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'[PIC]: Precision 0.5V reference for AD'
2002\07\02@131042 by Alvaro Deibe Diaz

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Hello.

I am planning to read some 0.5V to 4.5V sensor
voltages, and need good 0.5V and 4.5V precission
references. I plan to use Analog Devices REF194
to get the 4.5V, but don't know how to get the
0.5V without using resistors, op-amps, etc.

Any ideas?

Thanks in advance,

Alvaro Deibe.

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2002\07\02@131903 by Harold M Hallikainen

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On Tue, 2 Jul 2002 19:11:52 +0200 Alvaro Deibe Diaz <spam_OUTadeibeTakeThisOuTspamCDF.UDC.ES>
writes:
> Hello.
>
> I am planning to read some 0.5V to 4.5V sensor
> voltages, and need good 0.5V and 4.5V precission
> references. I plan to use Analog Devices REF194
> to get the 4.5V, but don't know how to get the
> 0.5V without using resistors, op-amps, etc.
>
> Any ideas?
>

       How about just subtracting it out in software? If you have enough A/D
resolution to give up the bottom end, this approaches the ideal zero
parts solution.

Harold


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2002\07\02@144645 by Olin Lathrop

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> I am planning to read some 0.5V to 4.5V sensor
> voltages, and need good 0.5V and 4.5V precission
> references. I plan to use Analog Devices REF194
> to get the 4.5V, but don't know how to get the
> 0.5V without using resistors, op-amps, etc.

Use ground as the lower reference.  Do you really need the extra precision
from a 4V range instead of a 4.5V range?


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2002\07\02@150954 by Brendan Moran

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How about a heavily on diode through a resistor divider?


   +5V
    |
    |
    >
   <
    >
   <
    >
   <
    |
    +-----+ 0.65V
    |     |
    |      >
    |     <
   ---     >
   \ /    <  <----- Vref
   ---     >    |
    |     <    --- small C to reduce dynamic
    |     |    --- source resistance.
    |     |     |
    +-----+-----+
    |
  -----
 / / /

Shown here Potentiometer, instead of two resistors because I got lazy.

It would'nt be all  that accurate, due to temperature drift, but it might be
good enough for what you want...  And besides, the on voltage of a signal
diode doesn't drift all *that* much... well, best of luck.

--Brendan

{Original Message removed}

2002\07\02@154931 by Drew Vassallo

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>I am planning to read some 0.5V to 4.5V sensor
>voltages, and need good 0.5V and 4.5V precission
>references. I plan to use Analog Devices REF194
>to get the 4.5V, but don't know how to get the
>0.5V without using resistors, op-amps, etc.

I've used pressure sensors with this same output range.  I used a 5 V
reference (if you use a 4.5 V, that's even better) and just subtracted off
the zero offset (0.5 V nominal) to get an A/D range of 0-920 decimal
(nominal).  If you need more steps than this, then maybe you can't use this
method.  But I suspect it should be just fine for your application.

--Andrew

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2002\07\03@052309 by Alvaro Deibe Diaz

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First of all, thanks to all of you.

I need those 0.5V extra precission. Using 4.5V instead of 4V range means
losing about 12.5% resolution, and this is too much for my application (we
intend to validate a new vehicle dynamics mathematic model with the data we
collect).

I was looking for a good and stable 0.5V referece, because any error in the
references will get into the data. The 4.5V reference I plan to use has
under 10ppmV drift with temperature and load. But there is no 0.5V
references this good.

An obvious solution would be using a precission op amp (rail to rail input
and output, single supply) and a pot to divide the good 4.5V and get the
0.5V. But I don't like the pots very much (the final pcb will go into a 1:5
scale car, and there are too much vibrations, noise and temperature
variations).

Other possible solution is to use a good adjustable reference but the adjust
is, again, with a pot...

Perhaps any of you have a solution for this problem?

Thanks again,

Alvaro Deibe.

