RTD Temperature Sensors - jump/drop in readings

 I have 28 groups of temperature sensors made up of a mix of 2N2222 transistors and PT100 resistive thermometers. Each sensor is set up through a 2-wire lead to one of eight AD7124-8 boards and placed within a vacuum enclosure. I'm running into a periodic issue where after about 0.5-3 hours of runtime, most of the sensors will jump/drop dramatically, as shown in the attached image. The PT100 sensor temperature readings drop dramatically while the 2N2222 readings jump slightly. This all occurs simultaneously during the same reading interval. As far as I can tell, the sensor readings remain at the new reading unless a new run is initiated. The answer to why this error is happening is probably obvious, but I'm having trouble isolating it. My first thought was that the sensors were short-circuiting, but I don't know how to explain most of the sensors short-circuiting at the same time and why it would be occurring so regularly. Any insight into what might be going on would be useful. If more information is needed, I'll gladly provide it. 

  • 0
    •  Analog Employees 
    on Nov 19, 2020 2:45 AM

    Hi, 

    Can you share us your schematic? And configuration? I was wondering the method you used for this temperature measurements (i.e. exciting the sensor, ADC configuration, etc. I was wondering some self heating issue for the selection of excitation current or the drift of the IC sensor, or a crosstalk issues, or any possible offset or gain error or drift but we can't tell yet as there are lot of possible reasons. It is not yet clear to me what's happening, but if you can provide more details then that would be helpful. 

    Have you also tried to check one sensor at a time? So for example, test a single RTD circuit only or a single IC sensor only. This is just to understand the response of individual sensor and avoid the possibilities of affecting one another. 

    Thanks,

    Jellenie

  • Sorry for the later reply - I had to collect the information you asked for. Here is a schematic of the setup I have (sorry for the roughness of it). I'm reading from 8 AD7124-8 boards, each wired in the way shown below. I highlighted the connections which are shared across all 8 boards. I have 8 RTD temperature sensors connected to each board using the 16 analog inputs available. Each sensor's positive input is split into two when connecting to the analog inputs, one to supply an excitation current of 50microA and the other to supply a voltage input. All sensors on a single board share a common ground on the corresponding board. As for my ADC configuration, AVdd (3.3V) is set to be the reference voltage, the gain = 1, and a unipolar analog input range. The internal voltage is disabled, the ADC set to to low-power and single conversion mode.

    In fact, now that I'm writing this, I'm not sure why we had decided to set AVdd as the reference voltage and then also supply an external reference voltage. I also considered that the jumping issue could have been due to our external reference voltage possibly not being precise enough, but based on our configuration, the ADCs might not even be referencing the external reference voltage. 

    I also took your suggestion and tested a single diode sensor over night. Below is the result of that test. The jump still occurred around 2 hours into the run. 

    Thank you for your input. 

    Edit: I let the previous run keep reading and it turns out that the sensor readings don't tend to remain at the new reading, but will at some point revert back to the initial ranges. 

  • 0
    •  Analog Employees 
    on Nov 25, 2020 7:09 AM in reply to Lagreeni

    Hi, 

    I am not sure if I understand correctly your circuit configuration. However, the recommended circuit configuration and ADC configuration for RTD measurements are described in CN-0383 I think the circuit note also explained the importance of the ratiometric configuration. 


    We also have an RTD Calculator Tool that can help you design your RTD circuit using AD7124. Tried to configure your system using our recommended configuration and see if the performance improved. Another worth trying is to use a fixed precision reference resistor (100ohm) for example in replacement with RTD sensor. This is to ensure that the resistance value of the sensor is not varying.  

    As for the case of your other sensor, I believe that is a simple transistor so you need additional circuitry to turn it into temperature sensor? Is that correct? Can you also check if your circuit there is stable, I mean the output voltage is somehow stable even before going into the ADC. This is just to isolate the two circuitry and see where is the issue coming from. 

    Thanks,

    Jellenie 

  • I abandoned troubleshooting this problem for a while, but I returned to it earlier this week and believe I found the issue. I was using a raspberry pi's 3.3V output pin as a reference voltage, AVdd. Once I switched the ADC configuration to actually reference the high precision external voltage reference board we had set up, the jump error disappeared. 

  • 0
    •  Analog Employees 
    on Jan 8, 2021 2:08 AM in reply to Lagreeni

    Hi, 

    Good to hear you've found out the issue. A reference from digital supply isn't really recommended as digital supplies are a bit noisy and the GPIO might not be capable of driving the reference input as digital source/sink currents are usually lower. 

    Thanks,

    Jellenie