RTD linearization
in [Design]
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From: John Larkin on 13 Dec 2009 20:19 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: >> >>>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 > >How do you cancel the offset error from the opamp? Ignore it. The offset is 100 uV typ, and the signal is 5 millivolts per degree C. And I'm digitizing a 0.1% 1K dummy resistor as a reference anyhow. I'd go for an 12bit >A/D converter with internal mux. I'm using a purchased data acquisition board to talk serial to my PC http://www.rs-485.com/daqitemw.asp?record=95 which has a 10 bit ADC. It does have 8 analog inputs, but I need my own mux so that I can add the gain after muxing. 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. I'd only get about 1 degree C resolution without the amp, and how much fun would that be? I'll probably have enough noise that software lowpass filtering will be a good thing to do. Temperatures don't change all that fast anyhow. I just finished the signal-conditioning/control board layout. Two layers, thru-hole parts, a time warp for sure. John
From: John Larkin on 13 Dec 2009 20:29 On Sun, 13 Dec 2009 15:51:06 -0800 (PST), dagmargoodboat(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. >> 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. > Or two equal resistors in parallel, one switched and one not. >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. Oh. Right. I said that! Jim W does some weird stuff sometimes. I think he mostly fiddles until things sort of work. His stuuff tends to be "component rich" and the only way he ever compensates control loops is by adding big caps. John
From: John Larkin on 13 Dec 2009 20:31 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
From: Jim Thompson on 13 Dec 2009 21:00 On Sun, 13 Dec 2009 17:29:04 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 13 Dec 2009 15:51:06 -0800 (PST), dagmargoodboat(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 ???? > >>> 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. >> > >Or two equal resistors in parallel, one switched and one not. > > >>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. > >Oh. Right. I said that! > >Jim W does some weird stuff sometimes. I think he mostly fiddles until >things sort of work. His stuuff tends to be "component rich" and the >only way he ever compensates control loops is by adding big caps. > >John Those of us in the know actually run the equations and sort it all out. Ms prissy Pants needs staff ;-) ...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: dagmargoodboat on 13 Dec 2009 21:20
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 |