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Author Topic: PT100 and Thermocouple choice  (Read 12091 times)
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engamor
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« Reply #25 on: November 06, 2010, 01:38:54 13:38 »

Well, if I shal continue in public I will explain the reason why it is wrong to calculate the ADC requested resolution as the poster who called me "silly" thinks it should be done.
The guy says: " you only need 0.5 C resolution in 100 C span, that makes for 200 points wich is well covered by a 10 bit converter ". This is the OUTPUT resolution, it is NOT the input resolution necessary to measure the temperature when using a PT100.   OF course, if you ALREADY have a circuit which gives you the correct voltage as (say) 0-2.5 V from 100 to 200 Celsius than you can make your job with even less than a 10 bit ADC. The problem is that you must build such circuit first. The reasonable way to do this, nowadays, is to use a 24 bit sigma-delta and make some easy calculation. I really love analog circuits as the one proposed by "zuisti" ( I cannot read it but I do beleive it works) but they simply are no longer necessary and the "analog workmanship" required is no longer available (unfortunately).
Second point: yes you can do without 3wire compensation but the OP wants to build a professional system, he wants to be able to buy real world PT100 probes, available as 3 or 4 wire configuration ...  and I suspect would appreciate to be able to enjoy interchangeability of the probes.
I do not think that 24 bit sigma delta converters are exotic components, either, they are very common and very cheap, maybe that somebody can be happy to learn how useful they are.
Finally I give my excuses to the board, but I do not think that being called silly by somebody and accept the insult makes sense for me, so I simply wanted to avoid a discussion with such people.
regards
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« Reply #26 on: November 06, 2010, 03:25:08 15:25 »

I never asked you to accept an insult,in fact I allowed you to throw a dart too
what I asked was for you to continue contributating to this thread
being called silly is no worse than calling someone a dummy so what I'am saying is the score is even so put that behind you so you can continue to help the OP and other people but at the same time try and remember for some people here a 24 bit sigma delta converter would be exotic and impossible for them to obtain so when someone puts a lesser design on the table whether good or bad it might be the only option for them
we don't need useful threads locked because of differences of opinion no matter how silly or dumb they appear and I wasn't just addressing you
over the mud slinging it goes for everyone.
We are all prone to get in huff when we think our toes have been stepped on myself included but it's no reason to come out swinging or retreat
So why don't you guy's meet in the middle shake hands and make one with -- say a 16 bit convertor  Wink
zuisti
we are still having download trouble with files, could your re-post your file somewhere else like rapid or such
 
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zuisti
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« Reply #27 on: November 06, 2010, 03:39:10 15:39 »

zuisti
we are still having download trouble with files, could your re-post your file somewhere else like rapid or such
OK.
The link:
http://hotfile.com/dl/80867570/53fd6e2/circ.rar.html
pw:zuisti
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« Reply #28 on: November 06, 2010, 03:57:13 15:57 »

Well, mr. BBarney, the problem with 16 bit ADCs is that they are NOT easily available. Or there are some specialized and costly and very difficult to use ones. The sigmadelta variety is made exactly on the purpose of the above discussion (or similar application) but unfortunately most of them are (so-called) 24 bits. Other than for that I DO AGREE that you can do the job with a good stable 16bit.
The interesting point of this breed of converters is that they are designed to be slow ( as slow as 15 conversions per second) high resolution and benefit from a very high noise rejection . In the real world they are much easier to use than even the 10bit SAR type found in most microprocessors, which usually is very noisy.
I mostly use the family of TI but there are certainly several others.
Please have a look to the data sheet of the ADS1232
http://focus.ti.com/lit/ds/symlink/ads1232.pdf

You will find several interesting application circuits that exactly address the topic, and believe me they are really useful.
Please note: you do not even need a reference in the PT100 3-wire compensated circuit as proposed.
Unfortunately I did not post this reference before,  I apologize, simply  I did not had it in mind, since I use a different converter. This application circuit was not given in the datasheet of the converter I use, so I had no handy drawing to give.  My typical application uses 4wires and I have a different arrangement for it, but 4wire is not practical for the OP so I never insisted on 4wires.
I sometimes do use 3wires and that works and the circuit is veru similar to the example in the data sheet, so I can say that it works.
Thanks for your patience, I promise not to waste your time in the future.

