--- In "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
while test 1 -lt 2; do ../../cgi-bin/adc_logger 3,0 && sleep 20 ; done
# ch3
0.071
# ch3
0.000
# ch3
0.066
# ch3
0.065
# ch3
0.000
Every third reading comes out zero. The main data logger is running once a
minute!
The main logger uses an adc_logger command to continually log 6 channels to a
data file. This has been working solidly over an extended period but none of
the other channels go this low unless they are zero inputs.
$ ../../cgi-bin/adc_logger 4,0 3,0 && ../../cgi-bin/adc_logger 4,0 3,0
# ch4 ch3
1.111 0.000
# ch4 ch3
1.106 0.001
$ ../../cgi-bin/adc_logger 3,0 4,0
# ch3 ch4
0.076 1.112
$ ../../cgi-bin/adc_logger 3,0 4,0
# ch3 ch4
0.002 1.110
So it seems reading any channel other than #3 prevents #3 from sampling
correctly . A double read on #3 within <30s gives correct output.
I'll now bread board the op amp cct to make sure this is realated to the MAX197
sampling cycle.
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