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PT1000 sensor count bounce

SIVARAMA KUMAR PASUPULETI
Prodigy 160 points
Other Parts Discussed in Thread: ADS1248

Hi,

I am using ADS1248 for PT1000 sensor measurement. Below are the details of PT1000 configuration.

PGA gain = 8

SPS = 5

IDC source = 0.1mA

Reference resistor = 8.2Kohm

Reference = 8K2 * (0.1mA+0.1mA) =1.64V (Generated with the help of ratio metric measurement i.e with IDAC current source)

Problem:


 The ADC count is bouncing in PT1000 case.


Analysis Done:


We have taken the waveforms across the reference resistor. Here the reference voltage is oscillating. Please refer the attached image for the waveform.

I have used the same circuit for PT100 input.

PGA gain = 8

SPS = 5

IDC source = 1mA

Reference resistor = 820 ohm

Reference = 820 * (1mA+1mA) =1.64V

Here there is no count bounce or reference voltage  Oscillation issue. Refer the attached image for the waveform observed

Question:


 What is the reason for this Bounce in reference voltage?

Whether it is because of 0.1mA current source or something else?

Please find the attached circuit for your reference. The same circuit is used for PT1000/PT1000 sensor connection. For PT100 case 820E resistor is placed in place of 8.2K ohm resistor at reference section. Except this change the same circuit is used for both the sensors. We have observed some wave forms across 249Ohm series resistors, for PT100 case there is no bounce in the voltage measured across it but for PT1000 case there is a bounce in voltage measured across the 249Ohm series resistor.

PT1000_Circuit.pdf

Regards

P. Sivaram Kumar

  • 0
    TI__Guru* 97835 points
    Sivaram,


    If you look at the first scope plot, the oscillation looks to have peaks at every 20ms. This gives a frequency of 50Hz which I suspect is the same as the power line cycle frequency where you are. I'm not sure why you see it in the case of the PT1000 and not in the PT100. However, it just may be because of the lower amount of current used in the PT1000 case.

    If the power line cycle noise comes from some sort of EMI effect, you may be able to improve the performance by simple shielding. If the noise comes from the power supply connected to the device, then you will need to clean up the power supply, possibly with some sort of LDO. Another possibility may be that the noise comes from the ground moving with return current. This may be a problem with the layout in your board.

    Regardless, at 5SPS, the digital filter should be able to reject this 50Hz noise. It's likely that you wouldn't see this noise during operation.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 160 points
    Dear Joseph,
    We have used LDO as supply.
    If it because of the noise comes from the ground moving with return current it has to be there with the PT100 (1mA) also.

    When I took the readings across 249Ohm series resistor, for PT100 the voltage is fine but for PT1000 the voltage is bouncing. So I thought it will be the problem with 0.1mA current source that is coming from the ADC?
    Please confirm that whether it is from IDAC source or other?

    Regards
    P. Sivaram Kumar
  • 0 in reply to SIVARAMA KUMAR PASUPULETI
    TI__Guru* 97835 points
    Sivaram,


    Without knowing more about your circuit, it's hard to know where the noise is coming from. However, the noise is clearly a 50Hz frequency. Isn't that that the power line cycle frequency where you are? This is almost certainly the source of your noise.

    If your supply comes from the power line, the noise is likely coming from the power somehow. If you want, you could run this off of a battery power to test to see if the noise goes away.

    Additionally, if the device is open, you could work to shield the device in some sort of metallic enclosure so that EMI could be reduced.


    Joseph Wu