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Issue with Protection Diode placed on Analog input channel of ADC IC ADS1248IPW

Amit Patil32
Prodigy 50 points
Other Parts Discussed in Thread: ADS1248

Hi,

We are facing issue with Protection Diode placed on Analog input channel of ADC IC ADS1248IPW

Please refer attached schematic Image.

  1. When we give -5V as input in case of 1:1 channel protection diode connected to -2.5V conducts and hold 1:1 channel signal to -2.7V .-----This is Ok.
  2. When we give +5V as input in case of 1:1 channel protection diode connected to +2.5V conducts and but do not hold 1:1 channel signal to +2.7V .

It reaches to 5V and at this time Power rail voltage also reach to 4.9VDC. In attached image, I have shown you Power supply used for generating 2.5V and -2.5V.

When tested same diode with external adjustable power supply (+2.5VDC) instead of Power supply in attached image, then protection diode conducts and hold 1:1 channel signal to +2.7V and current flowing through diode is about 20.42mA@5V.

 The load on Power supply in attached image is ADC only.

 We will really appreciate  your suggestions on this issue

  • 0
    TI__Guru* 92655 points
    Amit,


    I don't think this is an issue with the ADS1248. I think this may be an issue with the power supply that you have on the board.

    My guess would be that the LDO on your board is able to source current, but is not able to sink current. When current goes into the LDO, it is unable to hold regulation and the output voltage goes up in response. I've looked at the datasheet for the TPS70025, but it doesn't say anything about the load range.

    I would post in the Linear Regulator Forum to see if the TPS70025 is able to sink current. You can find the link here:

    e2e.ti.com/.../

    If the device is unable to sink current, I would ask about an alternate device that is able to sink current. I would ask if there is a device with the same package and pin configuration that you use to quickly replace the TPS70025.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 50 points
    Hi Joseph Wu,
    Thanks for your feedback.
    We do agree that there is no an issue with the ADS1248.Alternate LDO with same package and pin configuration can make our life easy in present situation.
    We would like to know the parameter which need to be addressed while selecting LDO which can sink current.

    Have a wonderful day ahead.

    Regards
    Amit
  • 0 in reply to Amit Patil32
    TI__Guru* 92655 points
    Amit,


    I think that this question has been routed to us through FAE contacts, so you may already have some resolution on this question.

    Generally, you would want the LDO to sink the current that is being forced into the input. In your example, it would need to be the 20.42mA that you had mentioned in your first post.

    However, I think most LDOs are not designed to sink much current if at all. It would be unlikely to find something that could be a replacement that can sink current. In the end, series resistance may help the most. If you limit the input current with series resistance, it would do better to protect the devices on the board.


    Joseph Wu