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AD5342, question on reference input

Hi,

 

Please let me ask what is impedance value of reference input(VrefA and VrefB) of AD5342, when it is not powered ?

 

We are now studying one issue of our board, and it seems reference input is powered before the part is powered. To understand what is occured, we would like to know the value of impedance.  BUF pin is connected to +3.3V, but it should be 0V by connected to GND when it is not powered.

 

Thank you very much for your support,

 

Best Regards,

  • Hi Usaghi,

    We are looking into this and will get back to you.

    Regards,

    Mark

  • Hi Usaghi,

    the input reference pin is internally protected by ESD diodes.

    If the part is not powered, the protection diodes will clamp the voltage to Vdd - 0.3V...

    Vdd is not powered but probably connected to a supply so.. two things may happen,

    1- the supply is at high impedance, then teh part is powered indirectly by the reference.

    2-the supply is at low impedance, then the reference is probably short- circuited to gnd.

    Regards,

    Miguel

  • Hi Miguel,

    Thank you very much for your reply. I briefly understand what will be happen, but I need to understand further at  following points.

    - what is boundary level of high and low impedance

    - if Vref input is AC, not DC normally expected, how it will work

    Could you please let me know simplified internal schematic of Vref and VDD ?

    Best Regards,

  • Hi Miguel,

    We found that BUF setting is affected to what is happened at the reference input. Datasheet describes BUF setting for two different mode of buffered and unbuffered and their impedance differences, however nothing described in case of unpowered.

    Could you please add the matter of BUF setting into my previous questions, how is BUF setting effect to reference input, in regard to impedance of power source at VDD, input frequency for Vref, when the device is not powered.

    Sorry to these complicating query but your helps would be always appreciated

    Best Regards,

  • Hi,

    Do you mean max/min? depends on the gain but talking about process tolerance, you can estimate around +-20%.

    You can work with AC signal but the remember that the minimum voltage at Vref is 1V.

    You must care about the maximum frequency as well... this is a string DAC and it is designed mainly for DC.

    If your Vref will be AC, i'll recommend to use a multiplying DAC, http://www.analog.com/parametricsearch/en/10166#10166?p4387=4

    or digipot, http://www.analog.com/en/digital-to-analog-converters/digital-potentiometers/products/index.html#Digitial_Potentiometers

    depending on resolution requirements.

    No difference with buffers, the ESD protection is connected before the buffer.

    Regards,

    Miguel

  • Hi Usaghi,

    If your refer to my previous phrase,

    the supply is at high impedance, then the part is powered indirectly by the reference.

    I meant your LDO output impedance. there is not boundary limits....
    the problem is more simple and depends of your voltage reference maximum current.

    the answer is the sum of the current driven to GND by the LDO and the AD5342 current consumption.

    If your reference can drive both loads, you wont have problem (Say that I'd imagine that there more components sharing the Vdd net so you need to add the current consumption of all the components)

    If not, you need to understand the current path

    Hope it helps, let me know if something is not clear, please

  • Hi Miguel,

    In regard to "boundary level of high and low impedance", I was talking about which impedance value do you have in your mind, when you mentioned in case "the supply is at high impedance". It may be relative, but if you provide any guidepost when deciding whether it is high impedance or low impedance, it would be much appreciated.

    We do not intend to input AC signal into reference input. We are just studying accidental case if reference input was AC signal.

    It is good to know status of BUF pin may not affect to.

    Best Regards,

  • Hi Miguel,

    Please let me ask you to clarify again.

    When you talked as follows,

    > Vdd is not powered but probably connected to a supply so.. two things may happen,

    > 1- the supply is at high impedance, then the part is powered indirectly by the reference.

    > 2-the supply is at low impedance, then the reference is probably short- circuited to gnd.

    did you mean "high impedance" is equal to a state of insulated/isolated, and "low impedance" is equal to be connected or at least is equal to NOT "high impedance", didn't you ?

    If so, I have been misunderstood and please ignore my previous question.

    Then, when you talked about sum of the current loaded to the reference input, did you subject to both of above two cases, since anyhow current has to be consumed in both cases, so need to be calculated total loads, didn't you ?

    Even so, I still do not understand why "the reference is probably short- circuited to GND" when VDD supply is at low impedance. If you briefly provide simplified scheme/diagram, it would be very much appreciated.

    Thank you very much for your kind helps, as always.

    Best Regards,

  • Hi Usaghi let me ask one question,

    In your regulator, What's the Vout internal impedance if Vin = GND?

    regards,

    Miguel

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