--- In Jim Jackson <> wrote:
>
>
>
>
> On Sun, 24 Feb 2008, j.chitte wrote:
> >> On my 7250 I have an R2R network DAC running of 4 bits of portA .
> >> This is far more linear than I expected from such a simple
circuit.
> >> An on screen plot looks as straight as a die.
> >>
> >> However , I am monitoring it with one channel of the MAX ADC
> >> and there is some very bad distortion of the values it returns.
> .....
> >
> > Anyone have any idea what this gross distortion is about?
> >
> > If I run this on range 3 (0-5V) it bigger and uglier and right in
the
> > area where I need results.
>
> If memory serves, the input impedance of the MAX197 is about 17K.
> What resistors are you using for your R2R ladder? Do you have a
buffer
> between the ladder and the max197 input?
>
Sorry I thought I'd specified that , the voltage is buffered by an op
amp voltage follower (LM2902)
I have checked the voltage at the SBC header and the voltage going in
is good down to about 0.623V on range 3 where the op amp bottoms out.
On range 2 it goes down to 0.110V .
Open cct the MAX input is floating around 2.5V , it seems that this
"input" is requiring me to sink quite a bit of current and it is
distorting the op amp output at lower voltages. The datasheet for the
ADC indicates a dynamic res. of 21k in unipolar mode.
0.125 mA is about twice what the op amp is capable of sinking with V
out = 200mV.
It just about fulfills its claim to run from zero to supply-1.5V but
don't expect it to drive anything!
The MAX is a bit of a special configuration and it looks like it
needs driving fairly firmly. Pulling it's "input" down to deck with
1k seems to have brought things back into line subject to further
checking.
Thanks for your wise advice, it pointed my at the problem.
regards.
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