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OPA1632: Powerpad on SOIC8?

Jürgen Herrmann
Prodigy 10 points
Part Number: OPA1632
Other Parts Discussed in Thread: OPA1633, TPA3244

Tool/software:

The datasheet unfortunately is not clear about this. Samacsys says that the SOIC8 version of the OPA1632 does not have a power pad. Is this correct? Thanks in advance for your help!

  • 0
    TI__Mastermind 21477 points

    Hello Jürgen,

    The D (SOIC8) package of the OPA1632 does not have a power pad / thermal pad.  This is confirmed with the additional packing information at the very end of the datasheet.

    We do also have the OPA1633, a device with upgraded features if your design would benefit from these increases.  The changes include lower power, higher supply voltage, and lower noise.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 10 points

    Thanks very much for clarifying. I will have another question, wether the OPA1632 can do gain below 0dB, but it will probably be better to move this into a new question.

  • 0 in reply to Jürgen Herrmann
    TI__Mastermind 21477 points

    Hello Jürgen,

    I am glad to help out with your additional question.  You can ask a new/related question or add onto your question here.  I will refrain from answering here until you confirm if you want to ask it separately Slight smile.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 10 points

    OK, if it doesn't matter that we get a bit of a mishmash of topics in this thread, i will ask here: can the OPA1632 do negative gain, for example with input resistors 1k and feedback resistors at 470 Ohms? I'm currently designing a Class D amp and the TPA3244 has too much gain for my needed output power needs and desired input voltage. so in order to keep the noise low i'd like to have the input amps a gain of around -6dB.

    PS: I don't see a reason why this should not be possible, be want to be sure...

  • 0 in reply to Jürgen Herrmann
    TI__Mastermind 21477 points

    Hello Jürgen,

    We can certainly discuss here!  One of the key benefits in circuit building provided by fully-differential amplifiers over differential amplifiers (diff amps -> two op-amps used to go single-ended to differential) is the ability to have inverting (negative) gain.  Attenuation is very much allowed and encourage for FDAs.  For OPA1632 using the Resistor Matching guidelines will be helpful.  You may notice the Rg value, typically around 370-410 Ohms, is kept fairly constant while the Rf (feedback resistor) value is changed.

    For an attenuation (negative gain), you should consider an Rg resistor within the range of 350-500 Ohms.  You may also use larger resistors such as 1kOhm, but refer to Resistor Matching guidelines on how to calculate how Rg and Rf effect the low input impedance of an FDA.

    The nice part about setting Rg and Rf is you can always change and tune these resistors Slight smile.  A negative gain is perfectly acceptable for proper FDA use.

    Best,

    Alec

  • +1 in reply to Alec Saebeler
    Prodigy 10 points

    Hello Alec,

    first of all, thanks very much for your help so far!

    I might as well share my design, I make all my designs open source anyways. Maybe some of you will spot some FDA design beginner mistakes.

    Prerequistes:
    - unfortunately the soundcard I intend to use has single ended outputs, resulting in injected noise from ground loops if I don't use a differential amp on the frontend.
    - the soundcard does probably not like SE load impedance <1k, so I'm probably stuck with 1k Rg. Will confirm this in the following days with a loopback measuerement with varying load resistors. Another approach would of cause be to use noninverting, high input impedance buffers in front, but this would also add to the complexity, which I'd like to avoid.
    - I plan to couple the Cinch "ground" inputs to circuit ground with 22R resistors, to make currents on the SE input "ground" line lower. Additionally I use the two 22R resistors and a cap (33nF) between them as input EMI filter. Any objections so far?
    - OPA1632 outputs go to inputs of TPA3244 via 10u film coupling caps. I could use the VOCM pin at 1/2 AVDD of TPA3244, but this would require a separate buffering amp. I'd like to keep things simple.

    Here's a schematic of one of the four input sections (this will be a 4ch amp with 2x TPA3244 and 4x OPA1632):

    As The board house is taking their time to review my layout I thought I might as well post things here and not just rely on my experience. I desigend a lot of opamp circuits so far, but never with FDAs.

    Here's a preview of the physical design:

    Full kicad project can be found here, if anybody wants to look at my pcb design:
    https://gitlab.com/t-5/4ch-classd-amp

    Looking forward to your answers/suggestions!