--- In "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.
>
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