digital analog conversion

Dear newsgroup community,

recently I came across the following challenge. There are several digital
values which I want to convert to analog signals. Ok then, no problem.
Simply D/A conversion! But after converting the signals the general set up
requires that these values should be held for about - let's say - a period
of 5 minutes with practically no droop (decay of the analog value) at best!
The D/A conversion itself takes place in a 1 MHz period, the values to be
set have to pend for about 5 minues. I guess a hold-element (capacitor and
op-amp) would be the obvious choice. But how should I dimension the
capacitance and how can I affect the droop? Is it realistic to expect
virtually no droop assuming an optimal configuration ? Isn't it, that with a
large time constant the charging time would be endless, too? Please help me,
if you can. I am almost become desperate. I need this for my graduation
report.

Thank you in advance and many greets
Veronica

"Veronica Matthews" <ikeepthespiritalive@freenet.de> wrote in
news:cmvum3$ici$1@news.cs.tu-berlin.de: 

> Dear newsgroup community,
> 
> recently I came across the following challenge. There are several
> digital values which I want to convert to analog signals. Ok then, no
> problem. Simply D/A conversion! But after converting the signals the
> general set up requires that these values should be held for about -
> let's say - a period of 5 minutes with practically no droop (decay of
> the analog value) at best! The D/A conversion itself takes place in a
> 1 MHz period, the values to be set have to pend for about 5 minues. I
> guess a hold-element (capacitor and op-amp) would be the obvious
> choice. But how should I dimension the capacitance and how can I
> affect the droop? Is it realistic to expect virtually no droop
> assuming an optimal configuration ? Isn't it, that with a large time
> constant the charging time would be endless, too? Please help me, if
> you can. I am almost become desperate. I need this for my graduation 
> report.
> 
> Thank you in advance and many greets
> Veronica
> 
> 
> 

Assuming that the DAC you use has a DC response, there should not be a 
droop problem. The DAC should maintain its output indefinitely until you 
write a new value.

-- 
Al Clark
Danville Signal Processing, Inc.
--------------------------------------------------------------------
Purveyors of Fine DSP Hardware and other Cool Stuff
Available at http://www.danvillesignal.com

"Veronica Matthews" <ikeepthespiritalive@freenet.de> wrote in message news:<cmvum3$ici$1@news.cs.tu-berlin.de>...
> Dear newsgroup community,
> 
> recently I came across the following challenge. There are several digital
> values which I want to convert to analog signals. Ok then, no problem.
> Simply D/A conversion! But after converting the signals the general set up
> requires that these values should be held for about - let's say - a period
> of 5 minutes with practically no droop (decay of the analog value) at best!
> The D/A conversion itself takes place in a 1 MHz period, the values to be
> set have to pend for about 5 minues. I guess a hold-element (capacitor and
> op-amp) would be the obvious choice. But how should I dimension the
> capacitance and how can I affect the droop? Is it realistic to expect
> virtually no droop assuming an optimal configuration ? Isn't it, that with a
> large time constant the charging time would be endless, too? Please help me,
> if you can. I am almost become desperate. I need this for my graduation
> report.
> 
> Thank you in advance and many greets
> Veronica

You might investigate the use of serial DAC's.  Depending on the
requirements, they have a small footprint and are relatively cheap. 
You could use one for each channel, and it should hold the value until
you change it.

Keep up the faith,

Newman

"Veronica Matthews" <ikeepthespiritalive@freenet.de> wrote in message
news:cmvum3$ici$1@news.cs.tu-berlin.de...
> Dear newsgroup community,
>
> recently I came across the following challenge. There are several digital
> values which I want to convert to analog signals. Ok then, no problem.
> Simply D/A conversion! But after converting the signals the general set up
> requires that these values should be held for about - let's say - a period
> of 5 minutes with practically no droop (decay of the analog value) at
best!
> The D/A conversion itself takes place in a 1 MHz period, the values to be
> set have to pend for about 5 minues. I guess a hold-element (capacitor and
> op-amp) would be the obvious choice. But how should I dimension the
> capacitance and how can I affect the droop? Is it realistic to expect
> virtually no droop assuming an optimal configuration ? Isn't it, that with
a
> large time constant the charging time would be endless, too? Please help
me,
> if you can. I am almost become desperate. I need this for my graduation
> report.
>
> Thank you in advance and many greets
> Veronica
>
>

Write the DAC once and stop clocking it.  Other than noise pick-up through
the power-supply, reference and any other analog signal conditioning, the
value should remain static.  This is assuming you aren't using some kind of
bizarre AC coupled output DAC.

