Welcome to the TI sponsored Iron Circuit Design Challenge Forum! This forum should be a great place to get some assistance with your ICD contest entry - especially the analog portions. I am a Signal Chain Marketing Engineer and have worked for TI analog for 8 years focusing in the performance analog area. Favorite TI amplifier today - the INA333 - the low power and zero drift instrumentation amp is going to make sensor signal conditioning a lot more interesting. If you could tag your contest forum questions with "Iron Circuit" that would be a great help, good luck designing and don't hesitate to ask if you need some help! .
Here is a short FAQ to get you started:
Details can be found at the EDN Iron Circuit Design Contest website
The contest is to find the best ideas for a low power wireless circuit that include at least three sensors of your choice, including at least three components from the TI approved parts list.
You will have to submit a block diagram and schematics, bill of materials and additional supporting information. Actual design does not have to be built or tested. For more details visit the entry details page
Visit http://www.edn.com/icd/ and click on the Register Now link
Bragging rights and …
In addition, random participants and voters will get surprise prizes!
7. When is it due?
The entries are due by May15th.
If you have any other questions - do drop us a line!
I am planning to enter the Iron Circuit Challenge and my entry will be a wireless biosensor device. I have been working on the design for some time now and for the most part have completed but of of the sensor circuits. The device is intended to measure record and communicate (O2 saturation - Pulse Ox, Temperature, Heart rate and SCL/SCR = Skin conductance level and variabiliy.) I am hoping to use the INA333 for the SCL/SCR measurement and was hoping to be able to get some design assistance.
Thanks in advance for any help and/or guidance.
If you are planning to use the INA333 to measure the skin conductance, you will probably need a very stable excitation source with which you either generate a known precision current or voltage divider with a known precision source and a known resistor. The INA333 can accept common input signal levels 100mV from either rail; however, I would recommend biasing the common signal levels such that it is mid supply about either 5V or 3.3V--whichever you prefer. You can scale the gain of the INA333 based on the maximum voltage that you anticipate on measuring such that it does not saturate the output into the positive rail. You will also need to apply a small voltage at the reference pin if you anticipate your minimum signal level to be very small. This will keep the INA333's output off the ground rail for those low input cases.
I hope this helps as a start.
I saw a voltage reference device REF50xx in the ist, and am wondering if that would work (per you suggestion) as a known precision voltage source? So basically the idea is to apply .5v across the skin and measure conductivity (in Micro Siemens ranging 1-50, or approximately .5 Micro Amps). I am also wondering if I might need a bridge on the input stage?
What is the best way to programatically adjust the gain? I am thinking a signal switch alon with a resistor bank?
Thanks again for all the help.
A bridge might actually be a better idea because it is easier to keep your signals of interest differential; likewise, using the REF50xx is a good idea to excite the bridge. If you anticipate having to dynamically adjust your gain to achieve the best impedance measurement, you might consider using the PGA308 for this end. Here is a link to the data sheet:
Great. Thanks again for all the help.
I managed to puttogether a design and simulate it in Tina using the PGA308 and it seems to work (at least theoretically).Now before I order boards I was hoping to be able to build a prototype. I just recieved some samples (PGA308AIDGSTG4), frankly I just picked what was available from the list and not quite sure what the G4 stands for. Anyway, is there an evaluation board (or prefereably a breakout board) that I could use to basically mount the device as a DIP? I have some breakout boards that are for SSOP type device packaging (I belive .63 pitch) and am not sure if that will work. Is the device mentioned .5 pitch?
Just want to re-iterate that for the Iron Circuit Design contest, we are not looking for a finished, working design. A concept will do – so you DON’T have to submit a detailed schematic along with detailed BOM down to the resistor level. Just get creative J
Voltaz, looks like you are on roll with your design. Good Luck!
Any chance you could explain what is going on here:
I was expecting to see 1.363v at the output not 743.68mV ! I am a bit confused as to why this is happening.
Thanks in advance for any help.
The problem is that you are saturating the output of the buffer amplifier (VOA1) off of the negative input. This is a limitation of a 3 OPA INA when the common mode voltage is not high enough. Where you bias your common levels from the bridge is dependent on how much your differential signal will change. You can determine where to set the common mode level, VCM, based on the equations below. Note that to get the voltage at Vin- and Vin+ into definition form, we denote the voltage as having both a common and differential component. Off the -input it is VCM -Vdiff/2 and off the positive input it is VCM+Vdiff/2. The output of each amplifier is VCM +/- (Vdiff/2)*Gain. Here you can see why in your case the amplifier was saturating--your differential voltage was 100mV; therefore, Vdiff/2 = 50mV and VCM = (271mV + 409mV)/2 =340mV. The output of each amplifier can only swing 75mV to either the ground or positive supply rail, so in your case the output of A1 was being driven to 340mV - 50mV*10 = -160mV, which is well below the ground rail.
If you expect large variation in the impedance you are trying to measure with your 100k ohm bridge, you might consider configuring it this way:
Also, if you want to decrease the sensitivity of the skin resistance, you can put a resistor in parallel with Rskin.
I hope this helps.
Perfect thank you very much. I realy appreciate the help Matt.
Heads up: We'll be opening up the voting session for the winners soon! Stay tuned ...
You can go here to see the top 3 circuits. Vote on who wins the $500 AMEX Gift Card and enter for a chance to win one of three Flip Video Series Camcorder. Happy voting and good luck!
Results are out Jul 1st. Good Luck to the finalists and registrants!
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