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Discussion Groups | Piclist | AD with 12F675 fluctuations

A discussion group for the PICMicro microcontroller. Also called the Microchip PIC, this list is dedicated to the use and abuse of this fine, simple, microcontroller. Close to topic posts are welcome, ie. general electronics.

AD with 12F675 fluctuations - d1camero - Apr 8 20:46:00 2004

I am trying to measure 10-15volts to 0.01 volt accuracy. I am
using the built-in 10bit ADC on the 12F675 with 1% tolerance
resistor voltage divider.

I find that the values fluctuate way more than expected, +/- 0.03
volts. The voltage is coming from a regulated power supply and I
have verified it with 2 DVMs, and it seems quite steady.

Here are some things I have tried:

1. changed power supplies - no difference

2. put up to a 1 second sleep after going into AD read mode - no
difference.

3. averaged 5, 10 and 50 samples - this has reduced the quantity of
fluctuations, but not the magnitude.

4. Tried a different development board - no change. 5. Changed the sample rate of the ADC to its max (250us) no change. Any comments or suggestions?

thanks
d1





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Re: AD with 12F675 fluctuations - Don Hackler - Apr 9 1:45:00 2004


There are some app notes for the 12F675 for doing higher accuracy A/D
conversions here:
http://www.microchip.com/download/lit/pline/picmicro/families/12f6xx/40040b.pdf
Perhaps they might give you some ideas...
- Don

d1camero wrote:

>I am trying to measure 10-15volts to 0.01 volt accuracy. I am
>using the built-in 10bit ADC on the 12F675 with 1% tolerance
>resistor voltage divider.
>
>I find that the values fluctuate way more than expected, +/- 0.03
>volts. The voltage is coming from a regulated power supply and I
>have verified it with 2 DVMs, and it seems quite steady.
>
>Here are some things I have tried:
>
>1. changed power supplies - no difference
>
>2. put up to a 1 second sleep after going into AD read mode - no
>difference.
>
>3. averaged 5, 10 and 50 samples - this has reduced the quantity of
>fluctuations, but not the magnitude.
>
>4. Tried a different development board - no change. >5. Changed the sample rate of the ADC to its max (250us) no change. >Any comments or suggestions?
>
>thanks
>d1 >to unsubscribe, go to http://www.yahoogroups.com and follow the instructions
>Yahoo! Groups Links





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Re: AD with 12F675 fluctuations - Igor Janjatovic - Apr 9 3:39:00 2004

Hello,

1k resistor and 0.01uF capacitor useally helps. Connect resistor to voltage
source and to A/D pin. Connect capacitor to A/D pin and to ground. That's
it. Make experiments with different values for both resistor and capacitor.
Make sure that total resistance of source and resistor don't exceed 10k (see
Data Sheet).

Regards,
Igor ----- Original Message -----
From: "d1camero" <>
To: <>
Sent: 09.04.2004. 3:46 AM
Subject: [piclist] AD with 12F675 fluctuations > I am trying to measure 10-15volts to 0.01 volt accuracy. I am
> using the built-in 10bit ADC on the 12F675 with 1% tolerance
> resistor voltage divider.
>
> I find that the values fluctuate way more than expected, +/- 0.03
> volts. The voltage is coming from a regulated power supply and I
> have verified it with 2 DVMs, and it seems quite steady.
>
> Here are some things I have tried:
>
> 1. changed power supplies - no difference
>
> 2. put up to a 1 second sleep after going into AD read mode - no
> difference.
>
> 3. averaged 5, 10 and 50 samples - this has reduced the quantity of
> fluctuations, but not the magnitude.
>
> 4. Tried a different development board - no change. > 5. Changed the sample rate of the ADC to its max (250us) no change. > Any comments or suggestions?
>
> thanks
> d1 > to unsubscribe, go to http://www.yahoogroups.com and follow the
instructions
> Yahoo! Groups Links





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Re: AD with 12F675 fluctuations - Igor Janjatovic - Apr 9 3:50:00 2004

Hello,

...and another reply:

> I am trying to measure 10-15volts to 0.01 volt accuracy. I am
> using the built-in 10bit ADC on the 12F675 with 1% tolerance
> resistor voltage divider.

