KISS. Why not use a single supply Op-Amp that goes rail to rail (LMC6462 comes to mind)? Use the + input to bias the output to + full scale for temp of 0 in (zero adj), and just invert the A/D conversion in software. Then you don't need a -5V supply. R Thomas McGahee wrote: > > Personally, I would use the AD590 to provide current to an op-amp > "current to voltage converter". Scaling is a function of the feedback > resistor. For example, you are interested in a range from 0 to 100 degrees > centigrade. That is a *change* of 100 degrees Kelvin, which would be > a change of 100 ua. Let's say you want to scale this to produce > an output that varies from 0-2.50 volts in 1000 steps: you would > get .1 degree for each .025 volts. (recall that you get 2048 counts for > a 0-5.12 volt input assuming your reference is 5.12 v) > > Note that using 1000 steps keeps the math simple, since only the > position of the decimal point has to be changed to accomplish the required > divide-by-ten function. 984 counts would display as 98.4 degreees celsius. > > Note that the AD590 operates based on the Kelvin scale where 273 > degrees K is [0 degrees centigrade]. To "bias" our "current to > voltage converter" circuit to operate in the proper way we must > provide a source of -273 ua to the input of the converter. Thus > when the AD590 is producing +273 ua, the -273 ua will cancel it > and a total of 0 ua will flow into the converter. > > Then an ADDITIONAL 100 ua must get converted to 2.50 volts by using a > 50K feedback resistor. Actually you would want the 50K feedback > resistance to be variable so you can tweak the gain since most > people will be using a 5 v reference instead of the 5.12 reference > assumed in this design example. A 47K resistor in series with a > 6.8K pot would allow variation from 47k to 53.8k which should be > adequate. > zero adj gain adj > 8.2k 2.5k 47k 6.8k > -2.5v ->-/\/\/\----/\/\/\------*-----------/\/\/\---/\/\/\---+ > | |\ | > | (-)| \ | > AD590 current source->---------*----| \----------------------*--->- to > inverter > | / > (+)| / op-amp > GND-----|/ > > Note that another inverting stage is required to flip polarity to positive. > Output of invert goes to 1k resistor to A/D i/o pin. > > Requires a +/- 5v supply or greater. > > Fr. Tom McGahee > > ----- Original Message ----- > From: Alan B. Pearce > To: > Sent: Friday, September 14, 2001 7:37 AM > Subject: Re: [PIC]: Minimal Vref+ value on pic16f877 > > > You have sent this question in while I was pondering on the lack of > > information on what one can do with Vref- :) > > > > What I am trying to do is use an AD590 temperature sensor as a temperature > > monitor. This gives 1uA/K and has a usable range from -55C to 150C (or > > thereabouts). My usage is for the range 0C to somewhere in the region of > > 100C (as I am looking for over temperature monitoring on resistive loads). > > Using a 10Kohm resistor as a load I will end up with a voltage output of > > 2.73V to around 3.73V, so I am looking to set Vref- to around 2.7V. I take > > it that this will then represent the 000 reading of the ADC, and that if I > > use 3.7V for Vref+, that will represent 7FF for the ADC. > > > > I am trying to simplify the conversion from binary to an ASCII string that > > represents the temperature by suitably scaling the conversion range (I can > > truncate the string being transmitted to stop unnecessary "accuracy"). > > > > However the questions that spring to mind come from what has been said > > previously about Vref+ needing to be greater than about 3V. Is this the > > minimum voltage Vref+ to Vref- that needs to be 3V? If this is the case I > > will have Vref- internally set to Vss and free up a pin, and have to > > subtract the 2.73V value from the ADC before doing the binary to ASCII > > conversion. What is does do is divide the accuracy by nearly 4 if I need > to > > do it this way. > > > > I do have another option which is to connect the current sink resistor to > a > > suitable negative voltage, and by using a larger value of resistor make > use > > of the ADC voltage range, but would like to use this method only if I > > absolutely have to. > > > > Does anyone have any real experience with trying to do things like this > and > > offer advice before I commit too deeply to unnecessary circuitry. > > > > -- > > http://www.piclist.com hint: PICList Posts must start with ONE topic: > > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > > > > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.