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« Reply #29 on: November 06, 2010, 05:05:29 17:05 »

engamor:  I appreciate you sharing your insights, and might even have uses for
an ADS1232 some time in the near future.

I'll not be stubborn like those who insist that Vinl sounds "right" and CDs sound
"artificial"; certainly this job CAN be done using opamps, precision resistors, etc.
Something as integrated as the ADS1232 clearly takes the prize.   The $11.75 Ea.
price tag at digiKey isn't terrible.

The controller for my furnace here uses a type K thermocouple and a MAX6675
to monitor firebox temperature.  It's a hard to beat combination for such a
project where it isn't rocket surgery and the most important things are not
freezing to death and not getting the firebox wall much above orange hot.   Cool

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« Reply #30 on: November 06, 2010, 06:26:15 18:26 »

engamor
The 16 bit referance was only a joke because it's 1/2 way between 10 & 24 (almost Smiley ) but thank you for the explanation and your not wasting my time your input on the subject has been very helpful to the OP and others reading this thread
That looks like a nice chip and oldvan is right $11 dollars for one is reasonable and if you were mass producing it's 1/2 that price , I also noticed that Digi-key has a demo board for $ 55 if anyone is interested in playing around with this chip it looks like the way to go
http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2622527&k=ADS1232
manual for the board
http://focus.ti.com/lit/ug/sbau120a/sbau120a.pdf
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« Reply #31 on: November 07, 2010, 10:38:57 10:38 »

I have got the file posted by ZUISTI but I cannot read it, I do not have a program to read it  Angry.
Thanks anyhow.
I agree that the evaluation board can be of great help.
The price tag of ADS1232 should be less than 4 $, but yes, this is the price when you buy (even small quantities) from a regular distributor.

Anyhow if "micromaster" decides to go that way, here is what he should do.
1) Implement the circuit as described in the datasheet. The 33KOhm resistors are not critical as for the accuracy, they shall be very good in terms of stability, 20ppm is ok. The original circuit diagram is rather simple and does not include protection resistors on the ic pins. You better place at least 1kohm ohm in series to the input pins and 1nF high quality capacitors in parallel. The exact values may have to be checked later, but you shall set up the place on your board, just close to the input connection block.   
2) Get the hardware working, the microprocessor shall communicate with the ADS1232 via SPI. Exercise a bit the different functions. You may have some specific question at that time, please contact me if in doubt.
3) Buy a few 100 Ohm resistors of high precision and stability , leaded old fashioned ones are best. Accuracy shall be at least 0.1%. If ever possible you might buy real high precision ones, that will cost a little, but you will use them for your lifetime.
4) The result of the conversion from the ADC is a representation of the resistance of the sensor, referred to the divider network made with the two 33K resistors. You can compensate the third wire as illustrated in the datasheet. You better calculate at this stage the real ohmic value from the equation of the divider network. This step is very useful to understand if your circuit is working as it should. Using the 100Ohm resistor(s) you can calibrate the precision ohmeter you have made!
Calibrate it at 100Ohm and at (say) 200 or 150 ohm . Your microprocessor shall be equipped with the ability to store non-volatile correction values and some calibration routine.
5) The circuit works because the two 33K do provide a stable current source even if the 100ohm nominal sensor resistance is changing with temperature. The output from the measurement is thus referred to the ratio as said above.
Of course the internal PGA is set at 128, so that the tiny changes of resistance are amplified and everything works as you had a 12800 ohm resistor driven by 5V/66Kohm ... Got the idea?
6) Once you have got the "3wire_ohmeter" calibrated in 2 points your job is easy. Use  a look up table approach and you have a good stable temperature measurement.

One very big advantage of the PT100 is that your system can get calibrated with just those 100 Ohm resistors. To calibrate a thermocouple system you would need a precision millivoltmeter and some delicate setup.

