• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 49 subscribers
  • Views 513 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

OPA2695: Unused Channel

Shinichi Yokota
Genius 9715 points
Part Number: OPA2695

Hello,

I understand that forming a voltage follower as the picture below is one of the ways to deal with an unused op-amp channel. But, is it OK to put +IN B (or +IN A) down to GND of OPA2695's unused channel? I'm concerned if it has a negative impact on the other operating channel since the +IN B voltage (GND) is not within its common-mode input voltage range. In this case, its supply is single +5 V.

Best regards,
Shinichi Yokota

  • 0
    Guru 48395 points

    Morning Yokota-san, 

    1st of all the OPA2695 is a dual current feedback op amp - you cannot operate those without a feedback resistor unless you are looking for oscillations. 

    For an unused channel, use a 499 ohm feedback R and bias the V+ input at midscale with reasonably low source R. I might also put a 499 ohm load to ground - this is all related to establishing a benign DC operating point for the unused channel and trying to have it see an adequate phase margin. But frankly, why use the dual version if only one channel is active? 

  • 0 in reply to Michael Steffes
    TI__Genius 9715 points

    Mike-san,

    Thanks for your quick feedback. I totally forget how to use a current-feedback op-amp since it was many years ago. The background behind the question is, a PCB is already built and it has no space to put additional resistors to fix the issue. I'm looking for a way to deal with an unused channel with as less resistors as possible.

    Now I remember thanks to you that a current-feedback op-amp requires a feedback resistor to be stable, but is it really necessary to put a non-inverting input of the OPA2695 to a mid-scale? The point is, whether this unused channel will have a negative impact on the other operating channel.

    Best regards,
    Shinichi Yokota

  • 0 in reply to Shinichi Yokota
    Guru 48395 points

    Your schematic shows single supply? If that is the case, grounding the V+ input will saturate the buffer across the inputs for that stage, quite possibly pull quite a lot of current. If you are in fact bipolar supplies, grounding is probably ok, although I put a suggestion of 20ohm to ground to squelch a >1GHz input transistor oscillation that is possible. 

    If you are singl supply, is there not possibly another reference voltage around that could be used, but again - trace length is problem on the V+ oinput DC biasing it. 

  • 0
    TI__Mastermind 28875 points

    Hello Yokota-san,

       As Michael and you have mentioned, if the amplifier is used in a single supply configuration, it will violate the common-mode input voltage range of the device and will saturate the unused amplifier which will increase current consumption (supply current will greatly increase), and may cause crosstalk interaction between the used and unused channels. Michael's comments can be summarized in this tech note. It covers the proper ways to configure an unused channel for a single supply rail system versus split supply.

    Thank you,
    Sima 

  • 0 in reply to Sima Jalaleddine
    Guru 48395 points

    Hey Sima, you have to be careful about using those PA documents - they don't have CFA devices so the one you sent shows a shorted feedback for the unused channel, exactly wrong for this CFA question. 

  • 0 in reply to Shinichi Yokota
    Guru 140200 points
    Shinichi Yokota said:
    I'm looking for a way to deal with an unused channel with as less resistors as possible.

    Then this might be an option:

    Kai