OK, so you have 15V maximum divided by 3 in order to have 0-5V range for AD,
right?

> I find that the values fluctuate way more than expected, +/- 0.03
> volts. The voltage is coming from a regulated power supply and I
> have verified it with 2 DVMs, and it seems quite steady.

OK, now lets do some math...

2^10 = 1024
5V / 1024 = 0.00488V

Now, we multiply that with 3 because of that divider:

0.00488V * 3 = 0.0146V

Now, we add AD error which is probably +/-1LSB and that gives us:

+/-0.0146V or approx. 0.03Vpp fluctuation

What it means is that fluctuation is quite normal... if I did this math
right :)

Regards,
Igor





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Re: AD with 12F675 fluctuations - Phil - Apr 9 4:59:00 2004

This seems to be a common problem with PICs. I've seen it on the
12F675, 16F88, 16F877 (A and non-A), 16F819. I too have tried the
same things that d1camero tried, also to no avail.

I've had some success with taking care to isolate the analog section
from the rest of the system but the PICs pin outs makes that a bit
difficult. There is a Microchip app note on this topic. Also, some
sensors produce a lot of noise. I've run tests where I used the
center tap on a pot as the input to an ADC pin and it was quite
stable. Hook up a sensor and it got 3+ LSBs of noise.

I've started looking into conditioning as a way to improve the
noise. I got some noise reduction on a temperature sensor by using a
cap and a resistor (LM34, as described in the app note). However,
the Sharp GP2D12 is one noisy sensor and probably needs it own power.

The tips app note is kind of funny in that they seem to be saying
that to get higher accuracy, dont use the ADC. Uh, yeah, sure, why
did I buy that chip???

--- In , Don Hackler <donh@s...> wrote:
>
> There are some app notes for the 12F675 for doing higher accuracy
A/D
> conversions here:
>
http://www.microchip.com/download/lit/pline/picmicro/families/12f6xx/4
0040b.pdf
> Perhaps they might give you some ideas...
> - Don
>
> d1camero wrote:
>
> >I am trying to measure 10-15volts to 0.01 volt accuracy. I am
> >using the built-in 10bit ADC on the 12F675 with 1% tolerance
> >resistor voltage divider.
> >
> >I find that the values fluctuate way more than expected, +/- 0.03
> >volts. The voltage is coming from a regulated power supply and I
> >have verified it with 2 DVMs, and it seems quite steady.
> >
> >Here are some things I have tried:
> >
> >1. changed power supplies - no difference
> >
> >2. put up to a 1 second sleep after going into AD read mode - no
> >difference.
> >
> >3. averaged 5, 10 and 50 samples - this has reduced the quantity
of
> >fluctuations, but not the magnitude.
> >
> >4. Tried a different development board - no change.
> >
> >
> >5. Changed the sample rate of the ADC to its max (250us) no change.
> >
> >
> >Any comments or suggestions?
> >
> >thanks
> >d1
> >
> >
> >
> >
> >
> >to unsubscribe, go to http://www.yahoogroups.com and follow the
instructions
> >Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
>



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RE: AD with 12F675 fluctuations - Charles Linquist - Apr 9 8:04:00 2004


I have had really good luck with PIC A/Ds by:

Bypassing the PIC with 6 or 8 .1uF caps close to the chip on a 4-layer
board.
Connecting the reference GND directly to the PIC GND pins.
Using 2K in series and .1uF directly to GND at the input pin.
Making sure that the source impedance (feeding the 2K resistor) is not more
than 2K.
Waiting 100uSec before beginning conversion after any channel change.
Averaging at least 6 samples.

When doing the above, I have found that the jitter is generally less than
+/- 1 LSB.