I am speaking of several 100Ohm resistors because you can buy a small lot and use them to make a reference 100 Ohm, a 150 and a 200. Use SHORT THICK WIRES for the series parallel connections!  ( You can buy different resistors of course ... )

There are many things that you may need to ask. No problem for me.   

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« Reply #32 on: November 07, 2010, 12:24:04 12:24 »

engamor
You need Proteus to read Zuisti's file here is a link to it just download the single file install at the bottom of the sticky (at the top of the thread)
you will find many use's for it I'am sure
http://www.sonsivri.com/forum/index.php?topic=31342.0
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MicroMaster
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« Reply #33 on: November 07, 2010, 11:36:53 23:36 »

I got zuisti circuit in Proteus and tempted by its seemingly simplistic nature!
But the resistance values and op amp specifications may be very stringent.

I have started looking for ADS1232, so far $13- but that is from farnell. General market I am searching now, should be low.
This is the data sheet circuit.

1K Ohm in series with input pins means 1 each for the probes.(?)
The cap in parallel is across each channel ( between AINN1- AINP1 and AINN2-AINP2) (?)
One channel will read lead resistance(Ch2) and other, lead plus RTD. (?)
Should this sort of high bit count ADCs need any special power supply for stable operation? (Noise problems?)

Only AD1232 and a couple of stable resistors - everything else is in software. That may be the beauty of the proposal!
Regards

« Last Edit: November 07, 2010, 11:40:05 23:40 by MicroMaster » Logged

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« Reply #34 on: November 08, 2010, 02:00:23 02:00 »

I have finally seen the circuit proposed by Zuisti. Thanks for the posting of course. This is a nice approach and it was possibly the simplest and smartest one possible some time ago. The circuit is still very useful if you can do without the 3wire configuration, which is possible if you keep leads short at application site, but it is not the general case with standard installation. For lower temperature applications the opamp and circuitry can be placed in the probe casing itself.
Now to the answers for the ads1232 circuit:

1Kohm resistors shall be placed exactly before the input pins:  AINP1+AINP2, AINN1, AINN2 , so that makes for 3 resistors.
Place the capacitors after the resistors, to ground.

AIN1 is reading RTD+2RL, AIN2 is reading RL (1x)

Decouple the power supply to the ADC, nothing special however, If possible use a linear power supply circuit.
The long conversion time makes for a good noise rejection. read carefully the data sheet. Also search for some tutorial on sigma-delta converters.

have a nice day!
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« Reply #35 on: November 08, 2010, 05:12:37 05:12 »

Hello,
It was interesting to read this thread. I got many new ideas. Wink

But I think solutions suggested are expensive when not so much accuracy is required. Also using built-in 10-bit ADC of uC seems reasonable for 100-200 deg C range with  0.5 deg C resolution is required. Using single supply op-amp such as 358 is also handy if no -ve supply is not available.

So I tried to design such circuit  Grin
Check it and see if there are any problems. Embarrassed.

Let me emphasize again, that circuit is NOT for high accuracy measurement but for simplicity and cost.

regards,
sam_des
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MicroMaster
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« Reply #36 on: November 08, 2010, 07:20:47 07:20 »

Thanks a lot sam_des.
I think this can help me a lot to learn also.
Also I am finding it hard to get the ADS1232 at market rate here. I have placed order with farnell even though the rates are high.(I need to make one for future needs and experience) It can be treated as a academic interest project now. The client also of the opinion that it might be a overkill with the prizing and specification.
I am loading that new circuit in Proteus and playing around.
I am afraid now that  I am becoming little bit greedy. I am thinking of changing my design specifications.
Please don't misunderstand, I am not asking to re-design the circuit - but trying to do it myself for learning.
If I can bring the range to 0 to 250 Deg. or 10  to 240 Deg or something it will be more useful as well as easy for testing and development. Since ADC has 1024 steps I think .25 Deg Celsius resolution and 256 degree span is possible. (Is there anything wrong in my judgment?) now the span seems to be more spread.

