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'0-1v analog output'
1999\09\30@015738 by Jon Petty

picon face
Hi

I want my pic to be able to output  8 bit (more doesn't hurt)  0-1 volt.

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?

Do I need a rail to rail DAC?
I want to keep component count low any ideas?

Thanks

Jon

1999\09\30@050627 by Bjarne Nebelong

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face
Jon,

Some suggestions:

1: Use PWM, filter it, current amplify it (i.e. OP-amp) and divide it.
2: Use a R/2R ladder, current amplify it and divide it.

HŠlsningar/ Regards,
Bjarne Nebelong

                       

                       Hi

                       I want my pic to be able to output  8 bit (more
doesn't hurt)  0-1 volt.

                       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?

                       Do I need a rail to rail DAC?
                       I want to keep component count low any ideas?

                       Thanks

                       Jon


'0-1v analog output'
1999\10\06@204103 by Mark Willis
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face
Could you use a "power ground" that's 0.5V below "Output Ground",
perhaps?  Either 5.5VDC or just plain 5V for Vcc, and you should be set.

 Mark

Jon Petty wrote:
{Quote hidden}

1999\10\06@221150 by Wagner Lipnharski

picon face
> > Hi
> >
> > I want my pic to be able to output  8 bit (more doesn't hurt)  0-1 volt.
> >
> > 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?
> >
> > Do I need a rail to rail DAC?
> > I want to keep component count low any ideas?
> >
> > Thanks
> >
> > Jon


A simple NPN transistor will solve your problem, with its base-emitter
voltage drop, 0.6Vdc, isn't?
Just connect the DAC output to any NPN transistor base and at the
emitter you will have DAC-0.6, and by free you have a buffered DAC
output.

                   + 5V
                     |
                     |
                    /
                  |/   NPN
DAC OUTPUT --------|    2N2222 or any other
0.5 to 4.5V        |\
                    v
                    |
                    o----------> Output 0 to 3.9V
                    |
                    R 500 Ohms to
                    R 20k
                    |
                   GND


You just need to find out which DAC output should be used to generate
the first positive voltage step at the transistor emitter, so that will
be your "zero + 1"

The resistor value should be to match your destination circuit impedance
balanced. If you don't know, just use something around 2k Ohms and it
will work ok.

Another suggestion is just use a silicon diode in series with the DAC
output, it will drop also around 0.6V.

1999\10\07@004439 by paulb

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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?

 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.

 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!

 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?
--
 Cheers,
       Paul B.

1999\10\07@015707 by Dennis Plunkett

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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

Dennis


>--
>  Cheers,
>        Paul B.
>
>

1999\10\07@041446 by paulb

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face
Dennis Plunkett wrote:

> This it would seem is a totem pole output and will drive in both
> directions, just like a CMOS output will

 Do you mean they (Maxim) are *not* using CMOS technology?  I would be
very much surprised.
--
 Cheers,
       Paul B.

1999\10\07@231841 by Jon Petty

picon face
Thanks for all the info

You guys/gals are great!


Jon


In a message dated 10/6/99 9:46:38 PM US Mountain Standard Time,
spam_OUTpaulbTakeThisOuTspamMIDCOAST.COM.AU writes:

<< 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?

  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.

  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!

  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?
--
  Cheers,
        Paul B.
 >>

1999\10\12@070433 by Roland Andrag

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face
Have you considered using a PWM output over a 4:1 resistive divider? If you
don't need a very fast reaction time this is possibly the way to go.

cheers
Roland


{Original Message removed}

1999\10\12@123420 by Morgan Olsson

picon face
-snip-

> >    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?

I sometimes use the trick of adding a pulldown resistor from the output down to some negative voltage.
The voltage do not need to be very clean as the DAC output usually regulates nicely; a simple diode pump from an available digital clock and RC filter usually works.

Design it to pull down enough so the DAC never needs to pull down, and thus the DAC + pulldown resistor seen as one, can drive your load down to GND with the current of the resistor without having voltage drop in DAC internal low side driver.

Regards
/Morgan
Morgans Reglerteknik, HŠllekŒs, 277 35 KIVIK, SWEDEN
  tel +46(0)414-446620, fax -70331,   .....mrtKILLspamspam@spam@iname.com

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