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ADS54J60: VCM changed by 4Fh register setting

Ryotaro Fukui
Intellectual 2345 points

Part Number: ADS54J60

Hello team,

I have received a question from my customer about the ADS54J60IRMP.

The datasheet states that the VCM terminal is 2.1V, but in their actual measurement it showed voltage value around 2.17V and 2.18V.

In their current setting, Register 4Fh is set to 0h, and according to Datasheet p. 55 "Table 32. Register 4Fh field descriptions", it states that in order to compensate for the 100-mV drop caused by resistor of 600 Ω, the device raises the VCM voltage by 100 mV by default.

Does this indicate that the Common-mode Voltage at the VCM terminal has become approximately 2.2V with a 2.1V of +100mV?

And if so, which should they think as a VCM for differential input VCM +/- 0.475V? Default 2.1V or compensated 2.2V?

Best Regrads,

Ryotaro Fukui

  • +1
    TI__Guru* 91947 points

    Ryotaro,

    The typical value for VCM is 2.1V. This can vary from device to device. The values recorded by customer are normal. What ever voltage they are reading at the VCM terminal will change by 100mV depending on the setting of address 4F. For the analog inputs, the VCM will be slightly lower due to the 600 Ohm resistor. This value is normally around 2V or 1.9V depending on the setting of address 4F. If they are AC coupling the analog input, I would suggest just leaving address 0x4F at 0x00. The part has room for error, so I would not be concerned about the VCM in an AC coupled application. They just need to make sure the input differential swing is not greater than 1.9Vpp.

    Regards,

    Jim 

  • 0 in reply to jim s
    TI__Intellectual 2345 points

    Hello Jim,

    Thank you for explaining about the VCM voltage change.

    The customer told their system was 0.95 Vpp instead of 1.9 Vpp because the INP side only contains + component of VCM and INM only contains - components.

    They had contacted us to know if the input tolerance would be narrowed to 0.85 Vpp because there was a difference of around 100 mV between the 2.1V and 2.2V VCM.

    It is said that it is a device that takes up a lot of room for error, so they understood that it could be set within 0.95 Vpp.

    I would like to declare this case closed. Thank you for your prompt response.

    Regards,

    Ryotaro Fukui