--- 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.
>
> 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.
>
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