Reply by Charles Linquist●February 9, 20042004-02-09
No. When I mentioned "-Vref", I didn't mean the -VREF on the
PIC. I simply
meant a negative voltage equal to VREF.
Using the technique that I described, you don't need -VREF. You use the
A/D
as you normally would. Use GND for -Vref.
I use a 2K from 4.096 V reference to the A/D input, and a 7.5K resistor
from the
A/D input to the measured voltage (-12V in my case).
You then have to subtract the A/D reading from a constant to obtain the
correct
reading, since the greater the negative voltage, the smaller the A/D output.
Using 1% resistors and the 10bit converter in most of the PICs, I find that
my
accuracy is usually within 15mV when reading a nom -12V level.
I should mention that I'm using a .2% tolerance part for my 4.096V
reference.
Charles Linquist
the data sheet pinout for the F87 implies the the 87 doesn't have
VRef+ and Vref-. It appears its brother the F88 does, though.
Also, you lose AN2 and AN3 when using Vref+/- although the data sheet
is not explicit on this point.
Phil
--- In , Chad Russel <chadrussel@y...> wrote: > Good idea, just keep in mind that the accuracy is
degraded by the ratio > and tolerance of the resistors.
>
> Chad
>
> --- Dave Mucha <davemucha@j...> wrote:
> > --- In , "Charles Linquist" <Charles@L...>
> > wrote:
> > >
> > > The A/D of a PIC can easily measure a negative voltage. Simply
> > build a
> > > 2-resistor voltage divider that is connected to Vref at one end, > > the A/D
> > > input at the middle and your voltage to be measured at the other > > end.
> > >
> > > For example: If you connect a 7.5K resistor between Vref the A/D > > channel,
> > > and another 7.5K resistor between the A/D channel and your voltage > > to be
Reply by Phil●February 9, 20042004-02-09
the data sheet pinout for the F87 implies the the 87 doesn't have
VRef+ and Vref-. It appears its brother the F88 does, though.
Also, you lose AN2 and AN3 when using Vref+/- although the data sheet
is not explicit on this point.
Phil
--- In , Chad Russel <chadrussel@y...> wrote: > Good idea, just keep in mind that the accuracy is
degraded by the ratio > and tolerance of the resistors.
>
> Chad
>
> --- Dave Mucha <davemucha@j...> wrote:
> > --- In , "Charles Linquist" <Charles@L...>
> > wrote:
> > >
> > > The A/D of a PIC can easily measure a negative voltage. Simply
> > build a
> > > 2-resistor voltage divider that is connected to Vref at one end, > > the A/D
> > > input at the middle and your voltage to be measured at the other > > end.
> > >
> > > For example: If you connect a 7.5K resistor between Vref the A/D > > channel,
> > > and another 7.5K resistor between the A/D channel and your voltage > > to be
> > > measured, then at 0 volts in, the A/D will read Vref/2. As the
> > voltage gets
> > > more negative, the A/D will produce a smaller and smaller result, > > and will
> > > read 0 volts at -Vref.
> > >
> > > The math can get a little messy, but I have done it many times. > > Contact me
> > > off-list if you need more information.
> > >
> > > Charles Linquist
> > > Charles@L...
> >
> > Hi Charles,
> >
> > Please keep this on-list. This is the great type of creative use
> > that makes this list so useful.
> >
> >
> > Dave
> >
> >
> >
>
> =====
> My software has no bugs. Only undocumented features.
>
> __________________________________
>
Reply by Chad Russel●February 9, 20042004-02-09
Good idea, just keep in mind that the accuracy is degraded by the ratio
and tolerance of the resistors.
Chad
--- Dave Mucha <> wrote: > --- In , "Charles Linquist"
<Charles@L...>
> wrote:
> >
> > The A/D of a PIC can easily measure a negative voltage. Simply
> build a
> > 2-resistor voltage divider that is connected to Vref at one end,
> the A/D
> > input at the middle and your voltage to be measured at the other
> end.
> >
> > For example: If you connect a 7.5K resistor between Vref the A/D
> channel,
> > and another 7.5K resistor between the A/D channel and your voltage
> to be
> > measured, then at 0 volts in, the A/D will read Vref/2. As the
> voltage gets
> > more negative, the A/D will produce a smaller and smaller result,
> and will
> > read 0 volts at -Vref.
> >
> > The math can get a little messy, but I have done it many times.
> Contact me
> > off-list if you need more information.
> >
> > Charles Linquist
> > Charles@L...
>
> Hi Charles,
>
> Please keep this on-list. This is the great type of creative use
> that makes this list so useful.
> Dave
>
>
=====
My software has no bugs. Only undocumented features.
__________________________________
Reply by Dave Mucha●February 9, 20042004-02-09
--- In , "Charles Linquist" <Charles@L...>
wrote: >
> The A/D of a PIC can easily measure a negative voltage. Simply build a > 2-resistor voltage divider that is connected to
Vref at one end, the A/D > input at the middle and your voltage to be
measured at the other end. >
> For example: If you connect a 7.5K resistor between Vref the A/D channel, > and another 7.5K resistor between the A/D channel
and your voltage to be > measured, then at 0 volts in, the A/D will read
Vref/2. As the voltage gets > more negative, the A/D will produce a smaller and
smaller result, and will > read 0 volts at -Vref.
>
> The math can get a little messy, but I have done it many times. Contact me > off-list if you need more information.
>
> Charles Linquist
> Charles@L...
Hi Charles,
Please keep this on-list. This is the great type of creative use
that makes this list so useful.
Dave
Reply by Charles Linquist●February 9, 20042004-02-09
The A/D of a PIC can easily measure a negative voltage. Simply build a
2-resistor voltage divider that is connected to Vref at one end, the A/D
input at the middle and your voltage to be measured at the other end.
For example: If you connect a 7.5K resistor between Vref the A/D channel,
and another 7.5K resistor between the A/D channel and your voltage to be
measured, then at 0 volts in, the A/D will read Vref/2. As the voltage gets
more negative, the A/D will produce a smaller and smaller result, and will
read 0 volts at -Vref.
The math can get a little messy, but I have done it many times. Contact me
off-list if you need more information.
Charles Linquist
-----Original Message-----
From: nibnewux [mailto:]
Sent: Sunday, February 08, 2004 9:20 PM
To:
Subject: [piclist] Can the ADC module of the PIC16F87 accept a negative
analog input?
If it can be done, what would be the equivalent digital representation, say
for -2V?
--- nibnewux <> wrote: > If it can be done, what would be the equivalent
digital
> representation, say for -2V?
It can not, directly, if it could, the output would depend on a lot of
things, references, bipolar AtoD, or single run at a lower supply.
Chad.
=====
My software has no bugs. Only undocumented features.
__________________________________
Reply by nibnewux●February 9, 20042004-02-09
If it can be done, what would be the equivalent digital
representation, say for -2V?