digital analog conversion

[Veronica Matthews]

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

 

[rickman]

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.

What you are looking for is called a track-and-hold circuit. You use a
switch to remove the driving voltage from the cap. So the RC constant
is very small when the switch is closed and very large when the switch
is open. If you need the cap voltage as a signal source, you will need
to buffer with an op amp or instrumentation amp to provide a very high
input impedance.

Is there a reason that you can't provide a DAC for every output? Then
you don't need a cap to hold the voltage. Oh, I almost forgot, with
that long of a hold time, you will need a cap with a very low leakage
current. I don't remember which caps are best, but they will not have
high capacitance values. It is a trade off where the high value caps
have high leakage. With a 300 sec hold time you will need a 100 uF cap
and over 300 Mohms for the total leakage to ground. I am not sure if
this is practical.

--

Rick "rickman" Collins

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[Pete Fraser]

Is there a reason that you can't provide a DAC for every output? Then
you don't need a cap to hold the voltage. Oh, I almost forgot, with
that long of a hold time, you will need a cap with a very low leakage
current. I don't remember which caps are best, but they will not have
high capacitance values. It is a trade off where the high value caps
have high leakage. With a 300 sec hold time you will need a 100 uF cap
and over 300 Mohms for the total leakage to ground. I am not sure if
this is practical.

A DAC for every output is the cleanest.
If only one DAC is available, use track and hold circuits
as suggested by Rickman, but refresh them frequently
so you don't need exotic capacitors and very high impedances.

 

[Veronica Matthews]

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

 

[Veronica Matthews]

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

 

[Markus Michel]

But obviously you have a board to place the analog-value-holders.
The best way to store values are digital registers.
So, why not use one ADC-Register-DAC chain for every channel (plus some
control logic)?
There are 8-channel ADC's and DAC's on the market.

Hope this helps.
Markus.

 

[Mark van de Belt]

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

If your concern is the number of output signals for each DA converter then
just consider a delta-sigma (or PWM) DAC for each channel. This will only
cost you one pin for each output channel, together with a simple R-C lowpass
filter.

Mark