RTD linearization
in [Design]
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From: Phil Hobbs on 14 Dec 2009 09:02 John Larkin wrote: > On Sun, 13 Dec 2009 17:40:15 -0500, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> 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 > > I got -2750 for -40 to 40C. > > Go back to the beach! Have one of those mango drinks with an umbrella > in the glass. I think that keeps the rum from getting diluted in case > it rains. > > John > > I'm just heading down there now. Occidental Grand Papagayo, Liberia, Guanacaste, Costa Rica. Nice place. Cheap, too, thanks to #1 daughter being in the travel business. (The 2.47k number is for a much wider range---50 to 250 iirc. It's from my thermal chapter on the web site.) 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: George Herold on 14 Dec 2009 11:45 On Dec 13, 9:20 pm, dagmargoodb...(a)yahoo.com wrote: > On Dec 13, 9:00 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- > > > > > > Web-Site.com/Snicker> wrote: > > On Sun, 13 Dec 2009 17:29:04 -0800, John Larkin > > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > >On Sun, 13 Dec 2009 15:51:06 -0800 (PST), dagmargoodb...(a)yahoo.com > > >wrote: > > > >>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 > > > >With those base resistor values on Q1, it's not a very good 2:1. > > > 4.9K vs 49.9K ???? > > No, that's worse! It'd have to be 49.9k and 499k to get the ~10uA-to- > ~100uA Williams wants. (And that doesn't yield an accurate 10:1 > either, obviously.) > > But @ i(c)=100uA, a 200-gain PNP will dump half a uA into 330k, > causing a ~170 mV error. On a ~4.4V emitter bias that's a 4% error. > > It doesn't matter much--as long as the current ratio is stable, the > circuit's initial trim will work across all future sensors. But > variations in the PNP's beta would affect the initial calibration, > plus, beta changes with temp. > > That compromises one of the circuit's appeals--that it can be > accurate /without/ calibration. > > -- > Cheers, > James Arthur- Hide quoted text - > > - Show quoted text - Hi James, I think delta-Vbe temp sensors are cool too! (But I'd rather call it a Ebers-Moll temperature sensor. The shorting of the base and collector is crucial. If you just fed current into the base the I-V slope is not as close to ideal.) I found that the collector resistance of Q2 also causes an error in this circuit. (Sphero mentioned this already.) I determined a collector resistance of about 1 ohm for the 2N3904 that I used as Q2. I wonder if this parameter also changes with temperature? Would running at 1uA and 10uA be better? George H.
From: John Larkin on 14 Dec 2009 12:05 On Sun, 13 Dec 2009 18:20:03 -0800 (PST), dagmargoodboat(a)yahoo.com wrote: >On Dec 13, 9:00�pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- >Web-Site.com/Snicker> wrote: >> On Sun, 13 Dec 2009 17:29:04 -0800, John Larkin >> >> >> >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >On Sun, 13 Dec 2009 15:51:06 -0800 (PST), dagmargoodb...(a)yahoo.com >> >wrote: >> >> >>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 >> >> >With those base resistor values on Q1, it's not a very good 2:1. >> >> 4.9K vs 49.9K ???? > >No, that's worse! It'd have to be 49.9k and 499k to get the ~10uA-to- >~100uA Williams wants. (And that doesn't yield an accurate 10:1 >either, obviously.) > >But @ i(c)=100uA, a 200-gain PNP will dump half a uA into 330k, >causing a ~170 mV error. On a ~4.4V emitter bias that's a 4% error. > >It doesn't matter much--as long as the current ratio is stable, the >circuit's initial trim will work across all future sensors. But >variations in the PNP's beta would affect the initial calibration, >plus, beta changes with temp. > >That compromises one of the circuit's appeals--that it can be >accurate /without/ calibration. If I buy 1K thinfilm RTDs and 1K 0.1% thinfilm resistors, I can hit a fraction of a degree C absolute accuracy with no calibrations whatsoever. The signal levels are millivolts per degree C, not microvolts, and emi hazards are esentially nil. The ADC only needs to be linear. The ceramic slab and 1206 platinum RTDs that we've tested are much better than their guaranteed specs. I can't imagine how they manage to trim them that well. John
