Post by thread:[PIC]: 16F877, RA3, Vref?

What is the purpose of having this Vref? Why not use the internal 5V reference all the time? If it is for accuracy reasons, then if your 5v line goes wacky... then can you count on the PIC during that time anyway? Thanks, Brandon -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Thu, Jun 27, 2002 at 10:22:42PM -0400, Brandon Stewart wrote: > What is the purpose of having this Vref? Why not use the internal 5V > reference all the time? Two reasons: Narrow the range for better resolution and to ease the process of conversion. > If it is for accuracy reasons, then if your 5v line > goes wacky... then can you count on the PIC during that time anyway? Doesn't matter. Here's a quick example: Say the range of what you're measuring goes from 0V to 4V. So you use a 4.096V reference. You get 3 instant effects: 1) You get nearly full range on your conversion: 0..1000 instead of 0..820 2) You get a smaller voltage step between steps (0.004V vs 0.0048828125V) and. 3) You get a much easier conversion since the steps are exactly 4 mV each. Hope this helps, BAJ -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Thu, 27 Jun 2002 22:22:42 -0400, Brandon Stewart wrote: >What is the purpose of having this Vref? Why not use the internal >5V reference all the time? If it is for accuracy reasons, then if >your 5v line goes wacky... then can you count on the PIC during that >time anyway? Quite a few sensors have analog signals in the 0-2.5 volt range. By using a different VREF you get finer readings. Tim -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

A 3rd reason to add to what Byron said: Most 5V regulators are +/- 5%, maybe greater. It's easy to find a 1% or less voltage reference. Your uncalibrated accuracy is much better in this case, even if you use a 5V external reference because the voltage spread between a number of units will be much less. Matt Pobursky Maximum Performance Systems On Thu, 27 Jun 2002 22:22:42 -0400, Brandon Stewart wrote: >What is the purpose of having this Vref? Why not use the internal 5V >reference all the time? If it is for accuracy reasons, then if your >5v line >goes wacky... then can you count on the PIC during that time anyway? > > > >Thanks, >Brandon > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> What is the purpose of having this Vref? Why not use the internal 5V > reference all the time? 1 - Accuracy. Precision voltage references are much more accurate than typical 5V regulators. A basic regulator like the LM7805 only guarantees 4.8 to 5.2 volts even at 25C. A PIC 10 bit D/A referenced to that isn't even accurate to 4 bits. 2 - Range. The Vref inputs can be used to narrow the range of voltages that map to full scale, within limits. 3 - V+ unknown. All PICs don't have to run exactly on 5V. The L versions have a wide voltage latitude, which can allow them to be powered from batteries without any voltage regulation. In that case the power voltage can't be used as a reference at all. You either have to make sure input voltages are meaningful as a fraction of the power voltage (using bridge circuits, for example), or you need a fixed external reference. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Brandon, Vcc is not always 5.0000 Volts. Take the following example: The a/d converter outputs a value between 0 and 0x3ff regardless of the reference used. If you use Vcc for the reference and it is exactly 5.0 volts, each step is 5.0 / 0x3ff = 4.88 mV per step. Now, lets say that you want to measure a 5.0 volt nominal input using a resistor divider consisting of 2 resistors of the same value (for simplicity). This means that the a/d actually sees 2.5 volts which would be converted to a value of 2.5 / 4.88 mV = 512. Now, lets assume Vcc is 4.5 V. 4.5 / 0x3ff = 4.39 mV per step, so measuring the same 2.5 volt input would result in a value of 2.5 / 4.39 mV = 569. This is a huge loss of accuracy which may or may not be acceptable. When the PIC software actually uses the returned value and it assumes the original 4.88 mV per step, the 569 returned value would be calculated as 569 * 4.88mV = 2.78V at the center of the divider or 2.78*2 = 5.56V at the input - an 11% error. If you use VRef with a tight tolerance reference, the number should be accurate within a bit or 2, depending on noise. With 2 bits of error, the dither (independent of Vcc) would be 4*4.88mV = .019V at the divider, or .019*2 = .039V at the input as calculated by the software. Which would you rather have, an error of 0.56V or an error of 0.039V? Again, it depends on the accuracy required. Also, remember that several PICs can run fine well below 4.75V. I've seen parts spec'd at 4.75V min run fine at about 2.0 Volts. Do you still want accurate reporting if Vcc is "out of spec"? Now, your turn. Do a similar analysis for the "valid" Vcc range of 5.25V to 4.75V with the software still assuming 5.0V nominal. Ken {Original Message removed}