Department of Electrical and Computer Engineering s8
University of Connecticut
Department of Electrical and Computer Engineering
ECE 4901: Fall 2015-Spring 2016
Team 1606 (Phonon): Current Monitor System
AJ Pikul (EE)
Barath Parthasarathy(EE)
Jason Stock (EE)
Maya Dubrow (EE)
Sponsor Contact Information
Statement of Need
Phonon Corporation has tasked us with the development of a calibrated DC/RMS current measurement system to test their Surface Acoustic Wave (SAW) devices. The current measurement system will attach to a power source in order to compile calibrated measurements of DC and PWM (RMS) current for SAW devices. In addition, the current system should have an increased voltage range, while maintaining low impedance. The focus should be on developing a flexible device which can give, calculate, and track accurate data.
In addition to the basic design, we are also tasked with adding additional design features should time pertain. These new features include intermittent current detection, voltage sensing, and a range selector to record data at different time ranges to track errors. The project will encompass circuit building hardware component with a software component which will be used to gather data.
This project is important for improving upon the testing equipment to better track current while working with SAW technology.
Specs/ Capabilities of Current Model
Past Model:
This was our initial design for our project; it was fairly complex and would take a fair amount of work to implement. The power supply circuit, which is outlined with a red box, consists of a rectifier, three series zener diodes, and a voltage regulator circuit. These components would act together to power our INA169 op-amp. The op-amp amplifies the signal and sends it to the output circuit, which consists of an output impedance, and a buffer circuit. This buffer circuit is another op-amp which was intended to keep the gain of the INA169 constant no matter what sense resistance the current is flowing over. The point of the sense resistances is to incorporate ranging into our project, such as the ranging feature on a multimeter. This is done by using four solid state relays and four resistors of values 0.05, 0.1, 0.25, and 0.5 ohms for our different current ranges. Each current range corresponds to a resistance value and a solid state relay. The relay allows current to pass through the resistor, and the idea here is to have a constant voltage drop across all of the resistors so we can get an accurate current sense reading once the signal goes through the op amp.
Current/Updated Model
Phonon’s original specifications were to have a voltage range from 5-50V and a current range from 0-10A. This made things a bit more difficult for us in terms of our system being able to handle that much power. After a brief video conference with the company, they altered their specifications to make it a bit more realistic. Now we have a voltage range from 5-30V and 0-5A, which makes life much easier for us. This is an updated and modified version of our project, which is a lot less complex and easier to work with. One of the main parts of this circuit is that we took out the entire power supply circuit because we learned that the op-amp will be powered by the signal we are measuring, which simplifies the circuit. This schematic does not have the ranging portion included in it, but it will still be included. We modified the ranging circuit to include MOSFETs in series with the resistors to utilize the low gate voltage aspect of the opto-isolator and the high gate current capabilities of the FET to pass current through the resistor. We are looking at testing the LTC6101 op-amp and using that as opposed to the INA169, if the LTC6101 works, it will be able to make our project much more efficient. The DUT (device under test) is connected to the op-amp, which will amplify the signal, and then from the op-amp, the signal goes right to the DAQ, where all of the measurements will be taken, and stored for a short amount of time on the computer. 
Alternative Designs
The ranging circuit allows us to control the current over our op-amp which will then give our DAQ a higher resolution. The two ranging circuits we are considering are parallel resistor and series resistor setups. The series resistor setup would allow us to use a few less resistors, but also has some unwanted resistance from MOSFETs that would cause voltage drops we could not ignore since we are using very small resistances. The parallel setup has a low impedance so it will drop less voltage which is useful to us. Within the parallel setup we can either use one op-amp over the whole circuit, or one over each resistor. We chose to put one at each resistor because the switches change resistance based on temperature, and with an op-amp over each separate resistor this change in resistance can be ignored. This option is less cost-effective, but is more useful for a circuit that we do not need to mass-produce.
