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AMC7812: Input impedance on analog inputs

Part Number: AMC7812

Hi ,

coudl you please confirm that the input impedance of each analog input is 50+40Ohm = 90 Ohm as hown in the Datasheet?

Why is the s&h cap in the picture 40pf and in the spec as well as in the thread 73pf ....typo?

Thansk in advance

Bertram

  • Hi Bertram,

    The figure shows only the typical value for the sample cap itself.  These can vary by process.  The spec table actually shows data from device characterization, which will include additional capacitance from various parasitics, such as the input pin, the mux, and sample switch.  You can think of the input capacitance as the following:

    For the 0V to 2xVref range: Cinput = Csample+Cpara = 40pF + 35pF ≈73pF

    For the 0V to Vref range: Cinput = Csample×2 + Cpara = 80pF + 35pF ≈ 118pF

    The math does not add up exactly as the actual switch configuration is a bit different in the two modes.  The values in the specification table are the most conservative and should be used when trying to calculate the desired input filter.

    Thanks,

    Paul

  • Hi Paul,

    thanks for the detailed explanation.  With this you also confimr that the Rsw  is 90 Ohm? 

    In regards to the supply I understand that there is no power sequence required this means the IOVDD with e.g  3.3 Volt  can be applied and than the ADC CHO - CH15  even could see 5V  even before DVDD/AVCC ANAVDD would see the 5.0 V because POR circuit would only turn active after seeing a valid Supply? Will POR only become active when  ALL supplies are within the specified operating conditions, or is this related just to a specific supply voltage?

    Thanks in advance, Bertram

  • Hi Bertram,

    The 90Ω is a typical switch impedance value, so it can vary, but 90 is a good estimate for your input filter calculations.

    There is no requirement for the sequencing of the supply pins, but if you bias the analog input before the AVDD supply you will violate the absolute maximums limits.

    This means that the ESD cells will activate as the input exceeds the supply value.  This is not always an issue if your ADC input is current limited.  For example, you have 66kΩ in series with the input, so there will be very little current flowing into the pin, so it will not damage the device.

    Thanks,
    Paul