Charles Linquist -----Original Message-----
From: Don Hackler [mailto:]
Sent: Thursday, April 08, 2004 11:46 PM
To:
Subject: Re: [piclist] AD with 12F675 fluctuations There are some app notes for the 12F675 for doing higher accuracy A/D
conversions here:
http://www.microchip.com/download/lit/pline/picmicro/families/12f6xx/40040b.
pdf
Perhaps they might give you some ideas...
- Don

d1camero wrote:

>I am trying to measure 10-15volts to 0.01 volt accuracy. I am using
>the built-in 10bit ADC on the 12F675 with 1% tolerance resistor voltage
>divider.
>
>I find that the values fluctuate way more than expected, +/- 0.03
>volts. The voltage is coming from a regulated power supply and I have
>verified it with 2 DVMs, and it seems quite steady.
>
>Here are some things I have tried:
>
>1. changed power supplies - no difference
>
>2. put up to a 1 second sleep after going into AD read mode - no
>difference.
>
>3. averaged 5, 10 and 50 samples - this has reduced the quantity of
>fluctuations, but not the magnitude.
>
>4. Tried a different development board - no change. >5. Changed the sample rate of the ADC to its max (250us) no change. >Any comments or suggestions?
>
>thanks
>d1 >to unsubscribe, go to http://www.yahoogroups.com and follow the
>instructions Yahoo! Groups Links

to unsubscribe, go to http://www.yahoogroups.com and follow the instructions
Yahoo! Groups Links




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Re: AD with 12F675 fluctuations - d1camero - Apr 9 8:46:00 2004

Igor,

Thanks for the quick feedback. I calculate the error:

The AD converter is measuring 0-5 volts, hence no division or
multiplication by 3 for the AD part.

1 LSB bit fluctuation is simply then:

5v / 1024 = 0.0048v

So I expect to see some fluctuation, but not as much as I have been
measuring. I will try the other suggestions and repot back.

thanks everyone.

Don
--- In , "Igor Janjatovic" <kodrat@p...>
wrote:
> Hello,
>
> ...and another reply:
>
> > I am trying to measure 10-15volts to 0.01 volt accuracy. I am
> > using the built-in 10bit ADC on the 12F675 with 1% tolerance
> > resistor voltage divider.
>
> OK, so you have 15V maximum divided by 3 in order to have 0-5V
range for AD,
> right?
>
> > I find that the values fluctuate way more than expected, +/- 0.03
> > volts. The voltage is coming from a regulated power supply and I
> > have verified it with 2 DVMs, and it seems quite steady.
>
> OK, now lets do some math...
>
> 2^10 = 1024
> 5V / 1024 = 0.00488V
>
> Now, we multiply that with 3 because of that divider:
>
> 0.00488V * 3 = 0.0146V
>
> Now, we add AD error which is probably +/-1LSB and that gives us:
>
> +/-0.0146V or approx. 0.03Vpp fluctuation
>
> What it means is that fluctuation is quite normal... if I did this
math
> right :)
>
> Regards,
> Igor




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Re: AD with 12F675 fluctuations - Scott Lee - Apr 9 10:26:00 2004

--- In , "d1camero" <a8723@c...> wrote:
> The AD converter is measuring 0-5 volts, hence no division or
> multiplication by 3 for the AD part.

But you said you were measuring 10-15V. How are you getting this down
to the 0-5V range for measurement?

Is your PIC properly decoupled? (That is do you have a .1uf cap as
close to Vss as possible?)

You said you were sleeping for 1 second after starting the conversion.
Why not just turn the interrupt on for A/D and go to sleep? (It will
wake up when the A/D is done that way.) If you are doing this in a
high level language are you sure that when you say to sleep for a
second that it's not just doing a 1 second timing loop? (I always
done everything in asm so I don't know anything about HL languages on
the PIC -- this is just a stab in the dark.) To truely sleep for 1
second you would need to set a timer so it would expire in 1 second
and it would need to be the only interrupt source -- that's why I
think it's reasonable that the HL language is simply doing a timing loop.

It sounds like you are only using one A/D channel so you would have no
need to change channels but if you are changing channels are you
waiting the necessary aqcuisition time (Taqc as described in the
datasheet) prior to starting the conversion?

--Scott



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Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - d1camero - Apr 9 13:44:00 2004

Thanks for all the input. I have tried your suggestions (averaging,
capacitors, sleeping) without any appeciable change in fluctuation.

