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Original question:

ADS1248: Detecting broken PT100 lead by using the External Voltage Reference Monitor

ADS124S08: Detecting broken PT100 lead by using BOCS

Nikolai Nikolov
Prodigy 160 points
Part Number: ADS124S08

Hi Joachim,

The old thread has been lock, so I had to open a new related one.

My work with ADS124S08 is progressing well and we are going to use the new HW (and the ADC) for several projects.

As advise by you in the past, I desired to test the use of BOCS to detect a disconnection of my sensing inputs of the PRT in 4-wire connection mode.

Up to now, I have tested the detection of a disconnection of the current-injecting wires in the 4-wire configuration, which does not need the involvement of the BOCS and the where the STATUS byte is sufficient for the detection of the condition. When I+ wire is disconnected, the STATUS byte becomes 20 (decimal), and when I- is disconnected, STATUS becomes 40 (decimal) (0 is the normal STATUS value).

However, without using BOCS, if S+ or S- wires are disconnected, STATUS does not change and stays 0.

In my measurement cycles reading of the 2x PRTs and the 2x calibration resistors, I introduced and extra measurement cycle, where I activate BOCS, by setting the SYS_MON[2:0] to 111. In this cycle I alternate the multiplexing, so every other time BOCS is connected to the first PRT, and the next to the second PRT. (I do not activate BOCS for the calibration resistors of course).

I discovered that by introducing this additional step in my SW, the STATUS byte I read, during the rest of the steps, where I measure the PRTs, is changing and detects the S+ and S- wires disconnection!? The STATUS value if S+ is disconnected is 36 and if S- is disconnected is 4.

It looks like there is some sort of memory, remembering that in the BOCS step failure has been detected and then in the next measurement cycles, the STATUS byte is reflecting it.

I could not find any explanation in the manual, how this might be so.

I do not make use the value measured during the BOCS cycle, which should be equal to the full-scale reading in case of disconnected wire, but rely on the fact that the STATUS byte in the following measurement cycles will reflect the detected disconnection as described above.

Would you please comment on how reliable this approach would be, I mean the fact that somehow the disconnection detected in BOCS cycle is remembered and reflected later in the corresponding STATUS byte?

Because this is not mentioned in the manual, I could not be certain that it is part of the expected official behaviour and will be maintained in the ADC design.

Thank you for your advice!

Best regards,

Nikolai

  • 0
    TI__Guru** 113765 points
    Hi Nikolai,

    Using the BCS to check sensor wires is a viable method to use. One thing that is most likely happening is the input filter caps are charging to levels outside of the PGA thresholds when a wire is removed. The current source will deliver charge to the caps and the charge will remain until it depletes or leaks from the cap. It will take some time for the charge in those caps to leak out sufficiently for the status byte to clear.

    If you detect a fault with the BCS due to a wire break, then the charge on the caps will remain for a relatively long duration as the input is now sampling the voltage across the differential cap. You will have faulty data as the sensor wire is broken.

    To some degree even with unbroken sensor wires you will see a similar analog settling as the cap will charge then slowly discharge through the series resistance used in the input filter. With the use of the BCS there will be some analog settling that will give the effect you are seeing. To minimize the effect, try using a smaller BCS setting instead of the 10uA setting.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Prodigy 160 points
    Hi Bob,

    Thank you very much for the reply!

    I manage today to test using 0.2uA BOCS current. I got almost the same result. The difference was that the STATUS value if S+ is disconnected was initially equal to 4 (decimal) and within a few seconds became equal to 36, which was the value I observed when the BOCS current was 10uA.

    I also do not see any deterioration of the measurement with or without BOCS cycle in my ADC SW, when I use the calibration precision-resistor array, which we use for checking/calibrating similar products we manufacture.

    The only thing that bothers me a bit and I forgot to ask earlier is that when BOCS is used, the manual does not specify whether IDAC1 (IDAC2 I don’t use) should or should not be disconnected.
    For that reason I did tests with IDAC1 connected and disconnected to I+ PRT wire. The results I mentioned earlier and in my previous message were captured with IDAC1 connected (the multiplexing is the same as for the normal measurement cycle).

    If the IDAC1 is disconnected in the BOCS measurement cycle, when S- PRT wire is disconnected, the STATUS byte does not change and stays 0. In all other cases of wire disconnection, the STATUS byte is non-0 and has the values already mentioned.

    Please, confirm whether in BOCS cycle IDAC should be connected as in the normal measuring cycle or not.

    Best regards,
    Nikolai
  • 0 in reply to Nikolai Nikolov
    TI__Guru** 113765 points
    Hi Nikolai,

    One suggestion I have is to take your schematic and use a colored pencil and mark the various current paths as both normal and with broken or missing wires. If you follow the current path with BCS in the path, you will see there is a voltage drop created across the filter resistors and the RTD and this will introduce an error. You should turn off the BCS when measuring the RTD.

    As far as the various sensor lead detection is concerned, the BCS only checks the S+ and S- leads. The IDAC source should have no effect. You need to make sure the configuration is PGA enabled (in PGA register 0x03) and that the PGA flags are also enabled (in IDACMAG register 0x06). For a missing S+ lead you should see the FL_P_RAILP flag set indicating that the AINP input of the PGA is pulled high to the AVDD supply. With the S- lead missing, you should see the FL_N_RAILN flag set indicating that the AINN input of the PGA is pulled low to the AVSS supply.

    When you turn off the IDAC (or it has a missing lead) the BCS will be routed differently. In this case there will be a current divider at the junction of the AINN input and the reference resistor. In this case if the S- lead is missing you will see the FL_P_RAILN and FL_N_RAILN flags set as the AINP BCS is being pulled to ground through the reference resistor.

    To determine which flags will be set for the various conditions, it is helpful in charting the current paths for the various sensor wires to see what the voltages will be for the various missing wires at the reference and analog inputs. You should be able to determine which flags should be set for the various sensor wire conditions before doing any testing.

    Best regards,
    Bob B