RTD & NTC

Hi Saeed,

Could you provide the design requirements or schematic you had with the RTD module.?

RTD is PTC, and thermistor is NTC. Are you referring to thermistor sensing element for the application? Please specify and I need to know the NTC type, accuracy and temperature operating range, available voltage rails current consumption etc.. Enclosed are 3 application notes that may be useful for your application. 

https://www.ti.com/lit/an/sbfa007/sbfa007.pdf?ts=1708967448356&ref_url=https%253A%252F%252Fwww.google.com%252F

sboa017 XTR105 application note.pdf

https://www.ti.com/lit/an/sboa323a/sboa323a.pdf?ts=1708967866889&ref_url=https%253A%252F%252Fwww.google.com%252F

Best,

Raymond

Hi Raymond, 

Really appreciate  your reply.

Yes, we are getting NTC thermistor as input like i told you about RTD module which we already made the module. I'm going to attach the Chart and specification of NTC as well. 

I explain about RTD module, module taking RTD inputs and process it through XTR105 and transmit 4-20mA as an output. 

Now I wanna make the NTC module which take NTC as input and process it through logic(not sure which IC i can use for NTC) and transmit 4-20mA as an output.  

Hi Raymond, 

In simple words, I got NTC sensor output which is actually my input because I wanna process it between 4-20 mA. Same like my module taking RTD input and through XTR105 gives 4 - 20 mA. Does any IC do that for NTC sensor? Another thing i have Fluke 726, there is an option RTD outputs PT 100 or PT1000 how can i get NTC inputs from Multimeter?

Hi Saeed, 

XTR105 can be used for measure thermistor and convert the voltage to 4-20mA. The circuit looks similar to the image below, which it use the IC's constant current source to drive the bridge or NTC to generate voltage difference and converted to current. The voltage difference is directly proportional to temperature via the NTC temperature curve or calibrated temperature range. 

We have XTR115/XTR116/XTR117 or XTR111 that can be used for convert the Voltage to 4-20mA. We can use a voltage source to drive NTC, where Vout is function of temperature via the voltage divider, and use XTR11X IC to convert to 4-20mA current loop.   

https://www.ti.com/lit/an/sboa323a/sboa323a.pdf?ts=1708969738498&ref_url=https%253A%252F%252Fwww.google.com%252F

mujtaba saeed said:

can i get NTC inputs from Multimeter?

I have not seen typical DMM that is taking NTC input and convert to temperature (I have seen the specialized NTC temperature monitor, but it is not DMM). You could use resistance scale and measure the NTC's resistance and use the NTC chart to obtain the measured temperature.  

If you use above circuit, Vout can be configured proportional to temperature, and read out the temperature directly from the DMM. The NTC can be used as is or NTC's temperature curve may be calibrated to improve the accuracy. In the example, TLV9002 is a general purpose op amp. For precision application, we can use low offset, zero drift op amp (5Vdc?) to sense the NTC thermistor. 

What is your temperature sensing range? What is available supply rail? Both XTR105 and XTR11X series require minimum of 7.5Vdc in order to power the IC. 

Best,

Raymond 

Hi Raymond, 

Temperature range -45C to +105C. Yes its 15V allocated to powerup the IC. 

Hi Saeed, 

There are many options to design the thermistor circuit, and convert the voltage to 4-20mA current loop. I need to have clear direction how you want to proceed. 

Your thermistor application is operating such a wide temperature range from ~700kΩ down to <100Ω range (-45C to 105C) per the thermistor curve. By the analog circuit alone, it is difficult to make accurate temperature measurement from the non-linear curve in the entire temperature range. 

mujtaba saeed said:

Now, i need to make the other module NTC module which will give precise output 4-20 mA same like RTD. 

All the V-to-I conversion is precise from the voltage to current conversion. So the this part of signal conversion is not an issue. 

Since your thermistor temperature is so wide, it is difficult to keep the thermistor temperature accuracy over the entire range like RTD. In other words, temperature to voltage conversion over from -45C to 105C will have measurement errors, and the temperature accuracy requirements are difficult to meet with a simple analog circuit. 

The accuracy requirements for the thermistor is shown below. You may have to perform the NTC curve calibration and obtain the thermistor's individual curve fit (or look-up table) in order to meet these accuracy requirements from -45C to 105C.  So I do not know how to proceed in order to meet the temperature accuracy requirements.  

Even you perform some kind of Thermistor linearization, you will get something like below, where you may obtain a good linearization near 25C, but the temperature nonlinearity near the low and high temperature will not be great. 

Sensors and Analog Interface.ppt

Please let me know why you are using thermistor to measure temperature in such wide range. XTR105 will work out great for RTD sensor, but it does not have a great advantage for thermistor application, since your sensing temperature range is so wide for thermistor application. 

Best,

Raymond

Yes please that would be great, if ill get the simulation.