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ISO224: Vdd2 split supply

Gerhard Kreuzer
Expert 2580 points
Part Number: ISO224

Hi,

my ADC has an input range of +-4V with Vcm at 0V, which I cant change.

Now I has to monitor a +-400V potential rail. Yes +- 400V, the polarity changes.

Is it possible to supply the ISO 224 with +-2.5V (as my ADC is supplied) and have Vcm at Vdd2/2 which is now ~0V ?

Thanks for helping!!!

Stay healty, with best regards

Gerhard

  • 0
    TI__Mastermind 47625 points

    Hi Gerhard,

    This may be possible but we cannot guarantee the device behavior when operated in this way since it has not been fully tested. I spoke with a member of the design team and there are some concerns, especially with the common-mode output voltage. If you look at the device datasheet, you will see that the common-mode output voltage is typically VDD/2, but has a minimum of 0.48 × VDD2 and maximum of 0.52 × VDD2. Any variation in common-mode output voltage would result in a direct offset error if sensing in a single-ended manner.

    To summarize, operating the device from unipolar supplies is best and using an operational amplifier for level shifting and differential to single-ended conversions is preferred. 

    There are several pieces of collateral under the technical document section on the product page for assistance when designing with this device, including differential to single-ended conversions. Please let me know if you have additional questions.

  • 0 in reply to Alexander Smith
    Expert 2580 points

    Hi,

    My ADC has a differential Input so I need a differential output ....

    If there is some offset, this can be corrected by software, no problem.

    At system start, the output voltage is 0V so just sample the voltage monitor output and we have a correction factor.

    If the system is working we have voltages between +-10V and -+400V to monitor. The ADC has 24 bit resolution and +-4V differencial input with Vcm at 0V aka ground of the analog sub system.

    Maybe there is a better solution around.

    The channel needs a isolation barrier.

    With best regards

    Gerhard

  • 0 in reply to Gerhard Kreuzer
    TI__Mastermind 47625 points

    Hi Gerhard,

    Apologies, you were clear about that in the OP and my mind wandered. 

    An offset calibration would solve the offset issue, but as I said we have not fully tested bipolar supplies so we cannot guarantee function as we do in the datasheet with uni-polar supplies. 

    Adding a fully differential amplifier (FDA) as a level shifter would work, as talked about in this blog: https://e2e.ti.com/blogs_/b/analogwire/archive/2016/07/13/how-to-use-a-fully-differential-amplifier-as-a-level-shifter

    If this is of interest, let me know and I can move this thread to their domain so they can help you with device selection.