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dbsjro
Joined: 31 Aug 2009
Posts: 17
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| A/D quick question |
 Posted: Sat Oct 24, 2009 5:38 am |
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Hello
I've used Vref_vref at a voltmeter I made and I came up with one question.
What's the difference between:
Vref_vref
Vref_Vdd
Vref_Vss
Thnx |
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Ttelmah
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| Posted: Sat Oct 24, 2009 3:41 pm |
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The ADC, internally has two connections that generate the 'range' over which it operates. VrefL and VrefH (the bottom and top of it's range). The definitons control what these are connected to.
VrefL, can be connected internally to the Vss pin, or to the separate VrefL external pin. Similarly, VrefH can be connected to Vdd or to it's external pin.
The data sheet shows details.
In the 'defines', the first value determines where VrefL comes from, and the second, where VrefH comes from.
Advantages of using an external reference, are:
You can select a voltage range that better suits your incoming signal (caveat, the PIC, normally needs something in the order of 2.5v between VrefL and VrefH to give it's full accuracy - data sheet again).
You can use a better regulated source.
You can maintain better isolation between the analog circuitry, and the digital stuff.
These all help accuracy. People who are genuinely getting 10bits or more from the ADC, _will_ be using these.
Best Wishes |
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Guest
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| Posted: Sat Oct 24, 2009 5:00 pm |
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Can you or anyone mention some examples ?
For instance, when do I connect Vref- or Vref+ (and where) ?
And when do I use Vref_vref or Vref_Vdd or Vref_Vss ?
Thnx |
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dbsjro
Joined: 31 Aug 2009
Posts: 17
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| Posted: Sat Oct 24, 2009 5:09 pm |
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So when I use Vref_Vref what does that mean?
The first Vref is for VrefL and the second for VrefL?
And when I use Vref_Vdd
The first Vref is for VrefL and Vdd for VrefL?
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sat Oct 24, 2009 9:31 pm |
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| Quote: | | So when I use Vref_Vref what does that mean? |
It means you are using external Vref voltages for both Vref+ and Vref-.
| Quote: | | And when I use Vref_Vdd |
It means Vdd is used for Vref+, and that you are supplying an external
voltage for Vref-.
| Quote: |
Can you or anyone mention some examples ?
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Suppose that you had an analog voltage that would never be higher
than about 4.0 volts. Suppose that you wanted to make one "step" on
the A/D be an even number of millivolts. Then you could use an external
reference voltage for Vref+ of 4.096 volts. (Using VSS_VREF). This would
mean that in 10-bit mode, one "step" on the A/D would be worth 4.0 mv.
Why is that nice ? Because the math is now very easy to convert from the
raw A/D value into a voltage. Let's say you read an A/D value of 525. To
convert that to volts, just multiply it by 4. That multiply operation can be
done by the compiler with two "left-shifts". It's quick and uses very little
ROM space. 525 x 4 = 2100 mv, which is 2.100 volts. |
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vinniewryan
Joined: 29 Jul 2009
Posts: 154
Location: at work
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| Posted: Sat Oct 24, 2009 9:35 pm |
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| dbsjro wrote: | So when I use Vref_Vref what does that mean?
The first Vref is for VrefL and the second for VrefL?
And when I use Vref_Vdd
The first Vref is for VrefL and Vdd for VrefL?
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Basically, when you're using an ADC, you need to tell it what it's maximum voltage is, as well as its minimum. Think of it as setting a threshold. When you say VREF_VSS, the vref is your maximum voltage, and VSS is your minimum voltage. Vref refers to the vref pin on your pic, vss refers to the VSS pin, and VDD refers to the VCC pin. So if your PIC was powered at VDD by 5.0V, and grounded at VSS, then VDD_VSS would mean that the maximim voltage is going to be 5.0v (VDD), and the minimum voltage will be 0V (VSS, or Ground). If want to use the VREF pin on your pic as your maximum voltage, then set it as VREF_VSS.
I believe this is inaccurate, but it should give you the basic idea of how it works. If you understand it better, you can look up the correct order of VREFL and VREFH, or someone here might correct me.
_________________
Vinnie Ryan |
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dbsjro
Joined: 31 Aug 2009
Posts: 17
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| Posted: Sun Oct 25, 2009 5:22 am |
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vinniewryan I think you are wrong.
First Vref is for Vref- and the second for Vref+.
Ok I understand it now.
I use
| Code: | | setup_adc_ports(AN0_VREF_VREF); |
But it shows an error when I change it to
| Code: | | setup_adc_ports(AN0_Vss_VREF); |
My program runs ok the way it is but I wanna know about the error. |
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vinniewryan
Joined: 29 Jul 2009
Posts: 154
Location: at work
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| Posted: Sun Oct 25, 2009 11:25 am |
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Try this:
| Code: |
setup_adc_ports(AN0 | Vss_VREF);
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I simply separated the AN0 from the Vss_VREF command. If you want to add more pins using the ADC, set it up like this:
| Code: |
setup_adc_ports(AN0 | AN3 | AN4 | VSS_VREF);
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Hope this helps! Let me know your result.
_________________
Vinnie Ryan |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sun Oct 25, 2009 12:16 pm |
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| Quote: | | setup_adc_ports(AN0 | AN3 | AN4 | VSS_VREF); |
He has never posted his PIC (or his compiler version). We don't know
what ADC constants are in the .h file for his PIC. |
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vinniewryan
Joined: 29 Jul 2009
Posts: 154
Location: at work
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| Posted: Sun Oct 25, 2009 12:29 pm |
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Good point, I simply assumed he/she was using a 16F628 based on his/her previous posts.
_________________
Vinnie Ryan |
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dbsjro
Joined: 31 Aug 2009
Posts: 17
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| Posted: Sun Oct 25, 2009 1:48 pm |
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I use 16f877
version 4.057
| Code: | | setup_adc_ports(AN0 | Vss_VREF); |
Error 12......Udefined identifier Vss_VREF
I thing it just dosen't support it right?(The pic) |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sun Oct 25, 2009 1:53 pm |
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Look in the 16F877.h file to see the list of constants that may be used
with setup_adc_ports() for that PIC. Here's the file location:
| Quote: |
c:\program files\picc\devices\16f877.h |
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dbsjro
Joined: 31 Aug 2009
Posts: 17
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| Posted: Sun Oct 25, 2009 2:03 pm |
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It doesn't support it.
Anyway my job is already done.
It was just a thought.
Thnx both  |
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