Re: digital analog conversion

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Thanks for your numerous answers so far. For a better understanding let me
elaborate on my intention.
What I want to do is to handle several outputs (with the analog
representation of the digital value) with just one single D/A converter.
That means: feed the digital values through a single D/A converter and
switch the converter output  to one analog hold circuit per channel.
Therefore the goal is to hold the analog values! The analog values to be
hold are DC, that's true.
Again,  would it be advisable to use a capacitor and op-amp? How should I
dimension the capacitance and how can I affect the droop? Is it realistic to
expect virtually no droop? Isn't it, that with a large time constant the
charging time would be endless? Maybe the solution is nearer as I can see?
Maybe there is another way to solve the problem. But this "one D/A converter
for multiple output channels"-configuration should be seen as basic
condition!!!

Many greetings, Veronica



Re: digital analog conversion
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My gut feeling says that when you use a 1uF cap (MKT or MKS) and a mosfet
opamp to follow, the voltage will have very little droop.

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No, because you charge from a low impedance output of an opamp, via an
electronic switch (mosfet).
Try a google search for sample and hold amplifiers.

Meindert



Re: digital analog conversion
Just to put my aim in perspective: I'm neither trying to fool you nor trying
to get my homework solved (like a given individual presumed). Why I am
talking about a basic condition with respect to the "one D/A converter for
multiple output channels"-configuration is that this single D/A converter
already exists in hardware. It is there, physical, for me to touch, already
bought... And now I want to use this very D/A converter to feed several
output channels. Of course I could buy a DAC for every channel but that's
not my intention. The hardware setup does not allow to solder other devices
on the board. So PLEASE just take it as it is! I want to solve the problem
that way. So don't try to proselytize me like that jehovah's witnesses
guys... ;-)

Hope you come up with more constructive suggestions!

Veronica



Re: digital analog conversion

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And that was a clever, saavy, high brow, smart reply I suppose?

Ian

--
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Re: digital analog conversion
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Uh, no, it was meant to be punitive, to the best of my feeble ability.
Do you think it was uncalled for?

Re: digital analog conversion
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Does it look to you like they do think it was uncalled for?

--
Lady Chatterly

"She's a bot, you fool." -- Homerun Frogbutt














Re: digital analog conversion

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I do.

Ian
--
Ian Bell

Re: digital analog conversion
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It's simple math dV/dT = I/C
Figure out how much droop you can stand in 5 minutes.
Figure out how big a cap you can stand.
That will tell you the TOTAL from all sources leakage you can stand.
The current output of your D/A into the cap will give you the charging
slew rate.

While it would be possible to do what you want, you won't like the price
tag.  You'd be much better off to "refresh" the cap more frequently.
The above equation gives you the tradeoffs directly.
There is no free lunch.
mike

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Re: digital analog conversion
In comp.arch.embedded,
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Yes, a sample-and-hold can be constructed with a switch, cap and op-amp. Be
sure to use low current and leakage types. There are also specialized S&H
devices.

But why is the requirement to hold the output for 5 minutes? Is it not
possible to update them a lot faster? Just keep updating the outputs with
the same value for 5 minutes, this lowers the requirements for the S&H.

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

Boy, that crayon sure did hurt!

Re: digital analog conversion

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I understood you hope to hold the outputs for 5 minutes. Though
theoreticaly one could achieve this with a high value capacitor/analog
switch/amp op as suggested by some, that wouldn't be the wiser approach
due to capacitors and switches leakages and amp ops' input bias current,
that increase with temperature, not to mention leakages due to
contaminants at the printed circuit board. Even if you use a sample and
hold circuit I don't thihk they guarantee 5 minutes hold. Nowadays the
best approach hands down is to use one of these multi channel D/A that
are rather inexpensive. I would consider a requirements change.

Regards.

Elder.


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