ADC

Started by nanoboticslover July 31, 2003
Hi!
I wonder what if I connect the load impedence of more than 10k
ohm to analog input of PIC 16F877. The recommended impedence is 10 k
ohm for PIC 16F877. I'd like to know what are the effect.

Regards.

bryan



Basically and crudely, you will lose accuracy because of input leakage
current, and set up time because of input capacitance. Some formulas
should be in app notes.

Chad

--- nanoboticslover <> wrote:
> Hi!
> I wonder what if I connect the load impedence of more than 10k
> ohm to analog input of PIC 16F877. The recommended impedence is 10 k
> ohm for PIC 16F877. I'd like to know what are the effect.
>
> Regards.
>
> bryan


__________________________________





You may increase the input impedance twice but must increase also the
aquisition time. The error will be negligible over the standard
temperature range.

top 10 wishes,
Vasile
http://surducan.netfirms.com On Thu, 31 Jul 2003, nanoboticslover wrote:

> Hi!
> I wonder what if I connect the load impedence of more than 10k
> ohm to analog input of PIC 16F877. The recommended impedence is 10 k
> ohm for PIC 16F877. I'd like to know what are the effect.
>
> Regards.
>
> bryan >
> to unsubscribe, go to http://www.yahoogroups.com and follow the instructions
>
> ">http://docs.yahoo.com/info/terms/




> I wonder what if I connect the load impedence of more than 10k
> ohm to analog input of PIC 16F877. The recommended impedence is 10 k
> ohm for PIC 16F877. I'd like to know what are the effect.

The accuracy will degrade. There was a thread about this on the MIT
piclist, you could search the archive.

Wouter van Ooijen

-- -------
Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products



Just for curiosity, have you measured how much lose of acuracy will
be hapened with 20K input impedance, in 0-20C temperature range ?

thx,
Vasile
http://surducan.netfirms.com On Wed, 30 Jul 2003, Chad Russel wrote:

> Basically and crudely, you will lose accuracy because of input leakage
> current, and set up time because of input capacitance. Some formulas
> should be in app notes.
>
> Chad
>
> --- nanoboticslover <> wrote:
> > Hi!
> > I wonder what if I connect the load impedence of more than 10k
> > ohm to analog input of PIC 16F877. The recommended impedence is 10 k
> > ohm for PIC 16F877. I'd like to know what are the effect.
> >
> > Regards.
> >
> > bryan
> >
> > __________________________________ > to unsubscribe, go to http://www.yahoogroups.com and follow the instructions
>
> ">http://docs.yahoo.com/info/terms/





Just grabbing a PIC spec sheet, input leakeage is +/- 1 microamp typ.
Therefore a 20K resistor = +/- 0.020 volts error. With a 3 volt
reference as an example, this is about +/- 2 lsb on a 8 bit A to D.

Chad

--- Vasile Surducan <> wrote:
>
> Just for curiosity, have you measured how much lose of acuracy will
> be hapened with 20K input impedance, in 0-20C temperature range ?
>
> thx,
> Vasile
> http://surducan.netfirms.com > On Wed, 30 Jul 2003, Chad Russel wrote:
>
> > Basically and crudely, you will lose accuracy because of input
> leakage
> > current, and set up time because of input capacitance. Some
> formulas
> > should be in app notes.
> >
> > Chad
> >
> > --- nanoboticslover <> wrote:
> > > Hi!
> > > I wonder what if I connect the load impedence of more than 10k
> > > ohm to analog input of PIC 16F877. The recommended impedence is
> 10 k
> > > ohm for PIC 16F877. I'd like to know what are the effect.
> > >
> > > Regards.
> > >
> > > bryan
> > >
> > >
> >
> >
> > __________________________________
> >
> >
> > to unsubscribe, go to http://www.yahoogroups.com and follow the
> instructions
> >
> > ">http://docs.yahoo.com/info/terms/
> >
>


__________________________________






Well, surprize !
Just grabbing the data sheet, the result is wrong...
First the typicaly leakage current in that temperature range is just
+/-0.5uA in the worst case. Then, for full AD and +5V reference, a
standard analog resolution is 4.8mV. With 20K input the error will be
10mV which means 2lsb.
Now, if you are an intensive user of AD, you know that 1lsb is a normal
error for most applications. So, you have add 1 lsb error with a 20K input
impedance which is acceptable. In practice is quite well.