BUT, when I removed the resistor divider network, and simply used
the 12F675 to measure 0-5volts, the ADC works great, with an
acceptable 1 bit fluctuation.

So my question has changed: How can I accurately measure 10-15volts
with the 675. Obviously a 1% resistor divider network introduces
too much noise. Other ideas?

thanks
d1




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Re: Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - Charles Linquist - Apr 9 14:36:00 2004


You don't mention what values of 1% resistors you use. It is important that
the parallel combination (thevenin equivalent) of these two resistors is
less
than 10K. Actually I prefer to keep it below 4K.

Charles Linquist ----- Original Message -----
From: "d1camero" <>
To: <>
Sent: Friday, April 09, 2004 11:44 AM
Subject: [piclist] Measure 10-15 volts ***accurately*** (was Re: AD with
12F675 fluctuations) > Thanks for all the input. I have tried your suggestions (averaging,
> capacitors, sleeping) without any appeciable change in fluctuation.
>
> BUT, when I removed the resistor divider network, and simply used
> the 12F675 to measure 0-5volts, the ADC works great, with an
> acceptable 1 bit fluctuation.
>
> So my question has changed: How can I accurately measure 10-15volts
> with the 675. Obviously a 1% resistor divider network introduces
> too much noise. Other ideas?
>
> thanks
> d1 >
>
> to unsubscribe, go to http://www.yahoogroups.com and follow the
instructions
> Yahoo! Groups Links





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Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - Scott Lee - Apr 9 15:25:00 2004

--- In , "d1camero" <a8723@c...> wrote:
> BUT, when I removed the resistor divider network, and simply used
> the 12F675 to measure 0-5volts, the ADC works great, with an
> acceptable 1 bit fluctuation.

It seems to me that Igor's explaination, which you seemed to
discount, may be at least some of the problem.

From what I can tell you are trying to measure 10-15V by using a
resistor divider which will scale the 15V down to the necessary 5V.
It will also scale that 10V down to 3.33V. Thus, although the A/D
is measuring over a 0-5V range, the data of interest is in the 3.33-
5V range.

You want 0.01V accuracy on the original 10-15V range. The resistor
divider is also scaling this just like it scaled the original
voltage range. Think about it, in the original 10-15V range there
would be 500 steps (501 levels) at the .01V increments you desire.
500 steps in the 3.33-5V range works out to .0033V increments.

Thus using your resistor divider to measure .01V increments over a
10-15V range will require that you measure .0033V increments over a
3.33-5V range. Given that the A/D can measure to a resolution
of .00488V but at best has an error of +-1 LSB this means that you
are going to be +-0.015V at best.

Now I know this doesn't explain all of it but you should see that
there is no way you are going to get the .01V accuracy you desire by
taking this approach.

One other point is you you say that you have verified the voltage of
the 10-15V supply using two DVMs and it seems quite steady -- have
you looked at it with a scope? There is a good chance you will find
some ripple which the DVMs are filtering out but that the A/D is
seeing due to the speed of the sample/conversion. As others have
pointed out, putting a cap close to the input to the PIC should help
filter this out. As others have also pointed out it is imperative
that you not have too high of resistance as viewed from the PIC
A/D. The fact remains that even following these guidelines you are
not going to acheive the accuracy you desire with a resistor divider.




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Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - rtstofer - Apr 9 15:47:00 2004


When I wanted to measure the voltage of a 24V storage battery (4
golf cart batteries in series) I used a rail to rail op amp to scale
the voltage. I was only interested in the range of 22 to 28 volts.

