--- In "mccarl1978" <> 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.
>
> Why do you say that the op-amp can't "sink" enough current near 0V? It
> doesn't sound like the output is going negative. If the voltage is still
> positive (even if only small) then it is still "sourcing" current. You
> shouldn't need any "pull-down" resistor at all. All you are doing is further
> loading the op-amp. ie. The load on the op-amp from just the MAX197 is
> normally 16k/21k depending on bi/unipolar mode, but with the resistor you
> speak of, the load is increasing to approx 2k.
>
>
> >
> > 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.
> >
> > $ ./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.
> >
>
Hi,
these op-amps have an output swing from low rail to high rail - 1.5V. Sort of
half R2R. At they can source 40mA but only sync 40uA.
If I understand the MAX197 in unipolar mode (I'm using 0-5V) it does an
internal offset. The input is floating arount 2.5V . If I feed a signal of 0.5V
that means my op-amp needs to sink 2/20k= 100uA.
Is that incorrect?
thanks for your reply.
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