RTD linearization
in [Design]
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From: Jim Thompson on 13 Dec 2009 16:04 On Sun, 13 Dec 2009 16:03:45 -0500, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Sun, 13 Dec 2009 13:24:49 -0700, the renowned Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: > >>On Sun, 13 Dec 2009 20:01:06 GMT, nico(a)puntnl.niks (Nico Coesel) >>wrote: >> >>>John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>>On Sun, 13 Dec 2009 12:20:05 GMT, nico(a)puntnl.niks (Nico Coesel) >>>>wrote: >>>> >>[auto-snip] >>>>>1k RTDs are easier to interface. I used one to control my floor >>>>>heating. 2k2 (IIRC) in series from 3.3V and then fed directly into an >>>>>ADC. In a limited temperature range, the output is quite linear so >>>>>there is not really a need for fancy math. >>>> >>>>I'm thinking along these lines... >>>> >>>>ftp://jjlarkin.lmi.net/RTD.jpg >>>> >>>>All the 1Ks will be 0.1%. >>>> >>>>The opamp will have a gain of about 8, input and output centered on >>>>+2.5. This should be accurate to a fraction of a degree C, so we'll >>>>know how to dress. The Z-wave home automation systems have a combined >>> >>>How do you cancel the offset error from the opamp? I'd go for an 12bit >>>A/D converter with internal mux. In my circuit the voltage across the >>>RTD varies 2.8mV per degree Celcius. With a 12 bit A/D referenced at >>>3.3V (same 3.3V that supplies the RTD so errors cancel) you'll get >>>806uV per bit so thats 3.5bits per degree. Since temperature changes >>>are slow you can take the average value of a lot of samples so >>>accuracy may be better than 12 bit. >> >>Why not a variation on Jim Williams' idea... >> >>http://analog-innovations.com/SED/TemperatureSensor.pdf >> >>I've recently implemented such a variation on a custom chip. >> >>Advantages... >> >>AC Gain avoids DC offset accumulation >> >>It's trivial to implement DC restoration (aka pick your own baseline) >> >> ...Jim Thompson > > >There's also a clever way to use a third current to compensate for the >base resistance of Q2 and lead wire resistance. Hard to filter the >snot out of it if you're switching currents around though, and >diode-connected transistors are reputed to rectify. > > >Best regards, >Spehro Pefhany On the chip I (naturally) used multiple devices switched in and out rather than changing the currents. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism properly!
From: Joerg on 13 Dec 2009 16:44 John Larkin wrote: > On Sun, 13 Dec 2009 09:41:02 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Sun, 13 Dec 2009 12:20:05 GMT, nico(a)puntnl.niks (Nico Coesel) >>> wrote: >>> >>>> John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>> On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <invalid(a)invalid.invalid> >>>>> wrote: >>>>> >>>>>> John Larkin wrote: >>>>>>> Does anybody remember the value of negative resistance that linearizes >>>>>>> a 100 ohm platinum RTD? >>>>>>> >>>>>> No uC at hand for this job? Maybe this helps: >>>>>> >>>>>> http://pdfserv.maxim-ic.com/en/an/AN3450.pdf >>>>>> >>>>>> But you don't have to use a Maxim opamp :-) >>>>> I'm thinking I'll use 1K RTDs for the automation project, and lay out >>>>> an interface board... easier than hand wiring. The little RS232 widget >>>> 1k RTDs are easier to interface. I used one to control my floor >>>> heating. 2k2 (IIRC) in series from 3.3V and then fed directly into an >>>> ADC. In a limited temperature range, the output is quite linear so >>>> there is not really a need for fancy math. >>> I'm thinking along these lines... >>> >>> ftp://jjlarkin.lmi.net/RTD.jpg >>> >>> All the 1Ks will be 0.1%. >>> >>> The opamp will have a gain of about 8, input and output centered on >>> +2.5. This should be accurate to a fraction of a degree C, so we'll >>> know how to dress. The Z-wave home automation systems have a combined >>> motion detector and temperature sensor (no temp-only nodes I know of), >>> but I don't trust their accuracy and they only go down to 40F, not >>> good for outdoors. >>> >> Add a BAV99 to +5V and GND at C1 through C4, and 100ohms or so each from >> there to the inputs of the 4051. Otherwise, if you have a thunderstorm >> in Truckee like we had one yesterday (flash, followed by loud bang in a >> couple hundred milliseconds) your 4051 might go *PHUT* > > The