Re: Analog Vecor Generator

Mendel Pearl wrote:

> >>
> >> When I examine the current outputs of the DACS, I found that the working, but
> >> distorted 'X' output was nice virtual ground, and after the current/voltage
> >> convertor I get a waveform. BUT, in the middle of the sawtooth there is a big
> >> triangular 'dip'. Could this be beause the DAC is bad, or is it more likely
> the
> >> DAC is getting wrong data? You can spot this dip in any test mode.
> >
>
> >Hope for wrong data. Fix the other output first and then see if this is still
> bad.
>
> Would that not be much harder to fix than a defective DAC or op-amp? I can
> imagine it will be a lot of trouble fixing the mathbox or something... What do
> you do to fix processor-related stuff, just check for activity around all logic
> gates/ram/rom?
>

I have a tendency to fix the worst problem (in your case the other channel) and am
SOMETIMES rewarded with the other problems going away by themselves as a bonus.

>
> >> For the 'Y' current DAC output, which is not working at all, I measure a 5
> volt
> >> p-p random-like sawtooth (which does not resemble the one you should get
> AFTER
> >> the current/voltage convertor) in stead of a virtual ground.
> >> So my guess is the DAC's bad, but I am not sure, because I cannot explain
> what
> >> the DAC08 is exactly doing (this one generates a 'BIPOLAR' signal, a current
> >> added to that of the X-Y DACs).
> >> AM6012 DAC's cost about $30 here in Holland, so I want to make sure before
> >> buying and blowing them again. :)
> >
>
> >If you don't have the virtual ground it probably ISN'T the DAC, the opamp is
> >probably bad. It's the output of the opamp, fed back through a resistor inside
> the
> >DAC, that gives the virtual ground. If the opamp is bad and has no output
> there is
> >no feedback and you get exactly as you describe.
>
> You are probably right there. It's the Op-Amp providing the virtual ground
> indeed. I've checked outputs and these are clipped to VCC, so it is the feedback
> network or the amp itself...
>
> >
> >> I tried to find exact information on this AVG circuit on the net, but I
> cannot
> >> find a really good description of it's priciple of operation, and why the
> analog
> >> multipliers MC1495 are in the circuit.
> >
>
> >The multipliers are a form of pincushion correction needed by certain monitors.
>
> With what signal do these multipliers multiply?
>

Not sure, haven't looked at the schematics. It will be some combination of X and Y,
though.

>
> Thanks for your info!
Received on Wed Nov 3 04:59:32 1999