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PGA2505: Question of Resistor Selection of Phantom Power Circuit of PGA2505

Wayne Chen (ex FAE)
Genius 15950 points
Part Number: PGA2505
Other Parts Discussed in Thread: PGA2500,

Hi Sirs,

My customer got noise problem with 6.81kR, 1% resistors with Phantom circuit as the below picture, while the noise problem could be improved by replacing high precision 6.81kR, 0.1% resistors. How can we explain this investigate in theory? 

Would you please provide tips how to modify the reference design if we need to keep using 1% resistors?  



Thank you and Best regards,

Wayne Chen
06/25/2019

  • 0
    TI__Guru 54426 points

    Hi Wayne,

    Most literature regarding Phantom power mentions that the current limiting resistors must have good matching to offer good performance.  Our evaluation board also recommends using 0.1% resistors.

    In general, the recommendation is to use precision matching components in the positive and negative input path.  Differential input pre-amplifiers gain up differential signals and tend to reject common-mode signals.  When using mismatched components or mismatched impedance at the device inputs, any extrinsic noise will show up at different amplitudes at the positive and negative path (due to the filters involved), and the difference in amplitude will be seen as a differential noise signal at the amplifier input.  In contrast, when using good impedance matching on the positive and negative paths, any extrinsic noise present on the positive and negative path will tend to have the same amplitude on both paths, and the noise will be seen as a common-mode signal at the amplifier input.  The common-mode noise will tend to be rejected by the PGA2505.

    Thank you and Regards,

    Luis

         

  • 0 in reply to Luis Chioye
    TI__Genius 15950 points

    Thank you for your advise, Luis...Wayne Chen

  • 0 in reply to Wayne Chen (ex FAE)
    TI__Genius 15950 points

    Hi Luis,

    Addition, can we use 6.81kR, 0.1% 1/8W resistors instead of 6.81kR, 0.1% 1/4W of the reference design?

    Thank yo and Best regards,

    Wayne Chen
    06/26/2019

  • 0 in reply to Wayne Chen (ex FAE)
    TI__Guru 54426 points

    Hi Wayne,

    The resistor power rating is dependent on the maximum current expected to flow through each current limiting resistor in the application.  This current is a function  of the phantom voltage used, and the current consumption of the microphones connected to the phantom power.  Most condenser microphones consume current on the 1mA - 2mA range; however there may be microphones that consume more current.  The correct approach is to consider the worst case current flow through the current limiting resistors during a fault condition where the cables connecting the microphone are shorted. 

    Assuming a +48V (nominal) phantom voltage source is used, the voltage source could be as high as +50V or +52V. The worst case current flow may occur when pin 2 of the mic pin connector is shorted to GND (or pin 3 is shorted to GND) due to a bad connection.  In this case, a current of about ~7.6mA may flow through the 6.8kOhm current limiting resistor, and the power dissipated on the resistor could be up to 0.39W.

    A resistor rating of 1/4 W (or 1/8W) will overheat (or burn) during the fault condition, drifting from its value (the 1/8W rated resistor may burn).  Using a 1/2W or 0.6W resistor will be a better approach for a robust design to survive the fault condition. 

    Thank you and Regards,

    Luis Chioye 

  • 0 in reply to Luis Chioye
    TI__Genius 15950 points

    Thanks a lot, Luis.

    We have another questions as a following of your reply:

    1. The 1/4W resistors were presented in PGA2505 reference design, did you mean that we need to update reference design to from 1/4W to 1/2W resistors?
    2. It is difficult to find 1/2W 0.1% resistor in the market. Is it doable to update the other component values to use 1% 1/2W resistors?

    Thank you and Best regards,

    Wayne Chen
    06/27/2019 

     

  • 0 in reply to Wayne Chen (ex FAE)
    TI__Guru 54426 points

    Hi Chen,

    The PGA2500/5EVM were developed in 2003 and 2008 respectively. The evaluation boards are intended to demonstrate the performance/functionality of the PGA2500/5; and facilitate the evaluation of the devices. These evaluation boards are in the queue to a hardware and software re-design late this year.

    From the PGA2500/5 perspective, using well matched components will improve common-mode noise rejection and immunity to extrinsic noise, therefore, we recommend well matched components for these high-performance audio preamplifiers. 

    However, the PGA2505EVM board is not defined as a phantom power supply reference design, since the phantom power is not a direct function of the PGA2505.  When selecting the power rating of the current limit resistors for the phantom power supply design, a general recommendation is to choose resistors that will withstand the worst case current that is required in the specific application case without issue.   The phantom power supply designer will need decide what the max worst case current that needs to be supported in their end application, and choose the resistors with the proper power rating. Resistors of 0.4W or 0.5W power rating and 0.1% precision are available.

    Best Regards,

    Luis 

  • 0 in reply to Luis Chioye
    TI__Genius 15950 points

    Noticed and thanks for your elaborate, Luis...Wayne Chen