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OPA1622: Single-Ended Input Circuit for OPA1622

Jeffrey Bruton
Prodigy 100 points
Part Number: OPA1622
Other Parts Discussed in Thread: PCM5102, TPA6120A2, TINA-TI, BUF634

Hi TI Staff,

The data sheet for the OPA1622 shows an example circuit for driving this OPA1622 with the differential output of a DAC.

We are planning to drive the input of the OPA1622 with the output of the PCM5102.

Since the output of the PCM 5102 is single-ended, will you please reply with an example circuit for interfacing the output of the PCM5102 to the input of the OPA1622?

Thank you and kind regards,

Jeffrey Bruton

  • 0
    TI__Genius 10850 points

    Hi Jeffrey,

    Thanks for your post. You can use the OPA1622 in a single-ended application. I would recommend that the DC resistance at both op amp inputs is matched so that the bias current of the amplifier doesn't create too much offset at the output. But otherwise, there's no problem using the OPA1622 like any other op amp.

  • 0 in reply to Tamara M Alani
    Prodigy 100 points

    Hi Tamara,

    Thanks for the reply and info!

    I'm a much better recording engineer than a circuit designer, so would you please reply with a schematic (or point me to a similar one) that shows the components needed to match the DC resistance at the input as you suggest?

    Also, we are considering and OPA1622, because we need the ability to drive a wide range of headphones (8 ohm units all the way to 600 ohm units) to a very loud level if needed. Because of this we are considering the OPA1622 as opposed to the TPA6120A2. Opinion?

    Thanks again

    Jeffrey Bruton 

  • 0 in reply to Jeffrey Bruton
    TI__Genius 10850 points

    Hi Jeffrey,

    Certainly we can help you. The audio expert on our team is out on Holiday. He will be back after the new year, I will ask him to support you here. Please accept our delay.

  • 0 in reply to Tamara M Alani
    TI__Expert 6400 points

    Hi Jeffrey,

    If you are looking for the headphone diver up to 8ohm load, TPA6120A2 should be the right alternative since it is difficult for OPA1622 to drive 8ohm load in low THD+N as you can see in the figure 17 in its datasheet, unfortunately.

    While, if you had any concern about the audio performance of TPA6120A2 such as distortion, you can try adding the output buffer for OPA1622 in as below.

    PCM5102+OPA1622+BUF634A.TSC

    If you don't see the above simulation in TINA-TI, please let me know.

    Best regards,
    Iwata Etsuji

  • 0 in reply to Iwata Etsuji
    Prodigy 100 points

    Hi Iwata,

    Thanks for your reply and wishing you and your family a wonderful 2021! I think it's going to be a great year!

    First, thank you for introducing us to TINA-TI! That program is going to be a huge help!

    We've updated our design spec to drive a wide range of headphones ((32 ohm units all the way to 600 ohm units) which it seems the OPA1622 would be well suited for.

    Once again we are planning to drive the input of the OPA1622 with the single-ended output of the PCM5102, and are requesting an example circuit to accomplish this.

    Could you provide provide a circuit showing this with TINA-TI? :-)

    Thank you and kind regards,

    Jeffrey Bruton

  • 0 in reply to Jeffrey Bruton
    Prodigy 100 points

    Hello TI Staff,

    Still waiting for a reply to my most recent post above from Jan 5, 2021.

    Kind regards, Jeffrey Bruton

  • 0 in reply to Jeffrey Bruton
    TI__Mastermind 22870 points

    Jeffrey,

    I re-attached the example circuit Iwata-san provided. Were you able to download the Tina example circuit?

    0121.PCM5102+OPA1622+BUF634A.TSC

  • 0 in reply to Chris Featherstone
    Prodigy 100 points

    Hi Chris,

    Just checking two things:

    1. We no longer need to go down to 8 ohm. 0ur spec is now for driving headphones 32 to 600 ohms.

    2. Is the buffer IC Iwata-san detailed in the circuit needed to simply run the OPA1622 with a single ended input, or was that included to accommodate our previous spec of driving 8 ohm headphones and can now be moved because our new spec is 32-600ohm headphones?

    Very kind regards,

    Jeffrey Bruton

  • 0 in reply to Jeffrey Bruton
    TI__Mastermind 22870 points

    Jeffrey,

    The BUF634 is used for a couple reasons. Basically it is decoupling the precision amplifier (OPA1622) from the load (headphones). With its large slew rate the BUF634 is able to reproduce the signal seen at it's input coming from the precision OPA1622 and drive the heavy load of 8,32, or 600 ohms. It is not necessary depending on the conditions you are running the OPA1622. For example, we specify the OPA1622 with a 32 ohm load at a supply of Vs = +/-5V and the output swing 2V within each rail providing an Aol of 114dB with an output swing of +/-3V. We also provide a specification for 600 ohm loads. 

    The BUF634 also has high capacitive load drive such as driving long cables. This decouples the OPA1622 from seeing the capacitive load and also the resistive load. Capacitive loads can produce instability in any amplifier. We provide a phase margin vs capacitive load drive plot (Figure 24).

    One other benefit from decoupling the OPA1622 from the resistive load is that it keeps the OPA1622 at a cooler temperature. Hot temperatures can degrade output swing operation as the temperature increases amongst degradation of other specs. This is true of any amplifier and not unique to the OPA1622. In this case the BUF634 is driving the load and you get the benefits still of the OPA1622 precision and distortion characteristics. 

    So to summarize, you don't need the BUF634 as long as you adhere to the specified conditions for the OPA1622 but the BUF634 does bring a lot of benefits depending on your application.