piclist 1999\10\07\015707a >
Thread: 0-1v analog output
face BY : Dennis Plunkett email (remove spam text)

At 14:44 7/10/99 +1000, you wrote:
>Jon Petty wrote:
>> I was looking at a MAX522 dual DAC it said it could sink and source
>> 5mA to within .5 volts to ground or vdd.
>> I assume that means that the lowest vout is .5 volts?
>  No, I think that means it can sink or source 5 milliamps to within
>0.5V of either rail, just like a PIC can sink about 15 milliamps to
>within 0.5V of either rail.
>> Do I need a rail to rail DAC?

This will depend on the application if rail to rail is required. All DACs
have different output configurations, some have a totem pole driver stage,
and others are of the open collector type etc.
You will need to view the output level with respect to load graphs, and
also have a look at the equation to calculate the output level to see if it
sets an offset (Many do, so the 0.5V may be it, others have .25V etc)

>  I believe the device to which you refer *is* a rail-to rail DAC
>according to that specification.  If you expect a device to drive
>current with *no* voltage drop, you are going to be sorely disappointed;
>you require a special component called a "relay" to do that.

This it would seem is a totem pole output and will drive in both
directions, just like a CMOS output will (Except this is linear), so there
is to be an expected (Not always some will not, but very expensive) drop
for where it can not sink or source the current without some voltage loss
(As Paul states)

>  Generally, the load to your DAC will be a resistance to ground.  The
>current that resistance draws is proportional to voltage; the closer to
>ground it goes, the less current it draws.  If you need to drive 4.5 mA
>at 4.5V, you have  a load of 1 k ohm.  This load will draw 1 mA at 1V,
>100 µA at 0.1V and so on.  The DAC will drive to ground perfectly since
>at that point it delivers *no* current at all!

Perhaps the best thing that you can do, is connect this DAC output to a
Rail to Rail OPAMP (Yes even these will not be exact, but they will be
better with an offeset and gain to alter the values to rail to rail), or an
OPAMP driving a single transistor output within the feedback loop (Lets you
use another supply, but does get messey, as does offsets, but these can be
done if the Vref from the DAC is accessable)

>  If OTOH you need it to drive all the way to ground against a pull-up
>to the 5V rail, then you *are* going to have a problem and will need a
>negative rail to power the op-amp which buffers the ADC.  For accuracy
>of course, the ADC will still be connected to ground.
>  Have I missed something?

See comments


>  Cheers,
>        Paul B.


See also: www.piclist.com/techref/io/atod.htm?key=analog
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