Rob Young

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

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

"Veronica Matthews" <ikeepthespiritalive@freenet.de> wrote in message 
news:cmvum3$ici$1@news.cs.tu-berlin.de...
> Dear newsgroup community,
>
> recently I came across the following challenge. There are several digital
> values which I want to convert to analog signals. Ok then, no problem.
> Simply D/A conversion! But after converting the signals the general set up
> requires that these values should be held for about - let's say - a period
> of 5 minutes with practically no droop (decay of the analog value) at 
> best!
> The D/A conversion itself takes place in a 1 MHz period, the values to be
> set have to pend for about 5 minues. I guess a hold-element (capacitor and
> op-amp) would be the obvious choice. But how should I dimension the
> capacitance and how can I affect the droop? Is it realistic to expect
> virtually no droop assuming an optimal configuration ? Isn't it, that with 
> a
> large time constant the charging time would be endless, too? Please help 
> me,
> if you can. I am almost become desperate. I need this for my graduation
> report.
>
> Thank you in advance and many greets
> Veronica
>

I guess you agree that this is a rather strange requirment! As you said 
before, let's not talk WHY you need to do so in the first place, and just 
think of HOW you can do it.
The simplest solution would be to use a sample and hold IC. These ICs 
include a very low drift output op-amp, a very high impedance switch and 
rely on an external capacitor for the "sampling" of the input voltage. As an 
example, you can take a look at this:

http://www.national.com/pf/LF/LF198.html

The critical parameter for you is deltaV / deltaT for the hold state. 
Typically, this value is round one milivolts per second for a good quality 
1u capacitor! So for 5 minutes that would be round 0.3 volts. Maybe you can 
find a better part than LF198 but the big problem is to find a very high 
quality capacitor that it's internal impedance is actually much higher than 
the input impedance of the sample and hold IC.
The advantage of using ICs like that is their tiny size and small board area 
(you just need one small IC and one capacitor).

Regards
Arash Salarian 

"Arash Salarian" <arash.salarian@epfl.ch> wrote in message 
news:419496dd$1@epflnews.epfl.ch...

>
> The critical parameter for you is deltaV / deltaT for the hold state. 
> Typically, this value is round one milivolts per second for a good quality 
> 1u capacitor! So for 5 minutes that would be round 0.3 volts. Maybe you 
> can find a better part than LF198 but the big problem is to find a very 
> high quality capacitor that it's internal impedance is actually much 
> higher than the input impedance of the sample and hold IC.
> The advantage of using ICs like that is their tiny size and small board 
> area (you just need one small IC and one capacitor).
>

But she doesn't need to hold for 5 minutes.
The DAC can be updated at a 1 MHz rate.
If she has 10 outputs, she can refresh each output every ten microseconds.
As long as the DAC and t/h have no serious glitch this has no downside,
and allows the use of small caps without droop. 

"Veronica Matthews" <ikeepthespiritalive@freenet.de> wrote in message news:<cn1vol$r7k$1@news.cs.tu-berlin.de>...
> 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

ok, what you want is a sample-and-hold for each channel and yes that could
be a cap a switch and an opamp buffer, the "droop" will depend on the capacitor
and what's discharging it (don't forget that cap is self)

you say you can't solder anything on the board, but you can fit several opamps
high quality switches and caps?  what will control the switches? 

A multiple output DAC give you all of it in a single package, it so much
simpler and most likely better...

you do sound a little like the jehovas witnesses ;) 

-Lasse

"Veronica Matthews" <ikeepthespiritalive@freenet.de> wrote in message news:<cn1vol$r7k$1@news.cs.tu-berlin.de>...
> 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. 

Presumably you have space for the analogue sample/hold or demux
arrangements?

> 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... ;-)

Well, I won't try to proselytise, but you really need to know that
some problems just *can't* be solved "as it is"!  Maybe the solution
can be found only if you re-frame the problem somewhat...

Since you want DC-like output, it seems to me that the best way would
be to make a bank of sample/hold circuits, one per output channel,
and refresh them as frequently as possible.  The digital value
corresponding to each channel's output can be held in an internal
register or small RAM bank, and the one-and-only DAC can be cycled
round all these values continuously.  That way, the sample/hold
circuits don't need particularly special droop performance and
they could probably be built using simple analogue switches and
small-value capacitors. It would require just a little extra logic 
in your FPGA, of course, but it sounds like only half an afternoon's 
work to me.

Alternatively, why not abandon the dedicated DAC altogether, and
provide one single FPGA output per analogue channel?  These
digital outputs can then be pulse-width modulated by logic inside
the FPGA, and low-pass filtered on the output side by a simple 
single-stage RC.  Once again, the fact that you are trying to 
create "DC" outputs comes to your aid: the RC filter can be
quite brutal (have a very long time constant), simplifying
the PWM design.

I know of no straightforward analogue sample/hold that can give
you very low droop over a period of minutes.  Consequently, your
original approach to the problem is unworkable.  No proselytising,
just good old-fashioned experience!
-- 
Jonathan Bromley