If it is possible in principle with the given circuit topology, it will be helpful.Considering the flexibility of the arrangement I feel it can be done.
Please suggest

btw which simulator you suggest better Proteus or Multisim?
(I am not that much interested in that MCU simulation part because I use MCU debuggers for that section. for the other capabilities which is good?)

Thanks

 
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« Reply #37 on: November 08, 2010, 07:37:08 07:37 »

Please, mr.sam_des, do not think I want to deceive you. Your circuit has a few pitfalls. First of all: you cannot claim that it is made for simplicity and cost! Using 3 trimmer resistors is not cheap. Professional trimmer resistors are very very costly, especially after you spend time and resources to adjust them, not to speak of stability. Secondly, again, the circuit does not provide for 3wire compensation.
If you have a look to the ANALOG Device booklet ( at the page I indicated in a previous post) you can study the circuit in there and possibly see by yourself what I say. The OP wants a professional quality result (building a PID control), and lacks instrumentation and experience on the subject, there is little else that can help him but the solution I propose.
For micro-master ... a little last advice: never speak of technical options to your customer!
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« Reply #38 on: November 08, 2010, 07:45:38 07:45 »

Quote
btw which simulator you suggest better Proteus or Multisim?
(I am not that much interested in that MCU simulation part because I use MCU debuggers for that section. for the other capabilities which is good?)

Can't say anything about Multisim because I've never used it but just about everyone here use's Proteus , you only have to look at the downloads for each to see the difference in fact that's why most people show up here in the first place because their looking for proteus
which is better isn't always the right choice, if your looking to share designs around here Proteus is the one to go with as most people here use it

Quote
For micro-master ... a little last advice: never speak of technical options to your customer!

I couldn't agree more the seccond you put a bug in their ear they start to scratch and it always cost you time & money because now there indecisive over the design
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« Reply #39 on: November 08, 2010, 08:05:59 08:05 »

For micro-master ... a little last advice: never speak of technical options to your customer!
Very correct, but I realized it now only.
Learning lessons one by one.
As bbarney put it he is just behind me scratching and interfering. Sorry to confess but I revealed my weakness.
No problem, let me see!

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« Reply #40 on: November 08, 2010, 09:43:36 09:43 »

Hi,

@ micro-master
Quote
If I can bring the range to 0 to 250 Deg. or 10  to 240 Deg or something it will be more useful as well as easy for testing and development.
Well, that's possible. I've checked from 0 to 200 deg. in simulation. With 200 deg it gives o/p of about 2.2V, keeping 0.3V margin with full scale of 2.5V.

Quote
Since ADC has 1024 steps I think .25 Deg Celsius resolution and 256 degree span is possible. (Is there anything wrong in my judgment?) now the span seems to be more spread.
I think it's not possible to have 0.25 deg resolution as it is. Using TL431 ref of 2.5V & 10-bit ADC we have 2.44mV limit on ADC resolution. But note that internal ADCs have +/-1 Bit of accuracy hence we have to double the limit to 4.88mV to be on safer side. Now we barely have 0.5 deg temperature resolution.

@ engamor
Quote
Please, mr.sam_des, do not think I want to deceive you. Your circuit has a few pitfalls. First of all: you cannot claim that it is made for simplicity and cost! Using 3 trimmer resistors is not cheap. Professional trimmer resistors are very very costly, especially after you spend time and resources to adjust them, not to speak of stability.
Hardly true where I live.
Bourns 3006 10-Turn Trimming Pot - +/- 0.05% Resistance Adjustability
                                                 +/- 100ppm/Deg C Temp Coeff
costs nearly 0.5$. With 3, it costs 1.5$ even in very small quantities and is available easily.

ADS1232 - Well I haven't been able to get even a quote from my regular dealer, it's not available. The price mentioned earlier of 4$ is still makes huge difference if units are made in quantities.
But yes, time is certainly a factor to consider.

