--- In Jim Jackson <> wrote:
>
>
>
>
> On Sun, 18 Oct 2009, j.chitte wrote:
>
> >
> >
> > --- In "j.chitte" <j.chitte@> wrote:
> >>
> >>
> >> --- In "j.chitte" <j.chitte@> wrote:
> >>
> >>>
> >>> 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.
> >>>
> >>
> >> checking this again with a DVM I note that the op amp output is not coming
> >> down to match the input. With input around 0.06 to 0.11 V the ouput is at
> >> 0.365V . Once the 197 samples the output drops to what it should be and
> >> hence the subsequent ADC reads are OK.
> >>
> >> After about 45s without sampling the op-amp goes bad again.
> >>
> >
> > Hmm this is odder than I thought.
> >
> > As a quick hack to keep this channel live I set a loop running on a console
>
> You can't run 2 processes both trying to control the MAX197.
> Or more correctly you can, BUT you run the risk of the 2 processes
> interfering. This is just a specific example of a more general interfacing
> principle
Thanks Jim.
I appreaciate what you are saying but the chance that the third 20s sample was
syncronious with the 60s is pretty small. This is not really the issue.
Normally there is just one process and it does not work.
Could you comment on my understanding that the MAX197 in 0-5V mode is floating
the input at 2.5V and hence requiring the op amp to sink current when the input
is <2.5V ?
I think this is the root cause of the problem with the chip I used.
best regards.
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