplifiers

Operational Amplifiers are the building blocks for not only amplifiers but also for many other electronic applications.

When provided a voltage power supply, Op Amps take a differential input voltage, magnify its value by multiplying it by a certain value and returns it through an output. The value by which the voltage is amplified is called the open-loop gain.

Figure 6. These are the symbols used to signify Op Amps in circuit diagrams.

V₊ is the non-inverting input, V_- is the inverting input, V_S+,and V_S- are the positive and negative power supplies, and V_out is the output.

Inverting Amplifier

The Open Loop Gain of an Op-Amp can be very high. The problem with very high gain is that, in practice, the amplifier is both unstable and hard to control. This is because small input signals in the order of micro Volts causes the output voltage to saturate and yield a voltage equal to either the positive or negative voltage swings (Voltage Power Supplies) of the Op Amp. An inverting amplifier provides a solution to this problem.

Here we see an inverting amplifier circuit.

Figure 7. An inverting amplifier with gain of -2.

By connecting a suitable resistor across the amplifier from the output terminal back to the inverting input terminal, we can reduce and control the overall gain of the amplifier. This is referred to as Negative Feedback and it produces a stable form of inverting amplifiers and helps us compute the gain accurately. The term inverting refers to the fact that this amplifier changes a positive voltage to a negative one and the voltage and vice versa. The gain is referred to as the Inverting Closed-Loop Gain.

The inverting closed-loop amplifier gain is given by the formula:

You will be guided through a series of questions to compute both the low and high gains using the formula. Then, compute the same gains using the stimulus and output voltage indicators on the LabVIEW VI control panel. Finally, you will verify the results obtained using both methods.

The following is a schematic of the inverting amplifier circuit in your Automated Measurements Board.

Figure 8. NI Measurement Top Board Unity Gain circuit schematic. The 928Ω-resistor protects the output from short-circuiting.

The switch in the amplifier is software controlled. Closing and opening the switch yields two different possible gains.

2. Pre-Lab Quiz

Quiz Date: xxxxxx
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3. Experimental Procedure

Before programming with the NI ELVIS III, you must first create an NI ELVIS III project. With an NI ELVIS III project, you can group together all the files relevant to your application and run VIs on the NI ELVIS III.

Safety

Be aware of powering the NI board and be aware of not creating any short circuit on the board (connecting ground and power levels (+5V, 3.3V) is not recommended, and may create some damage on the electronic components, be careful about that.)

Hardware/Software Equipment Check

Prior to starting the lab, make sure the equipment is working by conducting the following steps:

Step 1: Make sure that the NI setup is open from the ON/OFF switch as it found at the back of the device.

Step 2: Make sure that you press the open button on the device.

3. Data Collection

Part 1 Low Pass Filter

In this experiment, you will use your NI Automated Measurements Board and LabVIEW to conduct measurements on a basic low pass filter. You will measure the stimulus voltage and compare it to the filtered voltage, for different frequencies of input.

The low pass filter has a cut-off frequency of 3.3 KHz.

Make sure your:
- NI ELVIS III is powered on.
- Connected to your computer.
- The NI Measurement Board is powered on.

Open the /Filter/Time Domain folder in the “Measurement And Instrumentation Software”.

Launch the LabVIEW Project named Filter Time Domain.lvproj.
Make sure all other projects are closed.
From the project window, configure the NI ELVIS III IP address to reflect the IP address of the actual NI ELVIS III your computer is connected to.
- You can find the IP address of your NI ELVIS III by clicking and holding the button on the left-hand side until the IP address is displayed on the LED screen.
- To configure the NI ELVIS III from your project window,
  - right-click NI ELVIS III (0.0.0.0) [Unconfigured IP Address]
  - click General in the window prompt you get.
  - In the IP address section enter the IP address of the NI ELVIS III connected to your computer.
- Save your project.
Open the RT Main (Manual).vi.

· Connect socket 20 (the input to the filter) to the Bank A analog input channel AI0 and also to Bank A analog output channel AO0.

· Connect socket 21 (the output of the filter) to Bank B channel AI0.

· Connect the Filter socket to the Analog Ground socket.

· In the VI, set the Filter Type to Low Pass.

· When you’ve made your connections, run the VI.

Part 2 High Pass Filters

Let’s repeat the previous experiment using High Pass Filters.

The onboard high pass filters have a cut-off frequency of 80 Hz.

· Connect socket 26 (the input to the filter) to the Bank A analog input channel AI0 and also to Bank A analog output channel AO0.

· Connect socket 27 (the output of the filter) to Bank B channel AI0.

· Connect the Filter socket to the Analog Ground socket.

· In the VI, set the Filter Type to High Pass.

· When you’ve made your connections, run the VI.

Part 3

Inverting Amplifier

Experiment with the inverting amplifier found on the application board.

Launch Instructions:

Make sure your:
- NI ELVIS III is powered on.
- Connected to your computer.
- The NI Measurement Board is powered on.
Open the /Amplifier/Gain folder in the “Measurement And Instrumentation Software”
Launch the LabVIEW Project named Amplifier Gain.lvproj.
Make sure all other projects are closed.
From the project window, configure the NI ELVIS III IP address to reflect the IP address of the actual NI ELVIS III your computer is connected to.
- You can find the IP address of your NI ELVIS III by clicking and holding the button on the left-hand side until the IP address is displayed on the LED screen.
- To configure the NI ELVIS III from your project window,
  - right-click NI ELVIS III (0.0.0.0) [Unconfigured IP Address]
  - click General in the window prompt you get.
  - In the IP address section enter the IP address of the NI ELVIS III connected to your computer.
- Save your project.
- Open the RT Main.vi.
- Connect both the output channel A/AO0 and A/AI0 to the input socket of the inverting amplifier (socket 13) on your measurement board.
  Note: The stimulus signal generated from channel A/AO0 is fed through the input of the amplifier. The same signal is read from the amplifier input using into channel A/AI0.
  This serves the dual purpose of generating the signal we want to pass through the amplifier on the board and reading that signal back into the NI ELVIS III to programmatically display it.
- Connect B/AI0 to the output socket of the inverting amplifier (socket 14).
- Connect AMP to analog ground.

Run the main VI and from the amplifier configuration dropdown list, choose inverting amplifier gain x2 (low gain).

Part 4

Non-Inverting Amplifier
Launch Instructions:
- Connect both the output channel A/AO0 and A/AI0 to the input socket of the non-inverting amplifier (socket 15) on your measurement board. Note: The stimulus signal generated from channel A/AO0 is fed through the input of the amplifier. The same signal is read from the amplifier input using into channel A/AI0. This serves the dual purpose of generating the signal we want to pass through the amplifier on the board and reading that signal back into the NI ELVIS III to programmatically display it.
- Connect B/AI0 to the output socket of the non-inverting amplifier (socket 16).
- Connect AMP to analog ground.
Run the main VI and from the amplifier configuration dropdown list, choose non-inverting amplifier x2 (low gain).

Responsible TAs:

Deniz Aktaş, denizaktas20@ku.edu.tr
Emine Bardakçı, ebardakci20@ku.edu.tr