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'[PIC] ATD question'
2006\04\27@213526 by tete etertet

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What is the reason for the voltage reference inputs of
the ATD module (18f4620) and when should I use them
(as opposed to simply using Vss and Vdd) ?
Thanks.

2006\04\27@215400 by Mike Singer

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precision

On 4/28/06, tete etertet <spam_OUTmaxmsc04TakeThisOuTspambtinternet.com> wrote:
> What is the reason for the voltage reference inputs of
> the ATD module (18f4620) and when should I use them
> (as opposed to simply using Vss and Vdd) ?
> Thanks.
> -

2006\04\27@221026 by Bob Axtell

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tete etertet wrote:
> What is the reason for the voltage reference inputs of
> the ATD module (18f4620) and when should I use them
> (as opposed to simply using Vss and Vdd) ?
> Thanks.
>  
This allows you to direct-connect a source, allowing very accurate
readings. Helps
a LOT when the PIC is attached to noisy peripherals.

--Bob

2006\04\28@035547 by Alan B. Pearce

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>> What is the reason for the voltage reference inputs of
>> the ATD module (18f4620) and when should I use them
>> (as opposed to simply using Vss and Vdd) ?
>> Thanks.
>>
>This allows you to direct-connect a source, allowing very
>accurate readings. Helps a LOT when the PIC is attached
>to noisy peripherals.

As well as reference accuracy that someone else noted, it also allows one to
set the span of the ADC, as well as choosing a reference amplitude that
makes best use of the maximum range of the ADC.

An example which I have used is with temperature sensors. By setting Vref-
to the voltage of the lowest temperature I wanted to measure, and Vref+ to
the voltage representing the highest temperature to be measured, I could get
the best temperature resolution in the digital value that the ADC gave me.
In practice I had to use a lower voltage for Vref- and a higher voltage for
Vref+ as there is a minimum differential required between Vref- to Vref+ of
about 2.5V. Check the datasheet for the actual voltage, as it varies between
chips. It is also a little hidden away as a parameter.

Some folk here have reported operating an ADC reliably with a lower
differential between Vref- and Vref+, but you cannot rely on the result if
you do, as Microchip do not guarantee the result in this situation.

2006\04\28@063235 by Nicholasdatt

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So - is it possible to extend Vref+ above 5V
And is it possible to extend Vref- to -5V ?

2006\04\28@065812 by Jan-Erik Soderholm

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nicholasdatt@yahoo.co.uk wrote :

> So - is it possible to extend Vref+ above 5V
> And is it possible to extend Vref- to -5V ?


No and no.



2006\04\28@072624 by Michael Rigby-Jones

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>-----Original Message-----
>From: .....piclist-bouncesKILLspamspam@spam@mit.edu [piclist-bouncesspamKILLspammit.edu]
>Sent: 28 April 2006 11:32
>To: Microcontroller discussion list - Public.
>Subject: Re: [PIC] ATD question
>
>
>So - is it possible to extend Vref+ above 5V
>And is it possible to extend Vref- to -5V ?


No, the datasheet is very clear on the allowable range of the Vref inputs, I'd suggest you read the relevant section thoroughly before you start any design work.

Regards

Mike

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2006\04\28@075238 by olin piclist

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nicholasdatt@yahoo.co.uk wrote:
> So - is it possible to extend Vref+ above 5V
> And is it possible to extend Vref- to -5V ?

Gee, d'ya think they just might put this sort of information in the data
sheet!!?


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\04\28@170634 by Steve Smith

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I have a alternative thought about this:-

Yes and no.

Providing you don't bias the internal protection diodes into conduction and
don't exceed the published specs

I use a nominal 5V from a 7805 on lots of designs for VCC with a 5.12v
reference from a TL431 to simplify the maths on lots of different pics none
of them have ever given trouble specs say that max pin volts is VDD + 0.3V
max (the protection diode forward voltage) so VDD + 0.12 is completely
within spec this gives 50mV / step on the A-D converter.

Regards Steve

-----Original Message-----
From: .....piclist-bouncesKILLspamspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu] On Behalf Of
Jan-Erik Soderholm
Sent: 28 April 2006 11:58
To: piclistspamspam_OUTmit.edu
Subject: RE: [PIC] ATD question

@spam@nicholasdattKILLspamspamyahoo.co.uk wrote :

> So - is it possible to extend Vref+ above 5V
> And is it possible to extend Vref- to -5V ?


No and no.



2006\04\29@011739 by Russell McMahon

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> Providing you don't bias the internal protection diodes into
> conduction and
> don't exceed the published specs

I like the AND.
Too many people use a logical OR there,

> I use a nominal 5V from a 7805 on lots of designs for VCC with a
> 5.12v
> reference from a TL431 to simplify the maths on lots of different
> pics none
> of them have ever given trouble specs say that max pin volts is VDD
> + 0.3V
> max (the protection diode forward voltage) so VDD + 0.12 is
> completely
> within spec this gives 50mV / step on the A-D converter.

To be picky, some spec sheets give allwable range as Vss-Vdd.

And 0.3V is meant to be safely less than the protection diodes forward
voltage (or at such a low current that they are essentially non
conducting).
But, you knew that.

I like that idea, even though you are skating close to the edge :-).