Quote
Secondly, again, the circuit does not provide for 3wire compensation.
That's absolutely true. But like I said, circuit is not meant for high accuracy.
Another thing is, I've seen countless Process Controllers in Textile industry, Dying industry which don't even use 3 or 4-wire connections to PT100. They simply ignore error of about 1-2 deg.

So I think that while your right, it finally depends on your application and cost your customer is ready to pay.

regards,
sam_des
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« Reply #41 on: November 08, 2010, 11:00:55 11:00 »

Thanks for the inputs.

Since this thread has become a source for PT100 interfacing material, here is one more.
edit: that info is already there
Microchip AN687
Thanks again
« Last Edit: November 08, 2010, 12:00:39 12:00 by MicroMaster » Logged

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« Reply #42 on: November 08, 2010, 01:01:42 13:01 »


@ engamor Hardly true where I live.
Bourns 3006 10-Turn Trimming Pot - +/- 0.05% Resistance Adjustability
                                                 +/- 100ppm/Deg C Temp Coeff
costs nearly 0.5$. With 3, it costs 1.5$ even in very small quantities and is available easily.

Uhm ... I would not claim I can make subtle discussions in English. But this is their price isn't it? I was speaking of cost, in my language it is something different. After you have spent 1.5$ the thing shall be mounted, will take room on your board, will drive you crazy to place all three in a way that people can adjust them ... will stay there waiting to be adjusted by a screw driver ... by some human intelligence ... driven by (written) instructions. And btw ... at 100ppm ... why don't we use them as the temperature sensors?
OK OK, I do not want to start again on a bad discussion, but I'll say the following as a last resource:
I have some 36+ years of experience and I ALWAYS try to make a new design with (at least) today's technology, for the simple reason that my design shall survive tomorrow ... I will not base it on yesterdays' ... despite the fact that I know better yesterday than tomorrow.
Please don't be mad with me!



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« Reply #43 on: November 12, 2010, 03:46:11 03:46 »

Back after a break Smiley

I have created a modified form of Microchip AN687 PT100 circuit.

The circuit works ( not in correct ranges) in proteus.

Don't know weather any practical difficulties or problems with the used op amps.

Please suggest. The output is supposed to be going to a PIC internal 10 bit ADC.

The range is -10 Degree to 246 degrees with .25 resolution. Regarding accuracy, Like to know how much I can expect?

Proteus file also attached.
Please advice
MM

Posted on: November 12, 2010, 03:04:26 15:04 - Automerged

Here is the pdf of the diagram
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« Reply #44 on: November 13, 2010, 02:04:00 14:04 »

The proposed circuit is possibly theoretically correct, but I have no time to analyze it in detail for you. The problems with precision=stability and accuracy=error are the same as you can suspect. 1 bit here is worth 0.25C. A 10 bit internal ADC will not give readings stable down to 1 bit. You will be obliged to  average several readings to overcame this problem. But this shall not be of any trouble. The real problems are twofold: 1) You have no room in the converter resolution  for calibration AND you have no calibration device at all. 2) The stability (mid and long term and environmental) relies on so many components that it is really difficult to make a calculation. You may try and change some critical values and see what happens in Proteus. Try to change the reference value by 0.5% or some resistor value etc. I think you will want your circuit to work on some quasi-industrial temperature range (-20 to 85 at least) that makes for at least 60 degrees of change and a standard 50ppm resistor will change 3000ppm or 0.3% (1024 points are 0.1%) ... well you know what I think of this kind of effort!
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« Reply #45 on: November 13, 2010, 03:11:40 15:11 »

With 10 bit ADC you better go back to your original range and resolution.

But, as you said if you are getting greedy than use uC with 12 bit ADC - many of them are cheap nowadays (dsPIC/XMega/LPC1xxx/MSP430/etc). For example dsPIC33FJ12GP201 has one channel 12 bit ADC with price tag about $3 or so for single quantity.

You will need to do filtering and averaging on adc reading later.

