Download Analog Sensor Amplification/ Attenuation 8th Order Low Pass Filter

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