--- In "Charles" <> wrote:
>
>
>
> --- In "j.chitte" <j.chitte@> wrote:
> >
> > Hi,
> >
> > I'm using an LED as a light sensor. It is shunted by 200k and buffered by a
> > voltage follower, then sampled by the MAX197 on the 7250. The follower has
> > a 2k pull down resistor since the op-amp can't sink enough current at
> > outputs near the 0V rail.
> >
> > All other channels seem to sample correctly but don't sample this low. This
> > one seems to need double read on low voltages.
> >
> > Subsequent samplings seem correct but after delay of about a minute the
> > initial reading is at or near zero.
>
> Do you have O_SYNC on your /dev/mem open call?
>
> regards, ......... Charlie
>
> >
> > $ ./adc_logger -c 4 3,0 && sleep 15 && ./adc_logger -c 4 3,0
> > # ch3
> >
> > 0.071
> > 0.067
> > 0.071
> > 0.067
> > # ch3
> >
> > 0.068
> > 0.070
> > 0.000
> > 0.070
> > $ ./adc_logger -c 4 3,0 && sleep 60 && ./adc_logger -c 4 3,0
> > # ch3
> >
> > 0.072
> > 0.070
> > 0.071
> > 0.071
> > # ch3
> >
> > 0.002
> > 0.071
> > 0.072
> > 0.076
> >
> > Testing the input to the follower shows the expected final value and is
> > constant.
> >
> > I'm having a bit of difficulty seeing what may be happening, any ideas?
> >
> > TIA.
> >
>
iofd=open("/dev/mem", O_RDWR | O_SYNC);
yes.
The odd thing is that other channels are fine (although all sampling >1.0 volt)
and this channel works once the voltage is higher.
It looks like a capicitative effect.
This is the op amp I'm using in the follower.
http://www.datasheetcatalog.com/datasheets_pdf/L/M/2/9/LM2902.shtml
It's sink capabilities is poor new the lower rail, this is why I needed a pull
down. Maybe the MAX197 is not liking that too much.
:?
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