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Original question:

PGA280: Worst Case Analysis Help

PGA280: SPI Signal ABS Min/Max

Patrick O'Berry
Prodigy 20 points
Part Number: PGA280
Other Parts Discussed in Thread: PGA849, PGA855

Tool/software:

Hello,

I am seeking guidance regarding the PGA280.  Specifically what the input min max voltage range is for the SPI signals.  The datasheet is somewhat ambiguous.  It states that Signal input terminals, voltage cannot exceed VSN - 0.5 to VSP + 0.5.  There is no distinction between the analog input signals or the digital input signals.  In my case this would be +/-15.5V, but my DVDD is equal to 3.3V.  It is uncommon to see digital signal's spec'd to tolerate voltages much beyond DVDD so this is suspicious.  I guess the question is if the absolute max rating listed as "Signal input terminals, voltage" applies to all inputs on the device, and if it does not apply to the SPI signals, what is the absolute rating?

  • 0
    TI__Mastermind 29076 points

    Hey Patrick,

    I will confirm internally, but I believe the maximum input voltage will be about 1 diode drop above and below the digital rails (DVDD+0.3V, and DGND-0.3V). Typically the inputs are diode clamped to their respective rails. If this is correct, we will work swiftly to update the abs max table in the datasheet.

    Best,
    Jerry

  • 0 in reply to Gerasimos Madalvanos
    TI__Mastermind 19612 points

    Hey Patrick, 

    The abs max table is referencing the signal input terminals: INP1, INN1, INP2, INN2.

    The digital IO specification is located on page 6 of the datasheet: 

    The max input is DVDD and min input is 0V. I recommend connecting the digital supply (DVDD) to the main digital supply of the system, thus matching the logic levels of the SPI signals.

    As for the internal diodes that Jerry mentioned (typical in op amp products), these are not on the DVDD and DGND pins: 

    (Page 39 of datasheet)

    For more resources regarding the SPI communication with PGA280, I recommend: PGA280 Communication via SPI. 

    All the best,
    Carolina

  • 0 in reply to Caro Walter
    Prodigy 20 points

    Caro, Jerry,

    Unfortunately I find these responses confusing as they are not in complete alignment with one another.  But that said it seems you both agree that the absolute min/max section does not apply to the digital signals.  

    Caro's response suggest that there should never be a time when the input voltages are over DVDD or below DGND.  This does not allow any headroom for overshoot/undershoot on the signals.  

    Is there an guidance to be had for this device regarding an allowance for AC transients above/below the DVDD and DGND thresholds?  

  • 0 in reply to Patrick O'Berry
    TI__Mastermind 29076 points

    Hey Patrick,

    Sorry for any confusion, I will test if these pins are internally diode clamped to DVDD and DGND, however, due to the US holiday, I will not be able to measure this until I am back in office and have samples. I'm placing the sample order today and will update you once I have the devices and again once I have the data. This will show the tolerance above/below DVDD/DGND, however,

    We appreciate your patience, our team is currently working to build up our internal sample inventory, and we apologize for any delays in support this may cause.

    Best,
    Jerry

  • 0 in reply to Patrick O'Berry
    TI__Mastermind 29076 points

    Hey Patrick,

    Samples arrived and were tested today, it seems that the digital IO pins are diode clamped to DVDD/DGND. However, the behavior of each pin seemed to be different. Our next step here is to reach out to the design team to see if they can generate an equivalent block diagram, similar to the one seen in other PGA datasheets.

    Here is one example from the PGA849 datasheet.

    Best,
    Jerry

  • +1 in reply to Gerasimos Madalvanos
    TI__Mastermind 19612 points

    Hey Patrick, 

    Good news! Jerry was correct and there is input voltage protection for all digital pins on the PGA280, therefore the pins can tolerate the following: (DGND) - 0.3V to (DVDD) + 0.3V. The design team has confirmed this, and we will update the datasheet to making it significantly clearer. 

    What I had referenced that there wasn't input protection, this is in reference to other PGA protection, like the PGA855/849 that can tolerate ±40V from the supply rails: 

    The PGA280 has diode input protection for INN, INP and that must not go about ±0.5V from the supply rails. 

    Thank you for understanding on conflicting responses and let me know if you require further explanation. 

    All the best,
    Carolina