ADC - DAC question

Started by Francisco Lobo December 31, 2003
A couple of questions from a newbie :

I can't make basicx keep a stable voltage on one of its pins, this is
because i have to be calling the routine to keep the voltage, but my
app does not allow for fast enough refreshes. I need to output 8 bit
DAC, does any one has a solution for this with an external IC or
something? Which IC and do you have any suggestions on how to hook it
uo? Another question. If i want to sample for a voltage in a pin at
accurate timing. Like say sample 100 times per second the voltage in
pin 5, or sample once every 100 milliseconds, how can i achieve this in
a doable fashion with the most accurate results?? Thanks in advance!!!



--- In , Francisco Lobo <francisco@f...> wrote:
> I need to output 8 bit DAC, does any one has a solution for this
> with an external IC or something? Which IC and do you have any
suggestions on how to hook it up?

I recently used the LTC1451 with good results. It is a 12-bit DAC
but you use it as an 8-bit by writing zeros for the 4 LSBs. You can
find information on it here:

http://www.linear.com/prod/datasheet.html?datasheet

This is a serial interface DAC. You need a data line, a clock line
(both of which could be shared with other devices) and a select
line. Worst case, 3 I/O lines.

Don



--- In , Francisco Lobo <francisco@f...> wrote:
> Is there something like a buffer that does not require a
> data line, but can hold the DAC i put in basic x into a pin?

The built-in DAC function needs to be refreshed because charge is
drawn off of the capacitor. If you had an ideal capacitor (zero
leakage) and an infinite load impedance, the capacitor would stay
charged and require no refreshing.

Alas, we don't live in such a perfect world so you have to work to
achieve the desired result. Firstly, choose a low leakage type of
capacitor (polystyrene, polypropylene if you can, mica, mylar or
polycabonate aren't quite as good). Secondly, design a high
impedance amplifier circuit to attach to the capacitor and drive your
load(s). This probably can be done suitably with an op amp in
voltage follower configuration.

Other design problems are introduced by these choices. For example,
the low leakage capacitor types are typically low capacitance value.
Also, the output of an op amp typically is not "rail to rail". If
you need the full 0-5V range you may have to use a bipolar power
supply or a separate supply.

Don


Is there something like a buffer that does not require a data line, but
can hold the DAC i put in basic x into a pin? Thanks for ur help!!!

On Dec 31, 2003, at 8:22 PM, Don Kinzer wrote:

> --- In , Francisco Lobo <francisco@f...> wrote:
>> I need to output 8 bit DAC, does any one has a solution for this
>> with an external IC or something? Which IC and do you have any
> suggestions on how to hook it up?
>
> I recently used the LTC1451 with good results. It is a 12-bit DAC
> but you use it as an 8-bit by writing zeros for the 4 LSBs. You can
> find information on it here:
>
> http://www.linear.com/prod/datasheet.html?datasheet
>
> This is a serial interface DAC. You need a data line, a clock line
> (both of which could be shared with other devices) and a select
> line. Worst case, 3 I/O lines.
>
> Don >
>


---------
Francisco Lobo de la Garza
CEO/Director General
movic Records
elcielo Recording Studio
Garza Garcia N.L Mexico
---------
Grupo Fusion Global
Copyright(c) 2003-2004
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