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Is the response time of VCA/TGC using the digital/SPI method the same as analogue/Vcntl method?

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Replies: 6

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Hi,

I see in AFE5809 datasheet (SLOS738E) that Figure 51 and Figure 52 shows the response time of VCA/TGC on the rising and falling edge of Vcntl.

I'm wondering if those two figures also apply to the SPI method?

Thank you.

Dehuan

6 Replies

  • Hi Dehuan,

    We have received your post regarding the AFE5809 and I have assigned it to the device application expert. He will get back to you with a response as soon as he can.

    Regards

    Praveen

    TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.

  • Hi Dehuan,

    From the AFE5809 data sheet, it only shows you the Vcntl response time

    as you can see from the Figure 51 and 52.

    The digital TGC structure is controlled by using the SPI register: DIG_TGC_ATT_GAIN.

    As you can see the following:

    The register control should be faster than Vcntl pin.

    However the total response time was not speced on the data sheet right now.

    Just let you know.

    Thanks and best regards,

    Chen

  • In reply to Chen Kung:

    Hi, Chen,

    I guess what I really want to ask is how smooth is the tgc control in SPI mode.
    Since the attenuation is only 7 levels rather than 7 bits, the TGC would show up as steps rather than a smooth curve.
    Figure 51 shows the response time is about 2.5us, which is about 4mm of imaging depth.
    We are concerned that the SPI method would be even faster and the TGC steps would show up in the image as bands and rings when the edge is too sharp.

    Dehuan
  • In reply to Dehuan Xin94:

    Hi Dehuan,

    Thanks for letting us know your concern about the details of using the TGC function inside AFE5809 device.

    When trying to control TGC (in AFE5809), it is controlled by SPI (by sending registers) mode.

    And we know SPI mode is not designed to be used as timing controllable purpose.

    So for AFE5809 TGC can be explained as Digital-by-SPI-TGC.

    However we do have real Digital-Control-Signal-TGC (DTGC) devices on the market.

    They are AFE5801, AFE5851, AFE5816 and AFE58JD16 devices.

    They are truly TGCs by using Digital Control Signal (you can predict the timing for the gain control).

    Also I will forward your question to other engineers in our medical group.

    Could you please send us your email address also?

    We could send you data sheets for other devices for you.

    Thank you very much for using our AFE5809 device.

    Best regards,

    Chen

  • In reply to Chen Kung:

    Hi, Chen,


    I took a look at AFE5809 datasheet again.

    I think where I got wrong is I thought SPI TGC and Vcntl TGC are both quantized to 7 levels, which is not the case.

    As is explained in 8.3.2, the Vcntl TGC is piecewise linear.

    So I guess AFE5809 is by design to use Vcntl as the primary TGC control input and use SPI method is for static/fixed TGC only.

    Dehuan

  • In reply to Dehuan Xin94:

    Dehuan,

    Yes, most of our parts have the Analog TGC  (Vcntl), and the SPI attenuation is meant for fixed attenuation or testing.  We do have some true DTGC parts as Chen mentioned.

    Chuck Smyth

    Chuck Smyth, Applications Engineering, HealthTech-Imaging

    Please verify post if question is answered.

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