We have chosen three options for our op-amp, and have tested one in our circuit. The first we tested was the INA169 which is the updated version of the op-amp used by Phonon, the INA139. This op-amp only operates properly at a high-side voltage of at least 3V, and the range for the sense voltage is very low, at 100-500mV, which requires a buffer circuit to make sure the sense voltage stays within that range. Our other two options for our op-amp are the INA282 and the LTC6101. The INA282 is a better option than the 169 because it allows a sense voltage of up to 10V, which means it would not require a buffer circuit, but it does require circuit ranging up to 18V. The LTC6101 has the largest input range of the three, and would also not require a buffer circuit. It has an adjustable input impedance, a plus because low impedance is one of our given requirements, and and very low offset voltage. We need to do extensive testing on the INA282 and the LTC6101 to further characterize these two op-amps and decide on one of the three for our circuit.
Benefits of Current Model
The specifications of this project required a current monitor with a large self-powered voltage range while maintaining a low impedance. The purpose of this project is to improve upon a previous current monitor. The first iteration of this circuit by Phonon used an INA139. While we are still in the testing process, all of our options, INA169, INA282, and LTC6101, all have much larger common mode voltages. This allows for power supply to use a larger range of power sources, to make our circuit more flexible. The current circuit model also allows for a .1 mA resolution for the 5 A range.
One of the bigger upgrades to our circuit takes place in the software component. Using Visual Basic in combination with a DAQ, 1408FS, we developed code that would compile current measurements from our circuit. The software is impressive as it track number of samples, as well as the index sample at a very fast rate. Even more impressive, the data is being stored in binary in a separate excel document. While the binary is not useful on its own, if converted can create and store the data while still maintaining real time tracking of errors.
Budget/Component List
Note: Separate colors represent alternatives. The cost of the final product will only use one of these. The total below is including all three of them to accurately represent the total cost of this entire design iteration.
The buck-boost power supply was designed using TI.com’s workbench program which can computationally design buck-boost accuracy. It was then verified by the engineers. The current guard is used to protect the main sense amplifier, and the resistors change based on the maximum ratings of that amplifier. The range station is the most complex design consideration and involves range resistors, MOSFETS, SSR’s, and one high voltage gate driver to change the circuit being used. The amplifiers are described above. The output circuit is used to provide a second stage amplification to utilize the full range of the DAQ and to sum all the sense amplifiers. Summing all the sense amplifiers as opposed to using multiplexers is a theoretical design which should work because any sense amplifier switched “off” should have a 0 output. Using a summing amplifier over a multiplexer reduces cost, incorporates two functions (gain and mixing) in one, and reduces latency.
Timeline
Some of the highlights of the first semester include creating a base for our project. The base will be changed slightly to account for different op-amps to finalize a “best” option. In addition, the software has been put in a good prototype state, where it can track the data consistently which was the end goal. Despite this, we will still improve upon it to make it optimize the full capability of the DAQ. On a less technical side, all paper and written requirements were also handed in on time, with constant updates being streamlined using our website, google drive and Facebook group.
Looking to the future, we want to make sure the power source can be programmable while also ironing out any kinks with our current data acquisition code. While simulating the device may be difficult, we should still create a professional outline with data from our extensive testing on individual op-amps.
Future Plan Reiterated
As stated previously, we still have much to do in terms of testing and fine tuning our circuit to make sure it is up to the standard of Phonon and a good representation of student from the University of Connecticut. Our final To-Do list include:
● Spice Model and possible simulation.
● Decide on specific resistance values.
● Prototype and characterize LTC6101 and compare it to INA169.
● Prototype varying DUTs.
● Ranging options need to be tested.
● Extensive testing!
Acknowledgements
We would like to thank Phonon for tasking us with this project, and to the University of Connecticut for allowing us an outlet to work apply engineering concepts. Special thanks to Dr. Gokirmak for meeting with us every week to go over project progress. And special thanks to Lam Dinh, Paul Rago, and Scott Kraft for providing us with all the components and being very active in helping/ guiding us in this project.