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AMc7812 questions

TI Lady
Genius 4025 points
Other Parts Discussed in Thread: AMC7812, AMC7812B

A few questions on the AMC7812 

1)       Can the I2C address pins be changed dynamically?

2)      What is timing between CNVT pin going low and the first conversion starting?

3)      What is the minimum/maximum pulse width of the CNVT pin.

4)      Can AVDD, DVDD and IOVDD all be connected to the same 5V rail?


Thank you

  • 0
    TI__Genius 11750 points

    Howdy TI Lady,

    Thank you for the post regarding the AMC7812 device.  I'm currently gathering the information required to answer your post. I'll update this post once I obtain the answers to your questions.

    Best Regards,

    Matt

  • 0
    TI__Genius 11750 points

    1) The address of the part can be set through hardware, more specifically it is determined by the status of pins A0, A1, and A2, as shown in Table 8 of the AMC7812 DS.  I've included the table below for reference.

    2) In this question I'm assuming that the time from CNVT to the first sampled channel is what is in question.  In general, it is usually 2us, but should not exceed 3us.  

    3) Page 6 of the AMC7812 DS indicates that the minimum CNVT pulse width is 20ns.

    4) AVDD and DVDD should always be at the same value.  It is possible to connect IOVDD to AVDD/DVDD, but it should be understood that digital supplies are generally more noisy than their analog counterparts (SPI communication operates at a higher frequency) and can couple into the analog outputs,  this coupling is mostly referred to as digital feed-through.

    Best Regards,

    Matt

  • 0 in reply to Matthew Sauceda
    Genius 4025 points

    Thanks for your help Matt.

    i guess the first question was not answered:

    Can I change the connections of the address PINS while the part is powered on and thereby change the address it responds to without power cycling or resetting the part?  I want to use the address pins like chip selects.

    Let me know!

    Thanks,

  • 0 in reply to TI Lady
    TI__Genius 11750 points

    Hello TI Lady,

    More than likely a POR or reset should be invoked to incorporate the new states of the address pins, but I will check with design to verify my assumption.

    Best Regards,

    Matt

  • 0 in reply to Matthew Sauceda
    TI__Genius 11750 points

    Hello TI Lady,

    I stand corrected, I was able to verify through design that the I2C address pins are indeed capable of a dynamic change while the AMC7812 device is powered on.  I was also able to check this out on the EVM.

    Best Regards,

    Matthew Sauceda

  • 0 in reply to Matthew Sauceda
    Genius 4025 points

    I just noticed some strange behavior with the AMC7812B.  If I have it configured for auto conversion it works fine until there is a negative edge on the CNVT pin.  At that point the auto conversion stops until there is another negative edge on the CNVT pin at which point the auto conversion starts up again.  Is this the expected behavior?

     

    I see this in the datasheet:

     

     

    It looks like when the new conversion cycle starts it switches to direct mode and won’t convert again until the next negative CNVT edge at which time it switches back to auto mode.  I am hoping there is a register configuration that will prevent the auto conversion from stopping if the CNVT pin goes low.  If there is no way to keep auto conversion going while the CNVT pin is pulsed then I may have to disconnect the CNVT pin.   Is there a an internal pull-up or pull-down on the CNVT pin so that I can safely leave it disconnected?

    Thanks in advance,

  • 0 in reply to TI Lady
    TI__Genius 11750 points

    Howdy TI Lady,

    The ADC has the option of triggering externally vai the CNVT pin, or internally by writing to the ICONV bit in the AMC Configuration Register.  

    The information regarding internal/external triggering and it's relation with the ADC Conversion Sequence can be found in Figure 82 of the DS.  I have provided the image below for reference.

    The red boxes indicate internal or external triggers. Notice that you can exit (stop) "auto" mode by creating a trigger while in "auto" mode.  A secondary trigger is required to re-enter "auto" mode after the suspension.

    Therefore, you may be stopping and starting the ADC Conversion Sequence if you are externally biasing the CNVT pin with a pulsed waveform.

    I will have to check with the designers, but I don't believe the CNVT pin has an internal pull-up resistor, as we typically specify pins such as these to be pulled high externally.

    Best Regards,

    Matt