RCs (like R1, C1) ahead of the mux should take care of that. Using > grounded coax helps, too. > Well, it's your cabin :-) I've seen this sort of stuff go poof despite rather large caps. It was sometimes hard to convince the original designer (who has to ECO this) that the double-diode plus one more resistor would be good but when the new field failures numbers come in during the year they become believers. >> >>> I can run RG174 to the RTDs. No EMI/ESD hazards as with semiconductor >>> sensors. >>> >> Ain't that bad. Most semi sensors have a resistive output and you can >> bypass the dickens out of it. > > Accuracy is usually bad, 3C for many parts, and I'd need a 3-wire run. > The RTDs are nice. > Yes, they are nice. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: dagmargoodboat on 13 Dec 2009 16:59 On Dec 13, 3:24 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- Web-Site.com/Snicker> wrote: > On Sun, 13 Dec 2009 20:01:06 GMT, n...(a)puntnl.niks (Nico Coesel) > wrote: > > > > > > >John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > >>On Sun, 13 Dec 2009 12:20:05 GMT, n...(a)puntnl.niks (Nico Coesel) > >>wrote: > > [auto-snip] > >>>1k RTDs are easier to interface. I used one to control my floor > >>>heating. 2k2 (IIRC) in series from 3.3V and then fed directly into an > >>>ADC. In a limited temperature range, the output is quite linear so > >>>there is not really a need for fancy math. > > >>I'm thinking along these lines... > > >>ftp://jjlarkin.lmi.net/RTD.jpg > > >>All the 1Ks will be 0.1%. > > >>The opamp will have a gain of about 8, input and output centered on > >>+2.5. This should be accurate to a fraction of a degree C, so we'll > >>know how to dress. The Z-wave home automation systems have a combined > > >How do you cancel the offset error from the opamp? I'd go for an 12bit > >A/D converter with internal mux. In my circuit the voltage across the > >RTD varies 2.8mV per degree Celcius. With a 12 bit A/D referenced at > >3.3V (same 3.3V that supplies the RTD so errors cancel) you'll get > >806uV per bit so thats 3.5bits per degree. Since temperature changes > >are slow you can take the average value of a lot of samples so > >accuracy may be better than 12 bit. > > Why not a variation on Jim Williams' idea... > > http://analog-innovations.com/SED/TemperatureSensor.pdf > > I've recently implemented such a variation on a custom chip. > > Advantages... > > AC Gain avoids DC offset accumulation > > It's trivial to implement DC restoration (aka pick your own baseline) delta-Vbe temp sensors are cool. There are at least two amusing errors in that Williams article-- o as drawn the current ratio is 2:1, not 10:1 o the chopper amp switch only ever feeds the chopper 0v(!) But, we know what he meant... -- Grins, James Arthur
From: Phil Hobbs on 13 Dec 2009 17:40 John Larkin wrote: > > Does anybody remember the value of negative resistance that linearizes > a 100 ohm platinum RTD? > > John > About -2.47k depending on what range you want. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: dagmargoodboat on 13 Dec 2009 18:51
On Dec 13, 4:59 pm, dagmargoodb...(a)yahoo.com wrote: > On Dec 13, 3:24 pm, Jim Thompson wrote: > > > Why not a variation on Jim Williams' idea... > > >http://analog-innovations.com/SED/TemperatureSensor.pdf > > > I've recently implemented such a variation on a custom chip. > > > Advantages... > > > AC Gain avoids DC offset accumulation > > > It's trivial to implement DC restoration (aka pick your own baseline) > > delta-Vbe temp sensors are cool. > > There are at least two amusing errors in that Williams article-- > o as drawn the current ratio is 2:1, not 10:1 > o the chopper amp switch only ever feeds the chopper 0v(!) > > But, we know what he meant... Gee, no one wants to play? Another error: the LTC1150 feedback resistor should be 1 meg, not 1 ohm. (the stage needs a gain of about 504 to scale 198uV/K to 10 volts @ 100c.) Does anyone see the advantage to Q1? Two resistors in series to +15v, switch across the lower resistor, would be simpler, reduces the influence of Vbe on i(c), and minimizes the switch's on resistance. As it is, on the "100uA" setting, Q1 forces significant base current into the 330 k base network impedance, causing a beta and temperature dependence. That isn't obviously useful. -- Cheers, James Arthur |