7805 spec is 5V +/- 0.1V.
reference spec is far tighter.
So max variation is ~~0.22V.
That's safely under 0.3V, especially so  given that the 0.3V really
means "<<0.6V" for values of << that are meaningful in this context.



       Russell McMahon

2006\04\29@014021 by Steve Smith

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It struck me a few (lots of) years ago that if the FSD of an instrument is
256V or 51.2V or anything similar it turned all the maths into /2 /4 ect or
Logical shift one way or the other. This means no heavy maths in assembly
and only a few stupid resistors in the dividers and amplifiers. For safety I
use a 5.6v zener across the TL431 just in case the reference adjust knob is
completely wrong but never has any troubles with it so much so I now lay
boards out with fixed resistors and a pot for Vref adjustment cause 431's
just don't move except in the first 20 mins after power up...

Rgds Steve (rambling on about some trivial thing again..)

{Original Message removed}

2006\04\29@033302 by Jinx

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> I use a nominal 5V from a 7805 on lots of designs for VCC with
> a 5.12v reference from a TL431

Steve, I like my maths simple too. My choice is between 5.3V Vcc
and 5.12V reference or 5.12Vcc and using that Vcc as the reference.
As you say, can be had with a TL431, or a jacked-up 78L05 does
as well

2006\04\29@104752 by Picdude

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> -------- Original Message --------
> From: Russell McMahon <KILLspamapptechKILLspamspamparadise.net.nz>
lose to the edge :-).
>
> 7805 spec is 5V +/- 0.1V.

Depends on the 7805.  Since I use these quite a bit, and I usually reach for the lowest-cost ones when I order them, I previously checked out the datasheets for these from all the manufacturers I could find.  I've seen +/- 0.25V variations (ON-semi version IIRC).

Since I do a lot of averaging, I circumvent the complicated math most of the time by adjusting the number of samples I'm averaging to make the math easier.

Cheers,
-Neil.

2006\04\29@121450 by Wouter van Ooijen

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> > 7805 spec is 5V +/- 0.1V.
> Depends on the 7805.

Remember the 'generic components' discussion? There is no such thing as
'the' 7805's, there are only 7805's made by various manufactureres. So
if you find one datasheet that states +/- 0.1V, all you can say is that
7805's from *that* manufacturer (probably) are within those limits. IIRC
much wider limits (+/- 10%?) from some manufacturers.

And when saying '+/- 0.1V' you should probably specify the temperature
range.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\04\29@125003 by Peter

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On Sat, 29 Apr 2006, Jinx wrote:

>> I use a nominal 5V from a 7805 on lots of designs for VCC with
>> a 5.12v reference from a TL431
>
> Steve, I like my maths simple too. My choice is between 5.3V Vcc
> and 5.12V reference or 5.12Vcc and using that Vcc as the reference.
> As you say, can be had with a TL431, or a jacked-up 78L05 does
> as well

What would you jack a 78L05 up with ? (knowing that it drifts in
temperature like a real sensor)

Peter

2006\04\29@194106 by Jinx

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> What would you jack a 78L05 up with ? (knowing that it drifts in
> temperature like a real sensor)

Can't say I've ever found drift a huge problem. Taking the d/s figure
of 600uV/degC, over the temperature range I'd expect to be using
a circuit, 10C to 30C, the variation would be 5.111V to 5.123V.
In practice the variation I've observed is more like 0.2mV/degC or
5.117V to 5.121V. With a 10-bit ADC, 5.120V/1024 = 5mV/bit.
The variation due to temperature in the above range is 0.003mV/bit,
or 0.06%, which is in the region of 1 bit. Nothing wrong with that for
general purpose usage. I haven't tried tempco with a 7805, but a
quick look around

http://www.st.com/stonline/books/pdf/docs/2143.pdf

Fig 35

I don't do much ADC, and none that requires high precision, but if I
did, I'd be looking at more than 10 bits and a better Vref, eg

AD584

http://www.analog.com/UploadedFiles/Data_Sheets/115302565AD584_b.pdf

REF-02

focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=ref
02


2006\04\30@232051 by Dwayne Reid

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At 10:16 AM 4/29/2006, Wouter van Ooijen wrote:
> > > 7805 spec is 5V +/- 0.1V.
> > Depends on the 7805.
>
>Remember the 'generic components' discussion? There is no such thing as
>'the' 7805's, there are only 7805's made by various manufactureres. So
>if you find one datasheet that states +/- 0.1V, all you can say is that
>7805's from *that* manufacturer (probably) are within those limits. IIRC
>much wider limits (+/- 10%?) from some manufacturers.

I generally keep it even simpler: LP2950A for those projects
requiring decent accuracy, zener shunt regulator for those that don't.

LP2950A is a marvelous part.  Datasheet sez that initial accuracy is
plus/minus 25 mV @25C (5V version), plus/minus 50 mV over temp range
-25C through +85C, plus/minus 60 mV over temp range -40C through
+125C.  My experience bears this out.

I just don't do many designs that need the current capability of a
7805.  Most times, the 5V rail is less than 10 mA.  The output stages
might be handling lots of power but those supplies are separate from
the logic supply.

I mentioned zener regulators above.  I'd say that at least 90% of
what we build uses zener regulators.  The analog stuff is ratiometric
or there is no analog stuff.

dwayne

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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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