--- In "Yan Seiner" <> wrote:
>
> --- In Jeff Cunningham <jeffrey@> wrote:
> >
> > Yan Seiner wrote:
>
> > > The document that Jeff quoted shows two ways to wire a three-wire
> > > setup, both on page 7. I can't quite figure out what the difference
> > > is, which one I should use, and why there are 2 resistors in
> > > parallel, one 499 1% and the other 22K 5%.... Are they just trying
> > > to adjust the range of the sensor with the 22K resistor?
> > >
> > > What I have is 12 bit A/D that is currently set up for 0-10V; I can
> > > reduce that down to 0-3.3 if need be. I need to measure normal room
> > > temps; between 60 deg. F and 110 deg. F to an accuracy of about a
> degree.
> > >
> > > The sensors will be powered from the TS7300 5v supply. It will be
> > > remotely mounted (up to 10') in a low-noise environment. I have
> > > shielded twisted pair available which I'd like to use.
> > >
> > > Could someone give me clue?
> > >
> > > Thanks,
> > >
> > > --Yan
> > >
> > If you are powering your sensor from the same supply as the sampling
> > system, you will want to use the left circuit. The one on the left is
> > missing the comment that the 20k resister should be a pot. The whole
> > point of that resister pair is to allow you to calibrate the device.
> The
> > 499 ohm resister gets it in the ball park and the 20k (pot) is
> > used to fine tune. If absolute accuracy is not that important, you
> > can omit the 20k resistor. They are pretty close as is.
>
> I think what has me confused about that one is the + / - on either
> side of the top 499 resistor. The way I read it, I should bring +5v
> at the top, read the sensor where the schematic shows -, and
> shield/ground at the bottom. Right?
>
> Thanks for all your help.
>
> --Yan
>
Hi Yan,
I think everyone here has given you some good pieces of info, but let
me try to put it together a bit (plus add my own input).
Both circuits are as someone mentioned, using the 499ohm resistor
closest to the LM34 to convert the voltage output of the device to a
current output (ie. the current will change by a certain amount of mA
per deg. F). This is so the output wont be affected by changes in the
length of cable between sensor and the measuring device. The other two
(parallel) resistors are converting that current signal back to a
voltage signal for use by whatever, in your case an ADC.
Also, I guess it is worth checking that you understand exactly where
each component goes (I'm sorry if you do already, but it helps to
clarify). The LM34 and the closest 499ohm resistor are the only
components that are on the end of your twisted pair cable. The 5V
supply, 20k and the other 499 ohm are all back near where the ADC is.
The "twisted pair" label and arrows point to the two conductors of
that cable, in both diagrams.
As for the difference between the two circuits? There's not much. The
circuit on the right is what I would recommend you use, because that
would be the simplest to interface to the ADC. The circuit on the left
would be used if for example the ground terminal of the LM34 *HAD* to
be grounded. For example, if you were using the metal can version of
the sensor, it's body IS the ground terminal, and if that by necessity
was in contact with a metal object that is at the same ground
potential as you TS-7300 supply, then the circuit on the right won't
work. But from your earlier description, I doubt that is the case,
hence why I recommend it. As mentioned before, to use the left circuit
you would need either an instrumentation amp or two ADC inputs, etc
because you have to measure the voltage *ACROSS* the resistors, in
other words from the point marked + to the one marked -.
Also, I might add that I wouldn't worry about the 20k resistor at all.
As also mentioned elsewhere, since you are sampling the output and
using it in software, then you can just calibrate in software. All you
do is perform a two point calibration, ie. record the sampled value at
a known/accurate _low_ temperature and then record what the sampled
value is at a known/accurate _high_ temperature. With these two
temperatures and sample values, you can interpolate the temperature
for any other sample input that you get.
If you have any other questions, just let us know.
Cheers
Phil
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