Using this method you will get you 10bit ADC from 0V to 3.3V. You lose a little over 1/3 of the values, as it starts at 1.2V, so you're really look at about 650 ADC values that you will read. This method requires no other parts but and opamp, 4 resistors (2 the same, another of value x and a fouth of value 6x), and hook up wire. You will get as accurate of a result as you can without adding a voltage subtractor (which you could actually get a full 1023 value range on the ADC, but does add a little more complexity).
- run an opamp with a gain of 6. You accomplish this by having a 6:1 resistor value ratio on the opamp (any opamp in radshack or in the kits will do). they just need to be powered. this will give you a 1.2V to 3V range (6*200mA to 6*500mA). google opamps as amplifiers to see the two resistors' placement and proper wiring. Its REALLY easy
- Power the opamp it with your 5V line. Run the ground to GND
- Then, take your 3v3 line and connect it to aRef with two matching resistors going to ground. a good AREF tutorial is http://tronixstuff.wordpress.com/2010/12/07/tutorial-arduino-and-the-aref-pin/
EDIT: I do like johnwasser's post, but then you need an external power supply that is above 5V to power the opamp, or else clipping the sensor's voltage signal will occur too early (like 4.7V) since the range is 5V. You can't amplify up to or beyond the voltage you have powering your opamp
. Both ways, even if properly externally powered, will give you an identical amount of ADC discernible values of the sensor.
winner10920, the lowest AREF voltage is 1.1 according to the tutorial above
Please look up the voltage subtractor though. You will need to buy a dual or quad opamp. they are like $3. Thi also takes a bit of research and the ight resistor values.
If you want to do the voltage subtractor, you then will need to first subtract 200mV from the signal. find out how to do that using google.
Your desired span is 3.3V. your actual span is 300mV. you need to have a gain of 11 on your opamp. For a factor of safety though, as you may not get the resistor values 100% right with your resistors on hand with either the subtracting opamp or the amplification opamp, 10 is easiest and the best (like a 1kOhm and 10kOhm resistor) though it is simple to have 11 (a 1kOhm and a series like of a 1KOhm and a 10kOhm resistor, for 11kOhm). this will give you a 90-100% of your 0-1023 span. That is as accurate as you will get (period). Problem solved!
Transconductance...that might take the Vsense resistor, saving that value of the voltage drop to measure the change in current, and then finding the change in voltage and taking a ratio of the two. This one may get a bit complicated...of i'm just too tired.
gm = dI / dV
Here is a good debate. i'm sure someone got an answer eventuallyhttp://www.electro-tech-online.com/general-electronics-chat/19707-measuring-transconductance-gm.html
Please excuse typos...its 4AM...and i need to get up in an hour.