[Diy_efi] Question relating to old posts.....cheapdatalogger?

[Brian Dessent]

steve ravet wrote:
> 
> Mr Motor wrote:
> >
> > ok, bare with me here.....i'm no electrical guru. I now
> > understand how the IAT functions. I get THAT PART.
> >
> > Now, all this resistor stuff is where i dont know what you are
> > talking about. Are you telling me that i can wire up the IAT
> > sensor with some resistors to give it a constant 5v power supply
> > and then it will work with the dataq 194?
> 
> The IAT sensor is a variable resistor whose resistance varies with
> temperature.  Put it in series with a resistor as Perry indicated.  +5
> to the resistor, other end of the resistor to the IAT sensor, other end
> of the IAT sensor to ground.  Then read/log the voltage from where the
> IAT sensor is connected to the resistor.  There should be  table of
> voltage/temperature values in the service manual.

I think the real question here should be "What is the input impedance of
the Dataq board?"  As I understand it the confusion in this thread is
centered around the note from Dataq stating "this can't be used for
strain gauges, thermocouples, thermistors, etc."  Obviously, if you've
ever worked with strain gauges and the like, you know that they're very
low level signals and often require a differential instrumentation
amplifier with very good CMRR.  However, in the case of the thermistor
with the pullup to 5V, the voltage being read is certainly not small,
but the question of the internal impedance loading the voltage divider
could affect accuracy.  Worst case, insert an op amp follower (or
possibly just a common collector/emitter follower transistor buffer.)

Brian

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