{Original Message removed}

2002\07\03@053804 by Alan B. Pearce

face picon face
>I was looking for a good and stable 0.5V referece,
>because any error in the references will get into
>the data. The 4.5V reference I plan to use has under
>10ppmV drift with temperature and load. But there
>is no 0.5V references this good.

Check out the Analog Devices AD584 Precision Voltage Reference. I believe
you should be able to get what you want from this.

It starts off as a 5ppm reference, and you will need to divide from this to
get your 0.5V, but your reference output can be 2.5V, so you need only some
close tolerance resistors. By using 5 identical resistors in series, and
tapping at one resistor from ground you should eliminate any temp co-eff
problems, even if the resistors do not have a tight temp co-eff themselves,
and with 5 resistors from the same production batch you should get a close
tolerance divider.

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2002\07\03@063949 by Alvaro Deibe Diaz

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Yes, this is a good solution. Simple and easy. No calibration problems (once
selected the appropriate resistors), no temperature problems, vibration,
etc.

The only question is must I "isolate" the reference point from the (tree)
Vref- of the (tree) PICs that do the AD whith a good op amp?

Thanks again,

Alvaro Deibe.

{Original Message removed}

2002\07\03@064805 by Alan B. Pearce

face picon face
>The only question is must I "isolate" the reference
>point from the (tree) Vref- of the (tree) PICs that
>do the AD whith a good op amp?

Do I understand you correctly?

When you say "tree" do you mean "three" (3) ?

I think you are asking can you use the same reference for 3 PIC chips
simultaneously, is this correct? If so you may need to have buffer
amplifiers between the resistors and the reference inputs, but I have no
experience with this. However I would have thought any modern FET input
op-amp used in voltage follower mode would be satisfactory.

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2002\07\03@065842 by Alvaro Deibe Diaz

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Sorry for my english: I have three (3) PICs, not "tree"... :-)

----- Original Message -----
From: "Alvaro Deibe Diaz" <.....adeibeKILLspamspam@spam@cdf.udc.es>
To: "pic microcontroller discussion list" <PICLISTspamKILLspamMITVMA.MIT.EDU>
Cc: "Jesús Cardenal Carro" <.....jcardeKILLspamspam.....cdf.udc.es>
Sent: Wednesday, July 03, 2002 12:40 PM
Subject: Re: Re: [PIC]: Precision 0.5V reference for AD


> Yes, this is a good solution. Simple and easy. No calibration problems
(once
{Quote hidden}

believe
{Quote hidden}

close
> > tolerance divider.
> >
> > --
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2002\07\03@080139 by Olin Lathrop

face picon face
> I need those 0.5V extra precission. Using 4.5V instead of 4V range means
> losing about 12.5% resolution, and this is too much for my application (we
> intend to validate a new vehicle dynamics mathematic model with the data
we
> collect).

If you're that close to the edge, then you should be using an A/D with more
bits anyway.  If you use an external 12 bit A/D, then you can use the 4.5V
reference and ground, and still have plenty of precision left.


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2002\07\03@080152 by Andy Jancura

picon face
Hi Alvaro,

you didn't wrote nothing about your AD precision and sampling rates. This
two things give you the answer to your question and are the major criteria
by choice of the final solution.

Andy

------------------------------------------------------------------------

Hello.

I am planning to read some 0.5V to 4.5V sensor
voltages, and need good 0.5V and 4.5V precission
references. I plan to use Analog Devices REF194
to get the 4.5V, but don't know how to get the
0.5V without using resistors, op-amps, etc.

Any ideas?

Thanks in advance,

Alvaro Deibe.


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2002\07\03@084432 by Alvaro Deibe Diaz

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The project is almost finished. There are three PICs (16F877) talking each
other with I2C. Only one 16MHz clock. The master PIC talks (RF) to a remote
computer, sending the A/D data collected, 100 packets/second
(Manchester-like encoding). The remote computer sends (RF) commands
(Manchester encoding) to the master PIC, to control 6 RC servos (1us
resolution, 10ms period). This master PIC does other things at the same time
(like reading channels for steer and throttle/break coming from a standard
RC receptor).

