On Sun, 18 Oct 2009, j.chitte wrote:
> --- 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 ?
Sorry I'd not been following too closely as to what yuor problem is, I'm
only really an amateur when it comes to electronics.
However if you check out the max197 data sheet, it quotes values for min
and max input currents on the analog input. In unipolar 5v is says max
input current is 360uA (microamps). It doesn't quote a min. input current
which seems to imply it never sources current. In the bipolar ranges
it appears the max917 can both source and sink current on it's inputs.
> I think this is the root cause of the problem with the chip I used.
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