From: dagmargoodboat on 14 Dec 2009 12:36 On Dec 14, 11:45 am, George Herold <ggher...(a)gmail.com> wrote: > On Dec 13, 9:20 pm, dagmargoodb...(a)yahoo.com wrote: > > > > > On Dec 13, 9:00 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- > > > Web-Site.com/Snicker> wrote: > > > On Sun, 13 Dec 2009 17:29:04 -0800, John Larkin > > > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > > >On Sun, 13 Dec 2009 15:51:06 -0800 (PST), dagmargoodb...(a)yahoo.com > > > >wrote: > > > > >>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 > > > > >With those base resistor values on Q1, it's not a very good 2:1. > > > > 4.9K vs 49.9K ???? > > > No, that's worse! It'd have to be 49.9k and 499k to get the ~10uA-to- > > ~100uA Williams wants. (And that doesn't yield an accurate 10:1 > > either, obviously.) > > > But @ i(c)=100uA, a 200-gain PNP will dump half a uA into 330k, > > causing a ~170 mV error. On a ~4.4V emitter bias that's a 4% error. > > > It doesn't matter much--as long as the current ratio is stable, the > > circuit's initial trim will work across all future sensors. But > > variations in the PNP's beta would affect the initial calibration, > > plus, beta changes with temp. > > > That compromises one of the circuit's appeals--that it can be > > accurate /without/ calibration. > Hi James, I think delta-Vbe temp sensors are cool too! (But I'd > rather call it a Ebers-Moll temperature sensor. The shorting of the > base and collector is crucial. If you just fed current into the base > the I-V slope is not as close to ideal.) Yes, diode-connected NPN Q2 is the sensor. You can also use a straight diode there, but the diode-connected transistor behaves better. But I was questioning the PNP current-source (Q1) that feeds it. Q1's i(b) produces a significant error voltage in its 500k-1meg bias network, thus on its base node, which also injects a temperature dependence into the switched current ratio (it makes the high/low ratio increase with temp.). I was too lazy to evaluate all the higher-order effects of that. Maybe Williams is trying to compensate something, but first-order, it looks counter-productive. Replacing that Q1-based current source with a couple switched resistors avoids that problem, is more accurate, and it's simpler. > I found that the collector > resistance of Q2 also causes an error in this circuit. (Sphero > mentioned this already.) I determined a collector resistance of about > 1 ohm for the 2N3904 that I used as Q2. I wonder if this parameter > also changes with temperature? > Would running at 1uA and 10uA be > better? Yes, and reduces wiring resistance error too. -- Cheers, James Arthur
From: Joerg on 14 Dec 2009 13:30
John Larkin wrote: > On Sun, 13 Dec 2009 18:20:03 -0800 (PST), dagmargoodboat(a)yahoo.com > wrote: > >> On Dec 13, 9:00 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- >> Web-Site.com/Snicker> wrote: >>> On Sun, 13 Dec 2009 17:29:04 -0800, John Larkin >>> >>> >>> >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> On Sun, 13 Dec 2009 15:51:06 -0800 (PST), dagmargoodb...(a)yahoo.com >>>> wrote: >>>>> 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 >>>> With those base resistor values on Q1, it's not a very good 2:1. >>> 4.9K vs 49.9K ???? >> No, that's worse! It'd have to be 49.9k and 499k to get the ~10uA-to- >> ~100uA Williams wants. (And that doesn't yield an accurate 10:1 >> either, obviously.) >> >> But @ i(c)=100uA, a 200-gain PNP will dump half a uA into 330k, >> causing a ~170 mV error. On a ~4.4V emitter bias that's a 4% error. >> >> It doesn't matter much--as long as the current ratio is stable, the >> circuit's initial trim will work across all future sensors. But >> variations in the PNP's beta would affect the initial calibration, >> plus, beta changes with temp. >> >> That compromises one of the circuit's appeals--that it can be >> accurate /without/ calibration. > > If I buy 1K thinfilm RTDs and 1K 0.1% thinfilm resistors, I can hit a > fraction of a degree C absolute accuracy with no calibrations > whatsoever. The signal levels are millivolts per degree C, not > microvolts, and emi hazards are esentially nil. The ADC only needs to > be linear. > > The ceramic slab and 1206 platinum RTDs that we've tested are much > better than their guaranteed specs. I can't imagine how they manage to > trim them that well. > Wow, looks like it's turning into a major science project now. Pretty soon you can say with confidence that the temperature in Truckee is exactly 22.74 degrees Fahrenheit :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |