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ADS131A04: Connection of unused analog inputs

Stefan Olejniczak57
Prodigy 140 points
Part Number: ADS131A04
Other Parts Discussed in Thread: ADS131A02

Hello,

I use this device with an analog supply of AVDD = + 2.5V and AVSS = -2.5V. Chapter "10.1.1 Unused Inputs and Outputs" states that unused analog inputs shall either be left floating or tied to AVSS.

My question is: which one is the preferred method? Is there any advantage of using one over the other?

Only in case connection to AVSS is the preferred method:

In that case, due to a more convenient layout, i would like to connenct unused analog inputs to GND (which in my case is about half way between AVDD and AVSS). My understanding is that this should be equivalent to the connection to AVSS. Is this assumption correct? Or may there be unforeseen problems (e.g. during power up/down if AVSS temporarily goes slghtly above GND)?

As for the ADS131A02: Does it have the same die as the ADS131A04? Are the NC pins 5 to 8 internally bonded to the die?

Thanks and best regards!

  • +1
    TI__Mastermind 18900 points

    Hi Stefan,

    Stefan Olejniczak57 said:

    Chapter "10.1.1 Unused Inputs and Outputs" states that unused analog inputs shall either be left floating or tied to AVSS.

    My question is: which one is the preferred method? Is there any advantage of using one over the other?

    There's no functional issues with any of the options you've stated. In other words, we don't expect anything to happen to the device is any of the inputs are tied to those voltage during power up or typical GND bouncing scenarios. If you want to do GND, you can do that.

    The absolute maximum specifications should still be followed, this could be an advantage with connecting it to AVSS (or AVDD) as the input should follow the rails without much risk of breaking the device. GND still puts us in the middle of the range, but there could be issues, as you said, if there's a massive difference in AVSS vs. GND, but I think you have bigger issues than needing to change the inputs connections in the schematic if you happen to have massive GND bouncing. 

    As for floating, I think most would recommend tying the inputs to a known voltage so there's no unknowns and current isn't being pulled for no reason as the floating voltage fluctuates. That being said, design has put that it is fine to float the inputs, there will be no function issues with the device, and the increase in current will be on the nano-amps scale. 

    Stefan Olejniczak57 said:
    As for the ADS131A02: Does it have the same die as the ADS131A04? Are the NC pins 5 to 8 internally bonded to the die?

    I unfortunately, can neither talk about die information or what is going on inside the package without a manufacturer's level of a NDA. Anything reason in particular that I might be able to help without answering that question directly?

    Best,

    -Cole