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Hi all,
We are working on a design that needs to measure temperature at two points. We are using NTC thermistors (resistance at 25C is 10K) for it. In order to interface the NTC outputs with a MCU , we
have selected OPA333. The schematic with the NTC thermistors is attached. TI's reference design Temperature sensing with NTC circuit is referenced for our design.in this reference design the featured
product is TLV9002; but OPA333 can use as alternate(also in the reference design).
The design is such that to get a output swing of 0.05 V to 3.25V.(-10 to 60 degree.C)
When the temperature across the NTC is varied , we can observe the change in voltage at the terminal of the Op amp (non-inverting terminal. The input swing is between 0.5 V to 2.65 V. But at the output ,
we could not get the expected swing . At lower temperature, the output seems clamped at 1.35 V ( the reference voltage) and for higher temperature the output just rises to 1.5V. The reference voltage is
stable at 1.35V.
We have done the simulation in TINA with
and we have got the expected gain (1.5) and output. the simulated circuit and DC transfer characteristics is given below.
.
Request to help us understanding the cause for this issue and a solution for it.
I am also attaching the detailed design document with my simulation results and reference design.
With Regards
Adarsh
NTC thermistor with OPA333 design.pdfNTC temp_using TLV9002.pdf
Hi Adarsh,
We have done the simulation in TINA with
and we have got the expected gain (1.5) and output. the simulated circuit and DC transfer characteristics is given below.
I duplicated your plot as shown below. Here is the explanation.
You have two voltage sources and you need to write out the transfer function, where you may solve the Vout/Vg via superposition method. Here are the steps:
1. Short Vref of 1.36V to ground or Vref = 0, which Vo1 = Vg(1+RF/Rl) = Vg*(1 + 4.7kΩ/10kΩ) = 1.47*Vg
2. Short Vg source to ground, which Vo2 = -(RF/Rl) * Vref = - 0.47*1.36 = -0.6392V
3. Add Vo1 + Vo2 and Vout's transfer function is: Vout = Vg(1+RF/Rl) - (RF/Rl) * Vref = 1.47*Vg - 0.6392V.
In the example above, where Vg = 0.5V, Vout will be 1.47*0.5 - 0.6392 = 0.0958V = 95.8mV
Enclosed is the simulation above.
OPA333 NTC Thermistor 08062021.TSC
FYI, the plot below is Vout vs. Rntc, which Vout needs to be converted to temperature via a curve fitted equation (Vout vs. Rntc or Vout vs. Temperature)
OPA2333 Thermistor Plot 08062021.TSC
If you have additional questions, please let us know.
Best,
Raymond
Hi Adash,
The NTC thermistor curve is a nonlinear relationship with temperature, see the attached thermistor excel. There are many types of thermistors, where thermistor resistance vs. temperature are different. The below is 10kOhm at 25C thermistors, which the R coefficient is different in two types as well in 10kOhm @25C. Please check the chart that you are using. In any case, the shape of R vs. Temperature relationship looks similar.
I did check with the simulation, which it indicates Vout = 1.36V --> R=14.3219kohm (17ºC), and Vout = 1.35V --> R = 14.4445kOhm (~16.5 ºC). assume the above thermistor chart is correct, you are seeing temperature change in less than 1 ºC. where 1.36V is equivalent to 17C. What you are seeing is the nonlinear behavior of the thermistor as a function of temperature.
OPA2333 Thermistor Plot 08072021.TSC
10K 25C Thermistor R vs Temp.xlsx
If you are not convinced, place the thermistor is 17C environment or water, you should measure the resistance approx. 14.32 kohm with a DMM. If you have 25C water, the thermistor resistance should be measured at 10 kohm when it is immersed in H2O. You can back calculate Vout of the circuit based on the R_thermistor value using the Vout transfer equation I supplied or trace the points via the simulated plot.
There are several ways to maximize Vout vs. R_thermistor. For instance, if you are able to configure OPA2333P as a buffer, you will increase the changes in Vout; you may increase the voltage of the thermistor voltage divider etc.. Anyway, if you are able to tell me the complete design requirements, I may be able to recommend something.
Best,
Raymond
Hi Adarsh,
The problem is, when I increased the temperature across the thermistor about 70 - 80 C, from room temperature, no change in the output voltage from 1.36V. as per the simulation and design, we are expecting the output swing up to 3.25 V at 60 C.
If you increase the thermistor at approx. 70-80C, you do not see any changes at output, the following possibilities may happen.
1. you have a bad thermistor or poor connection at thermistor.
