Input data to LTC1298.....preamped electrodes?
Archiver
Posts: 46,084
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/
>
>
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/
>
>