Read chapter 4 of Op Amps For Everyone (available free at
www.ti.com) and just rescale the range of interest to cover the
entire range of the A/D convertor. --- In , "Scott Lee" <midl_man@y...> wrote:
> --- In , "d1camero" <a8723@c...> wrote:
> > BUT, when I removed the resistor divider network, and simply
used
> > the 12F675 to measure 0-5volts, the ADC works great, with an
> > acceptable 1 bit fluctuation.
>
> It seems to me that Igor's explaination, which you seemed to
> discount, may be at least some of the problem.
>
> From what I can tell you are trying to measure 10-15V by using a
> resistor divider which will scale the 15V down to the necessary
5V.
> It will also scale that 10V down to 3.33V. Thus, although the A/D
> is measuring over a 0-5V range, the data of interest is in the
3.33-
> 5V range.
>
> You want 0.01V accuracy on the original 10-15V range. The
resistor
> divider is also scaling this just like it scaled the original
> voltage range. Think about it, in the original 10-15V range there
> would be 500 steps (501 levels) at the .01V increments you
desire.
> 500 steps in the 3.33-5V range works out to .0033V increments.
>
> Thus using your resistor divider to measure .01V increments over a
> 10-15V range will require that you measure .0033V increments over
a
> 3.33-5V range. Given that the A/D can measure to a resolution
> of .00488V but at best has an error of +-1 LSB this means that you
> are going to be +-0.015V at best.
>
> Now I know this doesn't explain all of it but you should see that
> there is no way you are going to get the .01V accuracy you desire
by
> taking this approach.
>
> One other point is you you say that you have verified the voltage
of
> the 10-15V supply using two DVMs and it seems quite steady -- have
> you looked at it with a scope? There is a good chance you will
find
> some ripple which the DVMs are filtering out but that the A/D is
> seeing due to the speed of the sample/conversion. As others have
> pointed out, putting a cap close to the input to the PIC should
help
> filter this out. As others have also pointed out it is imperative
> that you not have too high of resistance as viewed from the PIC
> A/D. The fact remains that even following these guidelines you
are
> not going to acheive the accuracy you desire with a resistor
divider.



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Re: Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - Dwayne Reid - Apr 9 16:34:00 2004

At 12:44 PM 4/9/2004, d1camero wrote:

>BUT, when I removed the resistor divider network, and simply used
>the 12F675 to measure 0-5volts, the ADC works great, with an
>acceptable 1 bit fluctuation.
>
>So my question has changed: How can I accurately measure 10-15volts
>with the 675. Obviously a 1% resistor divider network introduces
>too much noise. Other ideas?

What value of resistors did you use? The specs state that the maximum
source impedance must be less than 2k5. You need a 3:1 divider where the
parallel value of the 2 resistors is less than 2k5 - this suggests the
divider resistors could be as high as 7k5 & 3k75.

The closest standard 1% values are 7k50 & 3k74. I'd put a 100R trimpot in
series with the bottom resistor and trim it for the correct reading out of
the A/D. This gives you an input impedance of about 10k.

I'd also add a cap (1u preferred, 100n OK) in parallel with the bottom
resistor if you can tolerate the slower response it gives you.

dwayne

--
Dwayne Reid <>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax

Celebrating 20 years of Engineering Innovation (1984 - 2004)
.-. .-. .-. .-. .-. .-. .-. .-. .-. .-
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Do NOT send unsolicited commercial email to this email address.
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Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - d1camero - Apr 10 9:11:00 2004

The source voltage under the scope looks fine. I am also using 1k
and 2k resistor for the voltage divider.

What alternatives are there?

Don

--- In , "Scott Lee" <midl_man@y...> wrote:
> --- In , "d1camero" <a8723@c...> wrote:
> > BUT, when I removed the resistor divider network, and simply
used
> > the 12F675 to measure 0-5volts, the ADC works great, with an
> > acceptable 1 bit fluctuation.
>
> It seems to me that Igor's explaination, which you seemed to
> discount, may be at least some of the problem.
>
> From what I can tell you are trying to measure 10-15V by using a
> resistor divider which will scale the 15V down to the necessary
5V.
> It will also scale that 10V down to 3.33V. Thus, although the A/D
> is measuring over a 0-5V range, the data of interest is in the
3.33-
> 5V range.
>
> You want 0.01V accuracy on the original 10-15V range. The
resistor
> divider is also scaling this just like it scaled the original
> voltage range. Think about it, in the original 10-15V range there
> would be 500 steps (501 levels) at the .01V increments you
desire.
> 500 steps in the 3.33-5V range works out to .0033V increments.
>
> Thus using your resistor divider to measure .01V increments over a
> 10-15V range will require that you measure .0033V increments over
a
> 3.33-5V range. Given that the A/D can measure to a resolution
> of .00488V but at best has an error of +-1 LSB this means that you
> are going to be +-0.015V at best.
>
> Now I know this doesn't explain all of it but you should see that
> there is no way you are going to get the .01V accuracy you desire
by
> taking this approach.
>
> One other point is you you say that you have verified the voltage
of
> the 10-15V supply using two DVMs and it seems quite steady -- have
> you looked at it with a scope? There is a good chance you will
find
> some ripple which the DVMs are filtering out but that the A/D is
> seeing due to the speed of the sample/conversion. As others have
> pointed out, putting a cap close to the input to the PIC should
help
> filter this out. As others have also pointed out it is imperative
> that you not have too high of resistance as viewed from the PIC
> A/D. The fact remains that even following these guidelines you
are
> not going to acheive the accuracy you desire with a resistor
divider.




