--- In Yan Seiner <> wrote:
>
> j.chitte wrote:
> >
> >
> >
> > This may also be a case of sinking current as well as sourcing
> > current. IIRC the 197 in unipolar mode is biased at half full scale so
> > voltages less than that will probably need the sampled source to sink
> > current. Many applications supply current better than they sink it.
> >
> > As all correspondents suggested last time : buffer with an op-amp.
> >
> > (and check the amp's ability to sink).
> >
>
> Hehe... I am to hardware design as Bozo the Clown is to Shakespeare. :-)
LOL. I think we'd guessed that by now.
As dirty Harry said: "a good man knows his limitations" ;)
>
> I can wield a soldering iron usually without injury but that's about
> it. Further, this is purely a hobby project so I don't have any
> resources to draw on.
>
> However, googling around I discovered an LM324, which looks like it will
> work - 5 VDC supply, 4 on a chip so I only need 4 chips, and a simple
> voltage divider to get the gain I need. Any hardware gurus confirm that
> choice?
>
> http://www.national.com/ds/LM/LM124.pdf
>
> I'm thinking the non-inverting DC Gain circuit on page 9 will work.
>
> No?
>
Yep, with R1=R2, that's the voltage follower configuration I recommended a few
posts back. In fact the LM2902 which that data sheet covers is what I use to
buffer my thermistor temperature sensors.
If you make the two resistors the same you will have a unity gain buffer which
if you use a large value (100k ?) will draw nearly nothing from you LM24.
However, current sink is an issue here again. These op amps can go "near" the
ground rail but cannot sink much current. See the data section. They are not
truly rail-to-rail output.
I found I needed to "help" them a little with a 2k pull down resistor on the
output.
Since your LM24 output is small, you'd probably be best biasing the output by
adding in half the supply voltage and subtracting this out in software.
Use the summing configuration also on page 9 , drop the negative inputs add
half the 5V rail to one input and your LM24 to the other.
If you don't have a clean , stable 5V or you want more precision use a bandgap
reference like LM317. The applications notes show how to increase the nominal
1.2V output. So you can double it to put your op-amp in the middle of the 197
input range and avoid the current sink limitations of the amp.
Again, using high value input resistors you will be able to use one reference
voltage for several temp sensor circuits but watch out for spikes if you are
anywhere near the HT circuit of the car. Large resistors will pick up EM spikes
and spark plugs make lots of those.
Use 1% resistors since all these errors add up quickly.
I hope that's enough to get you started.
If it's a secret, what's the project?
cheers.
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