broken link to .preamped electrodes
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Posts: 46,084
Sorry, this link works.
Dave
http://www.omega.com/toc_asp/frameset.html?book=Green&file=PHE6300_5300_2114
_1304
Original Message
From: <samhell@s...>
To: <basicstamps@yahoogroups.com>
Sent: Monday, July 30, 2001 10:52 PM
Subject: Re: [noparse][[/noparse]basicstamps] Input data to LTC1298.....preamped electrodes?
> these are advertised to overcome the issue Tracy spoke about.......any
> opinion?........
>
>
http://www.omega.com/toc_asp/frameset.html?book=Green&file=PHE6300_5300_2114
> _1304
>
> Dave
>
Original Message
> From: <samhell@s...>
> To: <basicstamps@yahoogroups.com>
> Sent: Monday, July 30, 2001 10:23 PM
> Subject: Re: [noparse][[/noparse]basicstamps] Input data to LTC1298
>
>
> > Hi Sid,
> >
> > It is a 2 wire device, connected via a BNC connector......it is the
> > "economy electrode" at http://www.appliedanalytical.com/
> > I have played around with it with a meter , and discovered that, like
> Tracy
> > said, it outputs both + and - values depending on the fluid its soaked
> > it....( ie. acidic CocaCola....or something alkaline like Texas tap
> water )
> >
> >
> > Tracy,
> >
> > in my searching, I stumbled across this page...to me it sortof
> descibes
> > the type of of amp circuit that may help me....
> > http://seniord.ee.iastate.edu/may9918/design_review.html ...but just
for
> my
> > knowledge.....you're saying that because of the impedence,
> > I must amplify and buffer the signal before the LTC1298 will respond
> with
> > useful data? I have a few questions ( If you dont mind )
> >
> > 1. would another ADC give better results? ( ie a MAX1287 or some other
> > part?_)
> > 2. do the pH mini testers that are commercially available, use op-amp
> > circuitry to alleviate the problems Im having?
> > 3. I also would like to add a disolved solids meter (ppm) to measure
the
> > consumption of nutrients by plants....has anyone had any experience with
> > these sensors? would they seem to be "easy" sensors like the DS1620 or
> > AD590 or more like what Im fighting now?
> > 4. heres a really dumb question....... what exactly is the data the
goes
> > acrosss the debug screen at the conclusion of the LTC1298 APP kit? The
> > comments say "sampling of channel 1 and 2" it makes plently of stuff
> even
> > though no inputs are connected.....
> >
> > Thanks for your help everyone
> >
> > Dave
> >
> >
> >
> > ////////////////////
> > Is your pH device a 2-wire or 3-wire device?
> >
> > Sid
> > /////////////////////////
> >
> > pH electrodes are like little batteries. The full scale output is
> > about +/- 0.4 volts for the full +/- 7 pH range. If you want to
> > cover both acid and base, you will need the converter to respond to
> > both + and - voltages.
> >
> >
> > > The kicker is that the output impedance of pH electrodes is extremely
> > > high. They act like a little battery, but with a 10 to 50 megaohm
> > > resistor in series. The voltmeter that measures the output of a pH
> > > electrode has to have extremely high input impedance, 1 teraohm or
> > > more. Even many digital multimeters, which have 10 or 20 megaohms of
> > > input resistance, will load down a pH electrode and give a reading
> > > that is much lower than it should be.
> > >
> > > While the LTC1298 has high impedance inputs, they recommend using a
> > > source resistance of 1 kohm or less.That is a long way below 10
> > > megaohms. It might be possible to make it work by putting a
> > > capacitor across the pH output, to absorb the charge that the LTC1298
> > > kicks out when it makes each reading. But that current would have to
> > > settle out through the pH electrode, which would mean a long settling
> > > time between readings, and a shortened lifetime for the pH electrode
> > > itself.
> > >
> > > The usual approach to pH electrodes is to amplify and buffer the
> > > signal, with an MOS or CMOS input operational amplifier. There is an
> > > example circuit in the CMOS cookbook, if you can get your hands on a
> > > copy of that.