The data collected comes from 1 triaxial linear accelerometer (Crossbow
CXL02LF3), 3 rate gyros (TOKIN 16D0), 1 triaxial magnetometer (Honeywell
HMC2003), 4 suspension and 1 steer position sensors (specialiced pots),
temperature sensors (LM335) and 4 wheel angular speed sensors (inductive
sensors).

The data is collected 100 times per second. Not all of the sensors are
relevant. Some are for reference purposes (temperature), and others need all
the precision we can get out of the PICs (accelerations, angular speeds and
magnetic fiels).

The software is running (about 95% finished), and the hardware needs only
defining the analog A/D part.

One of the goals was to keep the parts count as low as possible, to get
little PCB size.

I hope this defines the problem.

Thanks again,

Alvaro Deibe.



{Original Message removed}

2002\07\03@121403 by Harold M Hallikainen

picon face
I still think the solution to the 0.5V reference need is to just use the
4.5V reference and a higher resolution A/D. Any attempt at adjusting or
dividing down another reference will introduce errors due to the
temperature coefficient matching of the resistors in the divider.

Harold


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2002\07\03@121848 by Harold M Hallikainen

picon face
On Wed, 3 Jul 2002 10:38:47 +0100 "Alan B. Pearce" <@spam@A.B.PearceKILLspamspamRL.AC.UK>
writes:
> Check out the Analog Devices AD584 Precision Voltage Reference. I
> believe
> you should be able to get what you want from this.
>
> It starts off as a 5ppm reference, and you will need to divide from
> this to
> get your 0.5V, but your reference output can be 2.5V, so you need
> only some
> close tolerance resistors. By using 5 identical resistors in series,
> and
> tapping at one resistor from ground you should eliminate any temp
> co-eff
> problems, even if the resistors do not have a tight temp co-eff
> themselves,
> and with 5 resistors from the same production batch you should get a
> close
> tolerance divider.


       I've used standard resistor networks (with isolated resistors) for this
purpose, hoping the tracking would be better than discrete resistors and
that they'd all be at close to the same temperature. I've also used $5
resistors to get under 10 ppm/degree C tempco...

Harold



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2002\07\03@125839 by Brendan Moran

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If you use a surface mount IC (you mentioned that size is a consideration),
you could use a parallel DAC for this.  With an 8-bit DAC, using the 4.5V
reference, and GND as a reference, you can get 0.494V, and the drift should
be small.

I hope that that will help you

--Brendan

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2002\07\04@034147 by Alan Shinn

picon face
Why not just use 2 precision resistors to make a voltage divider off of
your 4.5 volts. If that's not close enough, just trim it in with your
voltmeter and 1 more small resistor in series with the appropriate
resitor (or a large value in parallel)
Or do you want to manufacture these circuits?


>
>Hello.

I am planning to read some 0.5V to 4.5V sensor
voltages, and need good 0.5V and 4.5V precission
references. I plan to use Analog Devices REF194
to get the 4.5V, but don't know how to get the
0.5V without using resistors, op-amps, etc.

>Any ideas?

>Thanks in advance,

>Alvaro Deibe.
>

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2002\07\04@075506 by Alvaro Deibe Diaz

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No. This is a prototype.

I think I will use a voltage divider made with resistors to get the 0.5V
from the 4.5V I have, as most of you suggested.

Thanks all for your time.

Alvaro Deibe.

{Original Message removed}

2002\07\04@135715 by Michael Rigby-Jones

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> -----Original Message-----
> From: Alan Shinn [SMTP:RemoveMEalshinnTakeThisOuTspamMINDSPRING.COM]
> Sent: Thursday, July 04, 2002 9:41 AM
> To:   spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU
> Subject:      Re: [PIC]: Precision 0.5V reference for AD
>
> Why not just use 2 precision resistors to make a voltage divider off of
> your 4.5 volts. If that's not close enough, just trim it in with your
> voltmeter and 1 more small resistor in series with the appropriate
> resitor (or a large value in parallel)
> Or do you want to manufacture these circuits?
>
>
The reference inputs on the PIC A/D can take up to 1mA during the aquisition
phase, so you'd want as low source impedance as possible if accuracy is
important.

