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ADS1248: ADS1248 used for high-side current sensing

user4700057
Prodigy 60 points
Part Number: ADS1248
Other Parts Discussed in Thread: ADS124S08, ADS1148

Hi,

I'm planning to use ADS1248 as the high side current sensing of multiple channels of 3.3V voltage supplies. The schematic is shown as below.

The range of the current is 0~600mA. (single direction), the desired sampling frequency is >=200Hz.

The filters are designed to suit this sampling frequency. The input low pass filter uses 1.2Kohm resistor, as the datasheet recommends <10kOhm.

I'm planning to use a PGA gain of 32, so for 0.1Ohm sensing resistor, the full scale input current range is -640~+640mA.

The AVDD of ADS1248 is supplied with 5V. According to the calculation, the allowable common mode voltage range is 1.124V ~ 3.876V, so 3.3V won't exceed this range.

Most of the reference circuit of ADS1248 is for RTD temperature measurement so I've never seen a similar current measurement circuit before.

So Please help me check if this circuit can work as desired. 

Any other recommendation on improving performance and reliability (such as PCB layout considerations, ESD protection measures) are welcomed.

Thanks for the help!!

Regards,

Shan Song

  • 0
    TI__Guru 55412 points

    Hi Shan,

    Thanks for your post and welcome to the forum! 

    I don't see anything wrong with the measurement. I do have a few other comments though. 

    I recommend removing the inductor from the 5V AVDD supply. We've seen issues arise when there is inductance on the supply line when the internal reference is turned on. If you're intended use for this as a filter, a small resistor should suffice. 

    In order to save board space, consider removing half of the capacitors for the input filters and doubling the resistance. 

    Last but not least, if this is for a new design I encourage you to take a look at the ADS124S08. It's our latest and greatest device for this space and offers better accuracy and resolution compared to ADS1248.

  • 0 in reply to Alexander Smith
    Prodigy 60 points

    Hi, Alex,

    thanks for the quick feedback.

    The said component is a ferrite bead. But I think what you said also applies to ferrite beads too, right?

    What is the value of resistor for the noise blocking? can you suggest one? 

    Thanks!

    Regards,

    Shan Song

  • 0 in reply to user4700057
    TI__Guru 55412 points

    Hi Shan,

    Yes, ferrite beads are similar to inductors so it still applies. The resistor should be just large enough to create a low-pass filter when combined with the decoupling caps, less than 100ohm, 10ohm should be fine. You will want to place the 10u cap close to the source of the power supply and the 100n cap as close as possible to the device pin. 

  • 0 in reply to Alexander Smith
    Prodigy 60 points

    Hi, Alex,

    Again, thanks for the useful information.

    I have two more questions considering the product life cycle support:

    1. Can ADS1148  be used as a drop-in replacement if lower resolution is needed? (I'm using TSSOP 28 package for both)

    2. Is there any long term support of both ADS1148 and ADS1248? (so that they don't go obsolete for next 5 years?)

    Many thanks!

    Regards,

    Shan

  • 0 in reply to user4700057
    TI__Guru 55412 points

    Hi Shan,

    Happy to help! 

    1. Yes - the software would need to be modified to reflect the lower resolution though (clocking fewer bits out).

    2.  Below is an excerpt from our policy on this: 

    TI does not have any longevity programs or make statements on how long a device's manufacturing lifetime may be. Here is a quote from a document located at www.ti.com/quality It is in the General Quality Guidelines document.

    TI’s Product withdrawal/discontinuance process complies with J-STD-048, latest issue. TI makes an effort to not obsolete products out of convenience. Convenience means: low running device, poor yields, limited customer adoption or similar items. TI’s obsolescence withdrawal schedule provides a longer lead time than the industry standard. TI allows 12 months for the last order and an additional 6 months to take final delivery of obsolete items. In rare circumstances, an accelerated withdrawal schedule may be necessary. In such cases, TI will communicate the last buy and final delivery dates in the EOL notice, along with an explanation of the circumstances necessitating the early withdrawal.

    I can't guarantee anything, but I do not see this part becoming obsolete in the near future. If you are concerned about it, then I would encourage you to consider the ADS124S08. 

  • 0 in reply to Alexander Smith
    Prodigy 60 points

    That's clear. Thanks again for the help!