> >
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the Subject
and
> Body of the message will be ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> >
> >
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
Dave
http://www.omega.com/toc_asp/frameset.html?book=Green&file=PHE6300_5300_2114
_1304
Original Message
From: <samhell@s...>
To: <basicstamps@yahoogroups.com>
Sent: Monday, July 30, 2001 10:52 PM
Subject: Re: [noparse][[/noparse]basicstamps] Input data to LTC1298.....preamped electrodes?
> these are advertised to overcome the issue Tracy spoke about.......any
> opinion?........
>
>
http://www.omega.com/toc_asp/frameset.html?book=Green&file=PHE6300_5300_2114
> _1304
>
> Dave
>
Original Message
> From: <samhell@s...>
> To: <basicstamps@yahoogroups.com>
> Sent: Monday, July 30, 2001 10:23 PM
> Subject: Re: [noparse][[/noparse]basicstamps] Input data to LTC1298
>
>
> > Hi Sid,
> >
> > It is a 2 wire device, connected via a BNC connector......it is the
> > "economy electrode" at http://www.appliedanalytical.com/
> > I have played around with it with a meter , and discovered that, like
> Tracy
> > said, it outputs both + and - values depending on the fluid its soaked
> > it....( ie. acidic CocaCola....or something alkaline like Texas tap
> water )
> >
> >
> > Tracy,
> >
> > in my searching, I stumbled across this page...to me it sortof
> descibes
> > the type of of amp circuit that may help me....
> > http://seniord.ee.iastate.edu/may9918/design_review.html ...but just
for
> my
> > knowledge.....you're saying that because of the impedence,
> > I must amplify and buffer the signal before the LTC1298 will respond
> with
> > useful data? I have a few questions ( If you dont mind )
> >
> > 1. would another ADC give better results? ( ie a MAX1287 or some other
> > part?_)
> > 2. do the pH mini testers that are commercially available, use op-amp
> > circuitry to alleviate the problems Im having?
> > 3. I also would like to add a disolved solids meter (ppm) to measure
the
> > consumption of nutrients by plants....has anyone had any experience with
> > these sensors? would they seem to be "easy" sensors like the DS1620 or
> > AD590 or more like what Im fighting now?
> > 4. heres a really dumb question....... what exactly is the data the
goes
> > acrosss the debug screen at the conclusion of the LTC1298 APP kit? The
> > comments say "sampling of channel 1 and 2" it makes plently of stuff
> even
> > though no inputs are connected.....
> >
> > Thanks for your help everyone
> >
> > Dave
> >
> >
> >
> > ////////////////////
> > Is your pH device a 2-wire or 3-wire device?
> >
> > Sid
> > /////////////////////////
> >
> > pH electrodes are like little batteries. The full scale output is
> > about +/- 0.4 volts for the full +/- 7 pH range. If you want to
> > cover both acid and base, you will need the converter to respond to
> > both + and - voltages.
> >
> >
> > > The kicker is that the output impedance of pH electrodes is extremely
> > > high. They act like a little battery, but with a 10 to 50 megaohm
> > > resistor in series. The voltmeter that measures the output of a pH
> > > electrode has to have extremely high input impedance, 1 teraohm or
> > > more. Even many digital multimeters, which have 10 or 20 megaohms of
> > > input resistance, will load down a pH electrode and give a reading
> > > that is much lower than it should be.
> > >
> > > While the LTC1298 has high impedance inputs, they recommend using a
> > > source resistance of 1 kohm or less.That is a long way below 10
> > > megaohms. It might be possible to make it work by putting a
> > > capacitor across the pH output, to absorb the charge that the LTC1298
> > > kicks out when it makes each reading. But that current would have to
> > > settle out through the pH electrode, which would mean a long settling
> > > time between readings, and a shortened lifetime for the pH electrode
> > > itself.
> > >
> > > The usual approach to pH electrodes is to amplify and buffer the
> > > signal, with an MOS or CMOS input operational amplifier. There is an
> > > example circuit in the CMOS cookbook, if you can get your hands on a
> > > copy of that.
> >
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the Subject
and
> Body of the message will be ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> >
> >
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>