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Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - Scott Lee - Apr 10 9:33:00 2004

--- In , "d1camero" <a8723@c...> wrote:
> The source voltage under the scope looks fine. I am also using 1k
> and 2k resistor for the voltage divider.
>
> What alternatives are there?

The cap that multiple people have suggested should smooth things but
you still have the problem that you will never get the accuracy that
you want.

You could always try the opamp as was mentioned here:
http://groups.yahoo.com/group/piclist/message/5438

--Scott




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Re: Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - Chad Russel - Apr 10 11:46:00 2004

What am I missing here. Can someone help???

I figure that measuring 15 volts with a .01 volt or 10 millivolt
accuracy is .067%. The Fluke 87 is .05% accurate and lists for
$349.00. A 12F675 costs how much?

My head hurts.

Chad

--- d1camero <> wrote:
> The source voltage under the scope looks fine. I am also using 1k
> and 2k resistor for the voltage divider.
>
> What alternatives are there?
>
> Don >
>
> --- In , "Scott Lee" <midl_man@y...> wrote:
> > --- In , "d1camero" <a8723@c...> wrote:
> > > BUT, when I removed the resistor divider network, and simply
> used
> > > the 12F675 to measure 0-5volts, the ADC works great, with an
> > > acceptable 1 bit fluctuation.
> >
> > It seems to me that Igor's explaination, which you seemed to
> > discount, may be at least some of the problem.
> >
> > From what I can tell you are trying to measure 10-15V by using a
> > resistor divider which will scale the 15V down to the necessary
> 5V.
> > It will also scale that 10V down to 3.33V. Thus, although the A/D
> > is measuring over a 0-5V range, the data of interest is in the
> 3.33-
> > 5V range.
> >
> > You want 0.01V accuracy on the original 10-15V range. The
> resistor
> > divider is also scaling this just like it scaled the original
> > voltage range. Think about it, in the original 10-15V range there
> > would be 500 steps (501 levels) at the .01V increments you
> desire.
> > 500 steps in the 3.33-5V range works out to .0033V increments.
> >
> > Thus using your resistor divider to measure .01V increments over a
> > 10-15V range will require that you measure .0033V increments over
> a
> > 3.33-5V range. Given that the A/D can measure to a resolution
> > of .00488V but at best has an error of +-1 LSB this means that you
> > are going to be +-0.015V at best.
> >
> > Now I know this doesn't explain all of it but you should see that
> > there is no way you are going to get the .01V accuracy you desire
> by
> > taking this approach.
> >
> > One other point is you you say that you have verified the voltage
> of
> > the 10-15V supply using two DVMs and it seems quite steady -- have
> > you looked at it with a scope? There is a good chance you will
> find
> > some ripple which the DVMs are filtering out but that the A/D is
> > seeing due to the speed of the sample/conversion. As others have
> > pointed out, putting a cap close to the input to the PIC should
> help
> > filter this out. As others have also pointed out it is imperative
> > that you not have too high of resistance as viewed from the PIC
> > A/D. The fact remains that even following these guidelines you
> are
> > not going to acheive the accuracy you desire with a resistor
> divider.

=====
My software has no bugs. Only undocumented features.

