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THS4541: current in negative rail

VAHID FARSI
Intellectual 290 points
Part Number: THS4541
Other Parts Discussed in Thread: LM7705

 Hi 

For enhancement of SNR I used LM7705 to drive VSS of THS4541 pin,, as TI recommended,

Bias on datasheet of  this switching capacitor regulator, the noise will increase at the out put  of regulator  as the  output current increase,

the maximum current that this device  can provide is 26mA

 cause I don't know how much current  is required in THS4541 VSS pin ,I am not sure this regulator could be able to drive 4 op-amp  or no?

Thank you all

vahid

  • 0
    Guru 47535 points

    So you are trying to supply 4-THS4551 with a single LM7705? 

    1. noise on the regulator output will be filtered by the caps and further rejected by the very high -PSRR of the THS4541

    2. I think the THS4541 is about 11mA, so maybe you can supply 2 of them per LM7705

    3. It is not so much to improve noise but swing to ground HD performance to supply a -0.23V on the negative supply -if you don't need to swing to ground (most ADC driver apps do not), then you may not need this.

  • 0 in reply to Michael Steffes
    Intellectual 290 points

    the datasheet of THS4541  mentioned that the question current of op amp is   about 11mA  but has not specified   how much  is  related to  positive rail  and how much is for  negative rail.

    what is important for me is negative rail current .

     I used capacitor and ferrit bead  at the out put but  still  there is noise which is not acceptable .

     

  • 0 in reply to VAHID FARSI
    Guru 47535 points

    The currents should be equal, 11mA in on the +Vcc and 11mA out on the -Vcc. 

    Yes, if the -Vcc has noise on it that is a concern, but how much is getting through to the differential output pins as a differential signal related to that switching supply. 

  • 0 in reply to Michael Steffes
    Intellectual 290 points

    I am not sure both negative and positive supply have same current , how can I make sure?

  • 0 in reply to VAHID FARSI
    TI__Expert 7875 points

    Hi Vahid, 

    If you can hook the board up to a bench supply that would be the easiest method. Otherwise you can try and measure a voltage drop across a small series resistor in the supply. 

    Regards,