Regards

Mike

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2002\07\04@140549 by Brendan Moran

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I still like the idea of using a smal DAC to provide the .5V reference.
With 4.5V as it's high-end reference, it would produce 0.494V

Pretty close, I think.

oh, well.

--Brendan

> The reference inputs on the PIC A/D can take up to 1mA during the
aquisition
> phase, so you'd want as low source impedance as possible if accuracy is

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2002\07\04@144620 by Alvaro Deibe Diaz

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Decisions, decisions!

Extending your idea a little more, I've searched Maxim's web for serial I2C
DACs (I already have I2C, so it makes little sense using a parallel DAC,
isn't it?) and found the MAX5822 dual 12-bit i2c DAC. It has very little
temperature variations (2.3ppm/ºC), and unity-gain precission rail-to-rail
op amps in the outputs. I'll look the datasheet in detail...

Thanks again,

Alvaro Deibe.

{Original Message removed}

2002\07\04@145445 by Brendan Moran

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> Extending your idea a little more, I've searched Maxim's web for serial
I2C
> DACs (I already have I2C, so it makes little sense using a parallel DAC,
> isn't it?) and found the MAX5822 dual 12-bit i2c DAC. It has very little

I was thinking that you could just hard-wire the parallel DAC, and not worry
about any software control of it.

--Brendan

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2002\07\05@133428 by Alvaro Deibe Diaz

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> I was thinking that you could just hard-wire the parallel DAC, and not
worry
> about any software control of it.
>
> --Brendan
>

Yes. That would be even easier (but bigger?) ;-)

Alvaro.

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2002\07\05@133905 by Alvaro Deibe Diaz

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> The reference inputs on the PIC A/D can take up to 1mA during the
aquisition
> phase, so you'd want as low source impedance as possible if accuracy is
> important.
>
> Regards
>
> Mike

Even more, because the acquisition is synchronized in all the PIC's, and
takes place at the same time. That's why I want an op-amp to buffer the
voltage refferences.

Alvaro.

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2002\07\05@135552 by Alvaro Deibe Diaz

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{Quote hidden}

Well, not so. Automotive sensors are, frequently, ratiometric with the
supply. But the ones I choose are not.

The triaxial accelerometer Crossbow CXL02LF3 (it has Analog Devices MEMS
accelerometers in it) gives a sensitivity of 1V/g, and it goes from -2 to
+2g. Erros like Transverse Sensitivity, Non linearity or Alignment are
large, but can be corrected in software. Errors like Noise Density
(130ug/Hz^(1/2)) or Noise (1.5mg rms) are in the range of the 10 bits A/D.

The triaxial magnetometer Honeywell HMC2003 has, again, its own internal
voltage reference, so it's output is not ratiometric with the supply. The
figures of this kind of sensor goes well beyond the resolution of the 10bits
A/D (using set/reset pulse circuitry to cope with problems like hysteresis).

I've tried Murata and Tokin piezo gyros, and will probably use Tokin CG-16D0
(the differences, however, are not that much). With this kind of sensors
would probably be enough 8 bits A/D.

Thanks for all,

Alvaro Deibe.

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2002\07\05@140007 by Brendan Moran

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> Yes. That would be even easier (but bigger?) ;-)

That is a question I leave to you (;

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2002\07\08@195356 by Andre Abelian

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LTC1728 has tolerance of 0.05%
http://www.linear.com/go/1389


Andre Abelian



-----Original Message-----



> I am planning to read some 0.5V to 4.5V sensor
> voltages, and need good 0.5V and 4.5V precission
> references. I plan to use Analog Devices REF194
> to get the 4.5V, but don't know how to get the
> 0.5V without using resistors, op-amps, etc.

Use ground as the lower reference.  Do you really need the extra
precision
from a 4V range instead of a 4.5V range?


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