Post by thread:[PIC]A2D Vref Diode

hello, Anybody can recommend 3 volt Vref Diode.. I cant use a 2.5volt such as LM336BZ-2.5 as the datasheet for pic18f242 says: (VREFH - VREFL) must be minimum of 3V when Vdd >= 3V.. is this correct as I've heard on other PICS its 2.5volts.. maybe I could use a zener but unfortunately I could not find any zeners here that has 1% tolerance.. thanks..

On 10/3/06, genome <spam_OUTjtroxasTakeThisOuTgmail.com> wrote: > > hello, > Anybody can recommend 3 volt Vref Diode.. I cant use a 2.5volt such as > LM336BZ-2.5 as the datasheet for pic18f242 says: > (VREFH - VREFL) must be minimum of 3V when Vdd >= 3V.. > is this correct as I've heard on other PICS its 2.5volts.. maybe I could > use > a zener but unfortunately I could not find any zeners here that has 1% > tolerance.. Your VCC is probably at least that accurate.

I'am trying to match 0.5-1.05 volt sensor output using a single supply opamp to a 0.05-3volt references.. the 0.05trim is to acount for the opamp output not being able to swing exactly to the voltage level on its powerrails.. i.e. I'm using a virtual ground.. > Your VCC is probably at least that accurate. I dont want to waste a2d counts as I need to have good A2D resolution so I cant exactly use the VCc as a reference. If I simply use 0-1volt sensor output and amplfy it to 0-5volts.. I would be losing counts and will not be able to detect voltages close to 5volts and 0volts as I know of no opamp that can swing to their rail voltages.. even those rail-rail opamps cant do that.. thanks.. "David VanHorn" <.....dvanhornKILLspam@spam@microbrix.com> wrote in message news:25b178740610031250n66006c55u996ef14d8d1bbc67KILLspammail.gmail.com...> {Quote hidden}> On 10/3/06, genome <.....jtroxasKILLspam.....gmail.com> wrote: >> >> hello, >> Anybody can recommend 3 volt Vref Diode.. I cant use a 2.5volt such as >> LM336BZ-2.5 as the datasheet for pic18f242 says: >> (VREFH - VREFL) must be minimum of 3V when Vdd >= 3V.. >> is this correct as I've heard on other PICS its 2.5volts.. maybe I could >> use >> a zener but unfortunately I could not find any zeners here that has 1% >> tolerance.. > > > Your VCC is probably at least that accurate. > --

16F series need somewhere between 2.5V to 2.8V. I doubt 18F philosophy on AD is different. The best choice could be a 3V linear regulator which gives you both the supply and the reference. Like LT1763-3 ? Vasile On 10/3/06, genome <EraseMEjtroxasspam_OUTTakeThisOuTgmail.com> wrote: {Quote hidden}> hello, > Anybody can recommend 3 volt Vref Diode.. I cant use a 2.5volt such as > LM336BZ-2.5 as the datasheet for pic18f242 says: > (VREFH - VREFL) must be minimum of 3V when Vdd >= 3V.. > is this correct as I've heard on other PICS its 2.5volts.. maybe I could use > a zener but unfortunately I could not find any zeners here that has 1% > tolerance.. > > thanks.. > > > > -

will it posible be posible to just amplify 2.5volt refrence to 3volts using an opamp.. If I do it this way will it not be unstable and susceptible to logic load noise.. anybody did somthing like this.. thanks.. "Vasile Surducan" <piclist9spam_OUTgmail.com> wrote in message news:@spam@5eeda4c20610032225k7d0f1b0bta4c7c1b24c2e7871KILLspammail.gmail.com...> {Quote hidden}> 16F series need somewhere between 2.5V to 2.8V. > I doubt 18F philosophy on AD is different. > The best choice could be a 3V linear regulator which gives you both > the supply and the reference. Like LT1763-3 ? > > Vasile > > On 10/3/06, genome <KILLspamjtroxasKILLspamgmail.com> wrote: >> hello, >> Anybody can recommend 3 volt Vref Diode.. I cant use a 2.5volt such as >> LM336BZ-2.5 as the datasheet for pic18f242 says: >> (VREFH - VREFL) must be minimum of 3V when Vdd >= 3V.. >> is this correct as I've heard on other PICS its 2.5volts.. maybe I could >> use >> a zener but unfortunately I could not find any zeners here that has 1% >> tolerance.. >> >> thanks.. >> >> >> >> --

On 10/4/06, genome <RemoveMEjtroxasTakeThisOuTgmail.com> wrote: > will it posible be posible to just amplify 2.5volt refrence to 3volts using > an opamp.. If I do it this way will it not be unstable and susceptible to > logic load noise.. anybody did somthing like this.. > > thanks.. > This will work fine. You can even buy a precision reference and amplifier combined in the same package. This combination is called a "linear regulator." Regards, Mark markrages@gmail -- You think that it is a secret, but it never has been one. - fortune cookie

I am not sure if this will help you... Connect the 2.5V reference to any AD pin. Take a reading. Use this as a reference standard. Lets say, your AD reads 500 (0x1F4) for 2.5V reference. At another AD pin, if you get a reading of 1000 (0x3E8), the input is 5.0V. HTH, Mohit. {Original Message removed}

> able to detect voltages close to 5volts and 0volts as I know of no > opamp that can swing to their rail voltages.. even those rail-rail > opamps cant do that.. Some r-r opamps have inbuilt power supply inverters to give them internal rails greater than rail so they can do the impossible. You can often do the same yourself if you don't mind being naughty or clever by having some dropping resistance in the supply or using voltage upstream of the regulator. Applicability varies with application. Russell