__________________________________


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Re: Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - David VanHorn - Apr 10 17:03:00 2004

At 09:46 AM 4/10/2004 -0700, Chad Russel wrote:

>What am I missing here. Can someone help???
>
>I figure that measuring 15 volts with a .01 volt or 10 millivolt
>accuracy is .067%. The Fluke 87 is .05% accurate and lists for
>$349.00. A 12F675 costs how much?

Is the reference voltage at least that accurate?
Most band-gaps in micros are pretty bad.

The pic probably doesn't do multiple functions, and true-RMS AC voltage reading.



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Re: Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) - Chad Russel - Apr 10 19:26:00 2004

I thought about this a bit. You can find many ap notes at maxim and
linear tech. Probably the cheapest way to do what you want is to use a
precision 5 volt or 10 volt reference to scale and level shift through
a good op amp/s. You should then be able to get by with a 10 bit AtoD
and 2 or 4 precision resistors depending if the reference is 5 or 10
volt.

Another way is to use a differential 10 bit AtoD with 10 and 15 volt
references. There are a few other ways, but probably more expensive.

Good luck,
Chad

--- Chad Russel <> wrote:
> What am I missing here. Can someone help???
>
> I figure that measuring 15 volts with a .01 volt or 10 millivolt
> accuracy is .067%. The Fluke 87 is .05% accurate and lists for
> $349.00. A 12F675 costs how much?
>
> My head hurts.
>
> Chad
>
> --- d1camero <> wrote:
> > The source voltage under the scope looks fine. I am also using 1k
> > and 2k resistor for the voltage divider.
> >
> > What alternatives are there?
> >
> > Don
> >
> >
> >
> >
> > --- In , "Scott Lee" <midl_man@y...> wrote:
> > > --- In , "d1camero" <a8723@c...> wrote:
> > > > BUT, when I removed the resistor divider network, and simply
> > used
> > > > the 12F675 to measure 0-5volts, the ADC works great, with an
> > > > acceptable 1 bit fluctuation.
> > >
> > > It seems to me that Igor's explaination, which you seemed to
> > > discount, may be at least some of the problem.
> > >
> > > From what I can tell you are trying to measure 10-15V by using a
> > > resistor divider which will scale the 15V down to the necessary
> > 5V.
> > > It will also scale that 10V down to 3.33V. Thus, although the
> A/D
> > > is measuring over a 0-5V range, the data of interest is in the
> > 3.33-
> > > 5V range.
> > >
> > > You want 0.01V accuracy on the original 10-15V range. The
> > resistor
> > > divider is also scaling this just like it scaled the original
> > > voltage range. Think about it, in the original 10-15V range
> there
> > > would be 500 steps (501 levels) at the .01V increments you
> > desire.
> > > 500 steps in the 3.33-5V range works out to .0033V increments.
> > >
> > > Thus using your resistor divider to measure .01V increments over
> a
> > > 10-15V range will require that you measure .0033V increments over
>
> > a
> > > 3.33-5V range. Given that the A/D can measure to a resolution
> > > of .00488V but at best has an error of +-1 LSB this means that
> you
> > > are going to be +-0.015V at best.
> > >
> > > Now I know this doesn't explain all of it but you should see that
>
> > > there is no way you are going to get the .01V accuracy you desire
>
> > by
> > > taking this approach.
> > >
> > > One other point is you you say that you have verified the voltage
>
> > of
> > > the 10-15V supply using two DVMs and it seems quite steady --
> have
> > > you looked at it with a scope? There is a good chance you will
> > find
> > > some ripple which the DVMs are filtering out but that the A/D is
> > > seeing due to the speed of the sample/conversion. As others have
>
> > > pointed out, putting a cap close to the input to the PIC should
> > help
> > > filter this out. As others have also pointed out it is
> imperative
> > > that you not have too high of resistance as viewed from the PIC
> > > A/D. The fact remains that even following these guidelines you
> > are
> > > not going to acheive the accuracy you desire with a resistor
> > divider.
> >
> > =====
> My software has no bugs. Only undocumented features.
>
> __________________________________
=====
My software has no bugs. Only undocumented features.

__________________________________





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