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PGA2505: question about passive component

user4474933
Prodigy 60 points
Part Number: PGA2505

hi team, my customer has some question about microphone preamplifier PGA2505. According to previous experience, there are two difference design that use PGA2505.

1. C234 C240 use ALUMINUM ELECTROLYTIC CAPACITORS it have polarity and VCOMIN is -5V.

 

2. C234 C240 use ALUMINUM ELECTROLYTIC CAPACITORS isn’t have polarity and VCOMIN is 0V.

 

Could you help to explain the relationship on the CAP and VOMIN ? Or can you share more details how to set the VOMIN pin. Thanks.

  • 0
    TI__Expert 6400 points

    Hi,

    As you can see in the subsection "BASIC CIRCUIT CONFIGURATION" in  the PGA2505 data sheet (page 12), the VcomIN voltage can set the output common mode voltage, i.e. (Vout+)/2 + (Vout-)/2 = VcomIN, and it should also cause the level shift at input side, i.e. (Vin+)/2 + (Vin-)/2 = VcomIN - 0.65V (please see the subsection "ANALOG INPUTS AND OUTPUTS" in the PGA2505 data sheet (page 9).

    Furthermore, please note that VcomIN is specified from 0V to 2.5V range, as you can see in the pin assignment table.

    Therefore, in the design 1, VcomIN is biased to -1.058V, i.e. this violate the VcomIN specification(0 - 2.5V). while, in the design 2, VcomIN is  biased to V, i.e. the output common mode voltage is biased in 0V properly.

    Could you please do double-check the biase voltage in the case of the design 1?

    Thank you and Best regards,
    Iwata Etsuji

  • 0 in reply to Iwata Etsuji
    Prodigy 10 points

    Hi Iwata Etsuji,

    Thanks for your reply.

    As I saw the feedback from you , I can understand how to set the VCOMIN pin.

    In fact , I also want to know what is the relationship whether I use ELEC CAP "have polarity" or "non-polarity" in position C234(C240).

    If you have new update from your inside , please kindly share the information to us.

    Thanks

  • 0 in reply to Huang Barry
    TI__Expert 6400 points

    Hi Barry,

    Regarding the polarity of DC blocking caps, if you could consider the case that the microphone bias voltage is obviously higher than PGA2505 inputs; e.g. biasing with a phantom power, you can use polarized caps.
    Actually, in your schematic, the input common mode voltage is biased in less than 0V, so if a phantom power was considered, the polarized caps  could be used.

    In general, the polarized caps are actually weak for reverse biasing, but not immediately broken with much lower reverse biasing than its working voltage rating.
    As you can see in the subsection 4.3 in the PGA2505EVM user guide , the much higher working voltage rating (e.g. 63WV) for the polarized cap than considerable signal amplitude or a phantom power would be recommended.

    That is, the capacitor polarity is not directly depending on VcomIN voltage but microphone bias voltage.

    Thank you and Best regards,
    Iwata Etsuji