best regards,
Vasile On Fri, 1 Aug 2003, Chad Russel wrote:

> Just grabbing a PIC spec sheet, input leakeage is +/- 1 microamp typ.
> Therefore a 20K resistor = +/- 0.020 volts error. With a 3 volt
> reference as an example, this is about +/- 2 lsb on a 8 bit A to D.
>
> Chad
>
> --- Vasile Surducan <> wrote:
> >
> > Just for curiosity, have you measured how much lose of acuracy will
> > be hapened with 20K input impedance, in 0-20C temperature range ?
> >
> > thx,
> > Vasile
> > http://surducan.netfirms.com
> >
> >
> > On Wed, 30 Jul 2003, Chad Russel wrote:
> >
> > > Basically and crudely, you will lose accuracy because of input
> > leakage
> > > current, and set up time because of input capacitance. Some
> > formulas
> > > should be in app notes.
> > >
> > > Chad
> > >
> > > --- nanoboticslover <> wrote:
> > > > Hi!
> > > > I wonder what if I connect the load impedence of more than 10k
> > > > ohm to analog input of PIC 16F877. The recommended impedence is
> > 10 k
> > > > ohm for PIC 16F877. I'd like to know what are the effect.
> > > >
> > > > Regards.
> > > >
> > > > bryan
> > > >
> > > >
> > >
> > >
> > > __________________________________
> > >
> > >
> > > to unsubscribe, go to http://www.yahoogroups.com and follow the
> > instructions
> > >
> > > ">http://docs.yahoo.com/info/terms/
> > >
> > >
> >
> > __________________________________ > to unsubscribe, go to http://www.yahoogroups.com and follow the instructions
>
> ">http://docs.yahoo.com/info/terms/





You are correct Vasile, except I do not see the +/-0.5uA in my 877
spec. I find +/-1.0uA and I assumed a 3.3 volt ref. Perhaps my spec is
out of date.

Chad

--- Vasile Surducan <> wrote:
>
>
> Well, surprize !
> Just grabbing the data sheet, the result is wrong...
> First the typicaly leakage current in that temperature range is just
> +/-0.5uA in the worst case. Then, for full AD and +5V reference, a
> standard analog resolution is 4.8mV. With 20K input the error will be
> 10mV which means 2lsb.
> Now, if you are an intensive user of AD, you know that 1lsb is a
> normal
> error for most applications. So, you have add 1 lsb error with a 20K
> input
> impedance which is acceptable. In practice is quite well.
>
> best regards,
> Vasile > On Fri, 1 Aug 2003, Chad Russel wrote:
>
> > Just grabbing a PIC spec sheet, input leakeage is +/- 1 microamp
> typ.
> > Therefore a 20K resistor = +/- 0.020 volts error. With a 3 volt
> > reference as an example, this is about +/- 2 lsb on a 8 bit A to D.
> >
> > Chad
> >
> > --- Vasile Surducan <> wrote:
> > >
> > > Just for curiosity, have you measured how much lose of acuracy
> will
> > > be hapened with 20K input impedance, in 0-20C temperature range ?
> > >
> > > thx,
> > > Vasile
> > > http://surducan.netfirms.com
> > >
> > >
> > > On Wed, 30 Jul 2003, Chad Russel wrote:
> > >
> > > > Basically and crudely, you will lose accuracy because of input
> > > leakage
> > > > current, and set up time because of input capacitance. Some
> > > formulas
> > > > should be in app notes.
> > > >
> > > > Chad
> > > >
> > > > --- nanoboticslover <> wrote:
> > > > > Hi!
> > > > > I wonder what if I connect the load impedence of more than
> 10k
> > > > > ohm to analog input of PIC 16F877. The recommended impedence
> is
> > > 10 k
> > > > > ohm for PIC 16F877. I'd like to know what are the effect.
> > > > >
> > > > > Regards.
> > > > >
> > > > > bryan
> > > > >
> > > > >
> > > >
> > > >
> > > > __________________________________
> > > >
> > > >
> > > > to unsubscribe, go to http://www.yahoogroups.com and follow the
> > > instructions
> > > >
> > > > ">http://docs.yahoo.com/info/terms/
> > > >
> > > >
> > >
> > >
> >
> >
> > __________________________________
> >
> >
> > to unsubscribe, go to http://www.yahoogroups.com and follow the
> instructions
> >
> > ">http://docs.yahoo.com/info/terms/
> >
>


__________________________________