Overview

During the school year, I work at the Electronics Technology Group (ETG). They are in charge of all the labs in the Electrical and Computer Engineering building. As part of that, we design equipment for students to use and new labs for students to do. I was put incharge of making new hardware for the Signals and Systems classes (EE 224 and EE 324). Previously, an senior design team worked hard to create a filter board, the CyDAQ 1.0, for this class to use in lab. But, the filters did not work as expected. We wished to continue using their software, but we needed to redo the hardware. The board I desinged is laid out in a way for easy testing. My goal was to create a board that the programmers could easily test. In other words, I designed this version of the board for debugging and testing while we developed new firmware.

The CyDAQ 2.0 test board is a data acquisition system with onboard filters that I designed. The goal of this data acquisition system is to help EE students make a connection between the filters built in EE 230 and the signals and systems concepts of EE 224. The CyDAQ 2.0 test board can be used to collect data in any of the EE controls or signals classes. The CyDAQ 2.0 test board will help bring theory and application together, solidifying the educational process.

With an input range of 5 Vpp, the CyDAQ can take in any signal from 5 volts to -5 volts. Since it has such a large range of operation, many different devices can be connected to it. Microphones may be connected to analyze audio signals. Accelerometers and other sensor may also be connected, allowing students to apply filtering in a real word example. When applying theory to real world applications, students will better understand the concepts of both noise and filtering.

System and Theory of Operation

The CyDAQ 2.0 test board has 2 onboard has 8 modes of operation. It can collect the pure signal, send it through a 1st order low pass filter, 1st order high pass filter, a 60 hz notch filter, a 2nd order bandpass filter, a 6th order bandpass filter, a 6th order low pass filter, or a 6th order high pass filter. Each filter has a unity gain and no distortion in the pass band. The filters have been designed as closely to an ideal butterworth response as possible. All filters are adjustable using digital potentiometers.

Data collection will be done using the TivaC. After the data is collected, it may be analyzed in any software, including Matlab. Digital filtering may be applied using software. Concepts of aliasing and and preventing it with low pass filtering circuits can easily be taught using the CyDAQ 2.0 test board.

The system is controlled by the Tiva C microcontroller from Texas Instruments. Through a computer, the user can communicate to the Tiva C through USB. The user can select the sampling rate of the ADC, the filter being used, and the desired frequencies to include or filter out.

After receiving instructions from the user through the computer, the Tiva C will select the one filters through the 4 bit wide Filter Select with enable shown in Figure 1. If the 6th Order High Pass or Low Pass filter is selected, the Tiva C will choose between the high pass or low pass option using the High Pass Low Pass Select. To change the cutoff frequencies or to adjust bandwidth, the Tiva C will change the value of the digital potentiometers through an I2C bus.

Once all the desired settings have been established, the Tiva C will begin recording the analog signal at the desired sampling rate. The signal will travel through the first buffer at Signal In as seen in Figure 1. Then, the signal will be sent to the desired filtering circuit using the Analog Multiplexer. After the signal has been filtered, it will be sent to the end buffer by the Analog Demultiplexer. The ADC will sample the analog signal and store the data. After sampling is completed, the Tiva C will send the data back to the computer over USB.

For more on the theory of operation, click here:

Basic Filter Layout

These filters are very advantageous for the CyDAQ 2.0. Since they can be highpass or lowpass depending on where they are cascaded from, they can easily be used for both with a simple analog multiplexer. By using multiplexer to change where the filters are cascaded from, multiple kinds of filters can be made using the same operational amplifier and potentiometer chips. Therefore, it greatly reduces the amount of chips needed and the overall cost of the board. Below is the basic structure that was used for most of the filters.

For more on the filter design, click here:

CAD Layout

I designed this revision of the board so that it would be easy to test. There are jumpers so that parts of the circuit can be taken out and isolated. Also, there are a lot of probe points. My goal was to design it in a way that we could easy trouble shoot hardware or software problems during the development process. After all the debugging, the plan is to make it more compact. I also included a truth table and addresses to help the programmers when they got their hands on it. I would also like to note that this was my first ever PCB layout.

To see the cad drawings, click here:

Testing

Below is a test of the 6th order low pass filter on a log scale. I tested the filters using a function generator and multimeter in conjunction with Signal Express, which is a UI for lab equipment. All filters were tested and worked, except for the 60 hz notch filter and the 6th order bandpass filter. However, I was able to identify a trace that was in the wrong spot and I corrected it. So after fixing the trace, the 6th order band pass filter was also able to pass the qualifications we needed.

For all the filter test results, click here:

For how I fixed the 6th order bandpass, click here: