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AGND and DGND problem for ads1240

user4242762
Prodigy 30 points
Other Parts Discussed in Thread: ADS1240, ADS1220, ADS1242

hi.

For some device, the AGND should be connected with DGND together. But for some devices, the AGND can be connected to negative supply.

ABSOLUTE MAXIMUM RATINGS if ADS1240 is as following:

DGND to AGND .................................................................... –0.3V to 0.3V

I don't know if I can apply a -0.23V negative supply to AGND. If i do so, what indexes will degrade? Will the device consume large current?

I had inquired chinese engineer. But their professional knowledge is too poor indeed.  They can not provide correct answer. their efforts tend to mislead us.

  • 0
    TI__Guru* 97655 points
    Hi,


    The ADS1240 was designed so that the AGND and DGND were at a common potential and tied together. I'm not sure what would happen if they were offset by -0.23V. It might work fine, but it has never been tested and certainly wouldn't be guaranteed to specification. The only thing that the Absolute Maximum Ratings show would be that the device shouldn't see any permanent damage. It does not guarantee operation.

    Internally, the AGND and DGND probably has some sort of back-to-back diode between the two pins. This would explain the -0.3V to +0.3V absolute maximum. If the voltage were larger, then there would likely be some sort of forward diode current between the two pins. This current would likely depend on the temperature.

    Additionally there are some logic gates between the digital section and the analog section. The link between the two may be disrupted and cause problems in operation.

    Again, operation with this split ground might work, but would not be guaranteed and has never been tested.

    Why are you asking about this type of operation? Is it because the input range with the buffer doesn't go all the way to AGND? If that is a problem, you may want to consider something like the ADS1220 which has similar features and can have an input range that is slightly outside the supplies when the PGA is bypassed.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 30 points

    I suspect your answer is not very authoritative.

    There is ADS1242, which is very similar to ADS1240, while the ads1242 has single GND and ads1240 has AGND and DGND. If the AGND and DGND of ADS1240 should be tied together, why does TI product two ads1240 and ads1242 at the same time?

  • 0 in reply to user4242762
    TI__Guru* 97655 points
    Hi,


    I recommend that you read this:

    www.ti.com/.../slyt499.pdf

    This article talks about grounding and having different analog and digital grounds and how they can be treated. It also mentions that analog and digital grounds are often separated to two pins because you don't want any of the internal digital return currents to couple into the analog section of the device.

    In many cases, we recommend laying down a common ground plane and have digital and analog return currents in the plane flow away from each other and the device. Generally having a ground plane is desired both for the a low impedance (and low inductive) current return path, but also for EMI shielding.

    You also asked about TI having two similar devices in the ADS1240 and the ADS1242. At one point we thought that the lower pin count of the ADS1242 could be useful for smaller, portable applications and we started removing different pins to fit a smaller package. The ADS1242 was the result.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 30 points
    hi, wu.
    ok, i have got it.
    thanks