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I'm working on a PT100 based temperature probe sensor to read temperature difference between two points inside a pipe with flowing water, the requierements are:

  • 1mK resolution over 0-45C range.
  • dual PT100 setup to detect calibration discrepancies.
  • 0.1C absolute accuracy would be optimal but what is actually important is getting reliable differential readings between matched probes.
  • RS485 communication.

Restrictions I have:

The PCB can't be bigger than 100mm diameter to fit into the probe's head and since I need a 2xPT100 setup o don't have space for signal conditioning + ADC for both so I have to rely on high resolution ADC only.

What I have on the design table so far:

I'll be using a 2xPT100 DIN 1/10 for max accuracy (+-0.006 C error at 45C) with a 22 bit delta-sigma ADC (ENOB:21.9 but actually 20.9 since it uses two's complement output). This is the simplified circuit of one probe that show how both PT100 interface witch the ADCs and the MCU:

enter image description here

EDIT: Just to clarify, the PT100s inside each probe need to be compared against each other (in the MCU) so it can be detected when the probe needs calibration. Then temperature between probes must be compared to know the temperature difference between points.

For this circuit I get a signal that varies 12.6572mV from 0 to 45C (100-117.47 Ohm), then I get 2.27mK resolution and a 1.54mA current through the PT100, If I wanted to go down to 1mK I only see two solutions:

  1. Use a lower value for R1 to get more resolution and also lower Vref/Vex to adjust the current trough the RTD

e.g: R1=500Ohm for 0.975mK resolution and Vref/Vex= 1.024V for 1.65mA

  1. Keep the R1 and Vref/Vex values and get more resolution trough Oversampling and decimation method. The PT100 response time T90 typically is no less than 3 seconds so even with the 13SPS offered by the MCP3551 I think I should be able to get those extra 1.2 bits I need but I'm not sure.

Is there anything else I could do to improve the resolution?

I've selected the MCP3551 ADC because it's ENOB is pretty high 20.9 against all those 24bit sigma-delta ADCs out there that have no more than 19 ENOB, and besides, It's just an 8pin SOIC IC so if someone can suggest a non massive ADC that outperforms this one I'm open to change it.

If you have any question about the problem, please ask, if I've left any important information behind or think I could elaborate more, please ask.

EDIT: I add the whole system sketch

enter image description here

EDIT2: I've been suggested to use ADS124S06, It provides around 23 bit resolution without enabling the PGA,I could also enable the PGA and meet common-mode specs because this IC allows to connect the PT100s in parallel and use the voltage through Rref as level-shift. The downside is that it's a 32pin IC so routing it could be a nighmare...

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  • \$\begingroup\$ Use an ADC with PGA? \$\endgroup\$
    – Jeroen3
    Commented Jun 4, 2018 at 10:45
  • \$\begingroup\$ I've explored that option but all ADC with PGA out there are 16PIN+ devices and I really don't have that much space. \$\endgroup\$
    – Bizcochito
    Commented Jun 4, 2018 at 10:59
  • \$\begingroup\$ Uhm, the AD1220 VQFN16 is the smaller than the MCP3551 SOIC8... And more channels. \$\endgroup\$
    – Jeroen3
    Commented Jun 4, 2018 at 11:23
  • \$\begingroup\$ @Jeroen3 that's a good IC but note than the more increase in gain the more ENOB you lose and since TI only provides a ENOB vs Gain vs Data rate for internal voltage reference I can't really tell how much resolution I get. I'm checking it with TI support \$\endgroup\$
    – Bizcochito
    Commented Jun 5, 2018 at 7:50
  • \$\begingroup\$ TI support verified that ADS1220 won't meet de requirements and suggested ADS124S06 instead. Also, you can set up to 4 gain with PGA disabled, once you need 16+ gain (which is the case) you have to meet the In.Amp common-mode requirements and I'm not 100% sure but I think they can't be met with such a tiny signal. \$\endgroup\$
    – Bizcochito
    Commented Jun 6, 2018 at 9:46

2 Answers 2

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I'm working on a PT100 based temperature probe sensor to read temperature difference between two points inside a pipe with flowing water

Consider going for a differential measurement as per the method used in a wheatstone bridge. The ADC you have chosen seems ideal for that: -

This product line has fully differential analog inputs, making it compatible with a wide variety of sensor, industrial control or process control applications.

enter image description here

This means you only need one ADC.

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  • \$\begingroup\$ Hi Andy, I think you got it wrong. Each probe has two PT100 inside, but the differential measure is not between those PT100 but btween probes. Each probe measures in a different point of the pipe, the reason for the 2xPT100 setup is that I need the probe to throw an alert if the temperature measured between the PT100s inside is greater than 'x' so I'll know that the probe needs to be calibrated again. I'll add a scheme of the system \$\endgroup\$
    – Bizcochito
    Commented Jun 4, 2018 at 10:38
  • \$\begingroup\$ My answer didn't assume two PT100s per probe - the picture is for one probe's PT100 and the other probe's PT100. If you use 2x PT100s per probe then you need to be clear how this interfaces with your ADC. \$\endgroup\$
    – Andy aka
    Commented Jun 4, 2018 at 10:42
  • \$\begingroup\$ I stated that the picture was the circuit to interface just one PT100, I've made it more clear and added an sketch \$\endgroup\$
    – Bizcochito
    Commented Jun 4, 2018 at 11:01
  • \$\begingroup\$ The system sketch doesn't mean anything much to me. If you want a differential temperature measurement then why not just use the two probes in a wheatsone bridge? It's not clear in your (modified) question why you cannot do that. \$\endgroup\$
    – Andy aka
    Commented Jun 4, 2018 at 11:18
  • \$\begingroup\$ I've updated the schematic. The reason why I can't use a whetstone bridge is that I need to process each PT100 separately for calibration discrepancies and then provide the temperature of both PT100 in the probe (or the average) to the system to compare it with the other probe, also, the probes could be 20+ meters away \$\endgroup\$
    – Bizcochito
    Commented Jun 4, 2018 at 11:55
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Do you think is possible to use a different IC as MCU? You can use ADuCM360. It has 2 programmable current source for 2 different PT100. Here some literature for this chip and PT100.

https://www.analog.com/en/analog-dialogue/articles/afe-design-considerations-rtd-ratiometric.html

I used it, not so easy as PIC but very powerfull IC. For RS-485 i used this schematic that working fine up to 1mbit.

RS-485

PS. You can also buy their Develop board to test the system.

https://www.digikey.it/product-detail/it/analog-devices-inc/EVAL-ADICUP360/EVAL-ADICUP360-ND/5861440

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