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ADS1148: ADS1148IRHBR

Diego Bolzoni
Prodigy 30 points
Part Number: ADS1148
Other Parts Discussed in Thread: ADS114S08

Good morning,

we are using the ADS1148IRHBR to measure the temperature of four PT 100 sensors.

 

In the attached file, you can find the schematic.

 

Our logic is the following:

-) we consider valid values of resistance if the measured resistor is 76,65ohm < value < 212ohm

-) if the resistor is >212ohm, we indicate "circuit open" on the respective input channel

-) if the resistor is <76,65 ohm, we indicate "short circuit" on the respective input channel

 

We are in the prototyping phase and we realized 7 samples of the hardware.

 

5 samples work fine, but we have problems on 2 samples.

 

On these 2 samples the behaviour is the following:

-) if we connect a resistor between PTx_1 and PTx_2 (where x is a generic channel) with a value between 76,65ohm and 212ohm the measure is correct

-) if we leave the circuit OPEN from PTx_1 and PTx_2 (channel open), the measure is very instable and the measure floats irregularly between values from 76,65ohm and 212ohm and values < 76.65 ohm.

This is incorrect because the detected values must be >212ohm (stable value) which indicate circuit open.

NOTE 1: the incorrect behaviour regards all the 4 channels.

NOTE 2: the other 5 samples works fine.

 

We tried to:

- remove all the clamping diodes LL4148

- remove all the TVS SMAJ12CA

- remove the filter capacitors

- substistute the filter capacitors

but all of these operations don't obtain any positive results.

 

Do you note some problems in the schematic?

From the described behaviour, can you understand the cause?

 

Please can you help to understand the cause? 5 samples at the end work fine

 

We soldered the board with a plate between 220°C and 240°C? Is it possible that the component was damaged during the soldering process? If yes, please can you indicate the instructions for the soldering with a plate.

 

Please let me know as soon as possible.

 

Best regards.

 

Diego Bolzoni

  • 0
    TI__Guru 52586 points

    Hi Diego,

    If you attached a file I did not see it unfortunately. I only see the image you posted in the thread.

    How are the RTDs being biased? It looks like the ADS1148 IDACs are "NC", which I took to mean no connection.

    I also do not see how the RTDs connect to the reference resistor (R18), which is labeled as "BIAS". Can you provide this info as well?

    Here is some information about the recommended solder reflow process: https://www.ti.com/lit/pdf/spraby1a

    You can also refer to the qualification summary by part number: https://www.ti.com/qualificationsummary/qualsumm/home?actionId=2800

    And the MSL info is included in the packaging info at the end of the datasheet

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 30 points

    Good morning Bryan,

    many thanks for your answer.

    I will complete the circuit with the external PT100 (between PT1_1 and PT1_2) and a lead between PT1_2 and BIAS.

    I drew only the channel #1 (PT1_1 - PT1_2). For the other channels the connection is the same 

    With an external PT100 the ADS11448 works well and it measures correctly.

    The problem is leaving:

    PT1_1 = open circuit

    PT1_2 = open circuit

    BIAS = open circuit

    so without external connections.

    In this case the measure is very instable and the measure floats irregularly between values from 76,65ohm and 212ohm and values < 76.65 ohm.

    This happens on 2 board samples. The other 5 board samples work fine and it is possible to detect the open circuit.

    I can't understand the cause of the difference of the behaviour. For this reason, I hypothesized the damage of the component during the soldering.


    Schematic remarks:
    -) NC_U1_26 and NC_U1_25 are not connected
    -) NMT means "not mounted"


    Please let me know as soon as possible.

    Many thanks.

    Best regards.


    Diego Bolzoni
  • 0 in reply to Diego Bolzoni
    TI__Guru 52586 points

    Hi Diego,

    I still don't see how the RTDs are biased, since you are not using the ADS1148 IDACs. Do you have any external IDAC circuit to bias each of the 4x RTDs?

    If the IDAC current is flowing through the RTD and then RREF under normal operation, and then you remove RTD, the IDAC current no longer flows through RREF. This means the voltage across RREF will be approximately 0 V. However, you are using RREF as your reference voltage, so you are measuring a floating input against ~0V. The value you measure could be anything at this point, depending on how long it takes for the charge across the RREF cap (C14) to dissipate as well as what voltages the inputs float to. It is not surprising then that this doesn't work, I am actually surprised it worked on the other boards. From a theoretical standpoint, this should not provide you any meaningful information.

    If you are doing something different, please let me know.

    Also, the ADS1148 has a VREF measurement feature in the MUX1 register using the MUXCAL bits. This features measures the REF0 voltage relative to the internal voltage. If you get a near-zero measurement, this indicates an open circuit on the RTD. The downside with this approach is that you need to take a separate measurement, so you need to stop taking RTD measurements, enable this feature, take a measurement, and then revert back to taking RTD measurements. We have newer devices e.g. ADS114S08, that offer constant VREF monitoring for this very application. You can also reference our RTD wire break app note for more information: https://www.ti.com/lit/an/sbaa483/sbaa483.pdf

    -Bryan