2. The op amp circuit is not working, or it is wiring issues.
To troubleshoot the first issue, I have suggested that you may immerse the same thermistor in water with different temperature.
at 70C water, the thermistor DC resistance should be approx. 1.75kΩ with DMM.
at 25C water, the thermistor DC resistance should be approx. 10kΩ with DMM,
The DC resistance will change as a function of temperature. If it does not change, that is the problem.
The circuit above does not support up to 70C in temperature measurement. It looks like the output of op amp has reached the saturation (3.3V - 70mV = 3.23V) at 70C.
Here is the hand calculation:
Vthermistor at 70C = 2.8085Vdc
From the Vout transfer function, Vout = Vout = Vg(1+RF/Rl) - (RF/Rl) * Vref = 1.47*2.8085 - 0.6392V =3.489Vdc, which means the circuit is unable to operate in op amp's linear region at 70C in thermistor.
Vthermistor (2.488kΩ) at 60C = 2.6425Vdc, Vout = Vout = Vg(1+RF/Rl) - (RF/Rl) * Vref = 1.47*2.6425 - 0.6392V =3.245Vdc, which means that 60C may be the maximum temperature that can be measured by the circuit (Vout = 3.245Vdc is the upper limit temperature that may be sensed by the circuit).
Anyway, you observed no changes at the output as temperature temperature varies, which means that the following option may be likely.
Please configure the following circuit below, and you should see Vout = 2.426Vdc when Rntc = 10kΩ. In this case, Gain = 1.47 V/V. If you do not see the output, then I will check the wiring connections etc..
If you have other questions, please let me know.
Best,
Raymond
Hi Adarsh,
we also connected the inverting terminal to ground and checked the output voltage. at that time I get Vout =40mV, at room temperature.
Could you provide me the voltage measurement in the following nodes that are circled in the circuit? I'd like to know the voltage in
1. Vin+
2. Vin-
3. Vcc
4. Vout
As simulated, Vin+ should measure approx. 1.65Vdc with thermistor at room temperature. Vin- should read also 1.65Vdc. Vcc is said to be 3.3Vdc. Per the simulation, the output is approx. 1.786Vdc.
I do not know where Vout = 40mV comes from. If you have input voltage at Vin+, say 1.65Vdc, the output can not be 40mV, since the circuit gain is 1.47V/V. The output should be 1.65Vdc * 1.47 V/V = 2.43V approximately, if Vref is grounded or Vref = GND=0.
Could you take a setup picture and share with us?
When I increased the NTC temperature, the Just raised to to 110mV at 70 C.
Regarding to this measurement, the output should be railed and saturated in the circuit above, and Vout should be measured close to 3.25Vd (saturated). Please also feed me the voltage node measurement listed above, and I will see where the issue may be.
Best,
Raymond
Hi Raymond,
We have done the measurements as you said. The results are shown in the table below.
| Temperature | Vin+ | Vin- | Vout | Vcc | Vref |
| Room Temp | 1.61 |
1.34 |
1.33 | 3.3 | 1.35 |
| 70C | 1.90 | 1.34 | 1.34 | 3.3 |
1.35 |
I am also attaching the setup image.
Here we have only populated the op amp part in our board and powered it using a power supply. The NTC is immersed in water with 25 C and 70 C. and measured voltages on a DSO.
Also attaching the PCB part with op amp.
I already sent the schematic part.
Thank you once again,
Hi Adarsh,
There may be a couple things going on in your setup.
1. In your Tamb reading, Vin+ =1.61V is within the room temperature range, but Vin- or the feedback voltage does not follow the Vin+ of 1.61V, which means that you op amp is not operating in linear region.
Question: Do you have something loaded down at the output of the op amp? What is OPA2333's load? If you tell me that you have no load at output and all wiring connection or PCB traces are good, then op amp is gone bad.
In the 2nd temperature measurement, 70C should have NTC thermistor value approx. 1751Ohm (the spreadsheet I attached previously). Therefore, if the measured T is indeed 70C, then the Vin+ should be measured at 3.3*10k/(10k+1.751k) = 2.808Vdc. You reported 1.90Vdc, something is wrong.
Assumed input is measured at 1.90V, the thermistor's temperature should be 3.3*(10k/(10k+X) = 1.90V or X = 7.368kΩ (that is approx. 32C) here is the simulation per the measurement condition.
Your op amp setup is not operating in a linear region, where Vin- has to follow Vin+.
So I believed that something is wrong with your op amp or PCB board or bad traces/connections. Since the output does not change, you may load down the output or there is a voltage presented at the output.
Please ground both inputs of OPA2333 and measure the output voltage. I think that you have to trace pin by pin with a conductivity/DMM meter.
By the way, I have indicated that 70C measurement will saturate the op amp circuit. In other words, the circuit may be able to measure the maximum temperature that is close to ~60C.
Best,
Raymond
