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Analog Sensor Amplification/ Attenuation 8th Order Low Pass Filter Analog to Digital Converter Figure 1: Block Diagram of the Analog Path Figure 1 above shows the pathway that the analog signal needs to take going through this system. The analog signal is first generated from the analog sensor. This sensor can be measuring anything from temperature, pressure, vibration or etc. The next step is the amplification or attenuation stage. The amplification or attenuation will be done by using a programmable gain amplifier. The next stage is the Low Pass Filter. The filtering for this system will be done using an 8th order low pass filter. The final stage is the Analog to Digital converter. This will be done inside of the micro controller that was chosen. The analog sensor will be connected to the circuit through the 2.5mm jack. From the customer requirements, the analog sensor can input an analog voltage anywhere between 0 and 10V into this system. Amplification and Attenuation The original customer requirement for this project was for a universal sensor input. This could not be accomplished while accomplishing the other requirements as well. So the input of the system was changed to be from 0-10V. The microcontroller that was selected only has an input range of 0-3.3V. To accommodate for this different range in voltages, the input range from the sensors will need to be attenuated. There are a few different ways that the voltage can be attenuated. One way that the voltage can be attenuated is a resistor divider circuit. This is not the best way to do the attenuation because the voltage division would change for different load impedances. With this type of circuit the load gain of the circuit, the output divided by the input, would change if the input voltage changed. Another reason why this is not the best way to attenuate voltage is because if another resistor was added in series or parallel at Vout, the gain would change. The gain of this circuit will change depending on the load impedances. One way to overcome this impedance matching issue is to use op amps to attenuate the voltage. Op amps have zero output impedance so there is no need to worry about any impedance matching. Op amps can be used to have a positive or negative gain in a circuit. Equation 1 shows the equation needed to find the gain of a non-inverting op amp. The gain of a non-inverting op amp can never be less than one. A programmable gain amplifier was used in this system. The chip that was chosen was the MAX9939 PGA. This PGA is a SPI PGA with gains from 0.2Volts/Volt to 157 Volts/Volt. The maximum input signal to this DAQ can be 16.5V because the input of the ADC on the microcontroller is 3.3V and the minimum gain is 0.2V/V. Low Pass Filter Design A low pass filter is used to pass low frequency signals through a system and filter out the unwanted high frequency components of a signal. The cutoff frequency is the frequency point where the magnitude of the signal begins to be attenuated. Any signal that is at a higher frequency than the cutoff will be attenuated. Low pass filters are used for smoothing data. For this design the MAXIM MAX7403 8th order filter low pass filter (LPF) was used. An 8th order filter attenuates the signal at -160dB per decade past the cutoff frequency. Since the 8th order filter attenuates faster than a first order filter, more of the unwanted high frequency noise will not be allowed to pass through. Figure 8 shows the frequency response of the 8th order filter. After the cutoff frequency, 10 kHz, the slope of the curve is 8 times steeper than the first order filter shown in Figure 7. Figure 8 shows that all signals with a frequency of 10 kHz or less will have a gain of 0dB. A gain of 0dB is the equivalent of multiplying by 1. Anything with a frequency higher than 10k Hz will have a gain of something much less than 0dB which is equivalent of multiplying by a number that approaches 0. Figure 8: 8th Order Filter Frequency Response Simulation Results Figure 9 shows the actual measurements that were taking using the MAX7403 8th order LPF that was on a breadboard. As you can see the actual measurements look very similar to the simulation shown in Figure 8. This proves that this chip will work in our PCB design as a very efficient low pass filter. Figure 9: 8th Order Filter Frequency Response Hardware Results Figure 10: Input and Output of the 8th Order Filter Simulation Results Figure 10 shows the input and output of the filter. The yellow curve is the input to the filter and the red curve is the output of the filter. There is some time delay between the input and output of the filter but it is in the µs range. The Input signal was changed and for all signals with a frequency below 10 kHz the output had the same amplitude as the input. For signals with higher than 10 kHz frequencies, they were attenuated and the output was zero. This helps prove that the filter is an efficient filter. The output of the filter cannot go below 0.25V. This can easily be fixed by using the offset pin on the chip. The signal will be offset by 0.25V but the ADC on the microcontroller can only have an input of 3.3V. This will change our maximum input signal to the whole system from 16.5V to 15.25V. Figure 11 below shows the measurement that was taken using the MAX7403 chip on a breadboard. Figure 11: Input and Output of the 8th Order Filter Hardware Results Power Management This DAQ device is being powered by a 3.7V Lithium Polymer battery. This battery will be charged using a USB. USB was chosen because USB has a 5V pin. Also the FRDM KL25Z board is charged via a USB. The MCP73831 chip will be used to charge the LiPo battery. This chip is a linear charge management in a small 2 mm x 3 mm package. This chip along with the USB charging will allow for quick easy charging of this DAQ device. None of the components in this device require 3.7V that the LiPo battery can supply. This is resolved by using a 5V boost converter. The converter that was chosen was the TI TPS61200. This chip was chosen for because it has high efficiency and it can operate with a varying input. This important because the voltage of the battery might not always be the same. When the battery is in use and not charging, the output voltage will decrease slowly over time. The MCP73831 LiPo charging chip and the TPS61200 were chosen because these chips were used on the power cell LiPo Charger/Booster. This charger/Booster was purchased to test these chips and charging a LiPo battery. PCB Design After going through a few iterations, the final PCB design for this will be a shield board that will go on top of the Freescale KL25Z devolvement board. The development board is used because it is reducing a lot of risk. The development board has the KL25 microcontroller already on the board. All of the pins are routed to the holes on the outside of the board. Pins will be put into these holes and these holes line up directly with our shield board. Figure 12 shows the final PCB layout for the shield board. Figure 12: Finalized PCB Layout