-ichan
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« Reply #46 on: November 15, 2010, 01:20:41 01:20 »

Thanks for the responses.
1) You have no room in the converter resolution  for calibration AND you have no calibration device at all.
You mean the variable resistors I have to use to trim that gain and offsetting resistance values?
Or anything else?
But, as you said if you are getting greedy than use uC with 12 bit ADC
You will need to do filtering and averaging on adc reading later.
-ichan
I can use 12 bit ADC at maximum, but controller only PIC16 or PIC18. MCP3402 12 bit ADC I can use. Or else I will stick with my .5 Deg resolution.
Thanks
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« Reply #47 on: November 15, 2010, 06:09:31 06:09 »

Hi MicroMaster;

There is an original Labcenter example project
  (using PT100, a cheap sigma-delta AD and a PIC18 with a GLCD).
  It's source level debuggable (written in Proton basic).
  You can see how to calculate the temp using a PT100.
  To use this you need a real high-accuracy 1 mA current source,
  the project uses only a simulated one.

It's in Proteus Samples\VSM for PIC18\MCP3421 & PIC18 folder

 
However:

I still think (though I only had 45 years of experience in there, engamor :-)
that it is more appropriate in your case a simple analog signal-conditioning
and linearizing circuit (like mine above).


.... a personal note:
  After a little "google-ing" I have found this appnote from MAXIM:
  http://www.maxim-ic.com/app-notes/index.mvp/id/3450
  It's dated at Feb 21, 2005. Have a look at the figure 4 and figure 8.

  It was interesting to see my own linearizer circuit (created in 1981),
  although mine is more complete :-).

  Unfortunately it was not patented (:-), was not possible here.
....


About your problems (with my circuit):

- all the extrem value resistors can be loaded with 2 pieces of
  1% metal film resistors (EIA E96 Codes). Tried!

- as OPAMP you can use for example LMP7701 from NS (or other modern
  low offset opamps), or - horribile dictu - you can use a TL071(081)
  IC with offset adjusting pot.  

- it's possible also the 3 wire measuring (one additional OPAMP
  completely compensates it (I found also the circuits made by me),
  or (small claims) the original circuit can be modified slightly to
  compensate the known resistance of the meas wires.

- I think your system will be used under normal conditions
  (20...25, max. 30 degC), and not under "quasi-industrial" conds ...


And now a little math:

The output of the analog circuit is 10 mV/degC
Using  a 10 bit PIC AD:
 
0.25 x 1023 = 255.75

Apply an external 2.56 (2.5575 :-) V to Vref+ in your PIC16 or PIC18 AD program
(created via 2 res and a pot from the +12v measuring bridge supply
to eliminate its accuracy and stability problems. If you want, substitute later
the pot with fix resistors :-).

So the meas. range is 0 ... 255.75 degC (1 LSB = 0.25 degC)

The displaying is easy:

Read AD (10 bit) to a word variable.
Use its highest 8 bits to get the integer degC values (0...255).
Then use the lowest two bits to add the fract. parts (only for displaying):
(00 = 0.00; 01 = 0.25, 10 = 0.5 11 = 0.75 degC)

For your PID controller you can convert it to float but I don't recommend
this: use the 10 bit value instead, do not use floats if possible.
      
Well, the PIC's AD is not very precise, it has also linearity errors, but
I think it's good enough in your case.


zuisti

PS:
If you want I will send to you the supplemented circuits, too
« Last Edit: November 15, 2010, 07:42:08 07:42 by zuisti » Logged
MicroMaster
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« Reply #48 on: November 15, 2010, 08:10:29 08:10 »

If you want I will send to you the supplemented circuits, too

Your observations about my project are reasonable.
The digital solution of 24 bit sigma delta ADC is not workable here.
I have a three wire PT100 and OPAMPs like OP07, TL084 etc available here.
The other limitation is how much time you can spare for this!
Thanks a lot.
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MicroMaster

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zuisti
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« Reply #49 on: November 15, 2010, 08:19:53 08:19 »

...
The other limitation is how much time you can spare for this!

Yes, unfortunately I don't have time enough now but I go back soon.

Best 73 :-)
zuisti
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