• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 3 answers
  • Subscribers 49 subscribers
  • Views 280 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

why OPA1632D got so hot?

THP210: Why don't you directly connect to GND instead of a 0V power supply?

Hongmei Wan
Intellectual 580 points
Part Number: THP210
Other Parts Discussed in Thread: TINA-TI

Hi,

Why not directly connect with GND in your simulation file you gave? a cap needed? We are updating our schematic now. We are wondering if we need a capacitor here even if Vocm is referenced to GND.

I am also to come to say I just verified that THP210 is very good in my application since I asked about this performance here before. Thanks for your help!

Thanks,

Hongmei

  • 0
    TI__Mastermind 49475 points

    Hello Hongmei,

    I assume this discussion is referring to the simulation on the post below, Let us know if you are referring to a different simulation.

    why OPA1632D got so hot?

    A bypass capacitor on Vocm is often used to reduce noise when is connected to a reference voltage (often there may be a resistive divider to generate this VOCM voltage). Also, in the case where VOCM is floated to obtain the default mid-supply bias, the decoupling cap is used to reduce noise due to the internal high-impedance divider.

    If you are connecting Vocm to analog ground using a short trace (assuming this is a clean, noise free analog ground plane/connection on an optimal/proper PCB layout), the bypass capacitor is not required.

    If you are referring to the stability analysis simulation that Kai provided in this post, there is only a 0V supply bias on VOCM (equivalent to VOCM connected to GND) but I don't see any capacitor on the VOCM pin

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    Intellectual 580 points

    Hi Luis,

    I use +15V and -15V, and the mid-supply bias is 0V ideally. I don't know if the analog ground is a clean, noise free or not. I guess it won't be in real life. Can I tie it to GND or I need a capacitor on VOCM then to analogy ground? I am just confused here. If I tie VOCM to ground, VOCM would pick up some noise because the ground in real life is not clean and noise-free and if I let VOCM float to the default mi-supply bias, then the VOCM is not 0V due to the difference between real +Vs and -Vs.

    Directly to ground or through a cap to ground for VOCM pin? Please advise.

    Thanks,

    Hongmei

     

  • 0 in reply to Hongmei Wan
    Guru 140200 points

    Hi Hongmei,

    section 8.2 of datasheet shows how VOCM is set at midsupply, by the help of two internal 5M resistors. So any noise coming from the supply rails will make VOCM look noisy. The best cure to decrease this noise is to connect a filtering cap from the VOCM pin to signal ground.

    But when you externally connect a voltage source to the VOCM pin, and do this low ohmically, example given by connecting VOCM to signal ground, no such filtering cap from the VOCM pin to signal ground is necessary, of course.

    That in my simulation VOCM was connected to a voltage generator sourcing 0V has no deeper sense. And in TINA-TI a voltage source also has a low ohmic impedance to signal ground.

    So, if you want to connect VOCM to signal ground in your application, do this. And you don't need a filtering cap at the VOCM pin. But be sure that you have a good signal ground in your circuit, by using a solid ground plane.

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 49475 points

    Hi Kai,

    Thank you for your response.

    Hi Hongmei,

    Using a solid ground plane is a good way to provide a good analog signal ground.  You can take a look at the THP210EVM User's Guide for a 4-layer PCB example, where the layer beneath the device is a solid ground plane.  Alternatively, it is also of course possible to use a GND copper fill on a 2-layer board layout.  You could refer to the OPA1622EVM User's Guide (different pinout device, but same principles).  Please keep in mind, there are several ways to accomplish this. These are just general examples. 

    Also take a look at the articles below.

    https://e2e.ti.com/blogs_/archives/b/thesignal/posts/grounding-principles

    https://www.ti.com/lit/an/slyt499/slyt499.pdf 

    https://www.ti.com/lit/an/slyt512/slyt512.pdf 

    Thank you and Regards,

    Best Regards,

    Luis