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Original question:

OPA3S2859-EP: TIA: Gain switching Saturation Issue

OPA3S2859-EP: One channel not used how to connect unused pins

Christian_Fischer
Prodigy 100 points
Part Number: OPA3S2859-EP
Other Parts Discussed in Thread: OPA3S2859, LMH32401

Hi everybody,

OPA3S2859-EP seems to a very nice part for application. So we are going to do some tests with that part. Unluckily we only need one channel. Is there any recommendataion how to connect pins of used channel so that they will not interfere or cause problems when using the other channel? Connect everything to GND and make a voltage follower?

By the way is there any plan at TI to bring out a part with only channel as well?

Thanks for your help.

Regards Christian

  • +1
    TI__Mastermind 22370 points

    Hello Christian,

       Thank you for your interest in the OPA3S2859-EP. Our industrial/catalog version, OPA3S2859, is in preview currently. 

        Unfortunately, you will not be able to shutdown only one channel of the device. That is a very good question. Below are some references on the topic of unused channels:

    1. https://e2e.ti.com/blogs_/archives/b/thesignal/posts/the-unused-op-amp-what-to-do
    2. https://www.ti.com/lit/an/sboa204a/sboa204a.pdf?ts=1652374024921&ref_url=https%253A%252F%252Fwww.google.com%252F

        However, I believe you might still need to include a 1kOhm feedback resistor. I will confirm my assumption with the design engineer, and get back to you shortly with an update. 

        For your last question, I would have to ask our system engineers. 

        Both questions will be answered in the new reply after confirmation with different members in my team. 

    Thank you,

    Sima 

  • 0 in reply to SimaJalaleddine
    TI__Mastermind 22370 points

    Hello Christian,

      Thank you for your patience. I have talked to both our design and system engineers:

    1. Best practice for unused channel for this device would be:
      1. Include a 1kOhm gain for one of the feedback path to avoid oscillations (avoid increased supply current + crosstalk) 
      2. Latch the channel to keep gain selection to 1kOhm path 
      3. IN+ would either be grounded or biased depending on your supply range to avoid violating the input common-mode range of the device.
      4. Also, a 4pF capacitor will have to be connected to the COM_x pin (23 or 8) of the unused channel. Normally this pin would have the diode connected to it. The diode has been assumed to have a capacitance of 4pF which factors into the stability equation of the channel. In the absence of the diode capacitance, the channel will appear to be in a gain of 1 and this would likely make the channel unstable. (In general, to avoid increased supply current + crosstalk) 

    2. We were looking into this as an initial release, but instead we went with dual channel. The team is contemplating creating single channel catalog versions of this device. I have passed along your interest. 

    Thank you,

    Sima 

  • 0 in reply to SimaJalaleddine
    Prodigy 100 points

    Hi Sima,

    thanks for the additional input that will help us for our decision.

    Could be quite interesing esspecially if the one channel derivate will be released quite soon. We looking forward to that.

    Christian

  • 0 in reply to Christian_Fischer
    TI__Mastermind 22370 points

    Hello Christian,

      Unfortunately, it wouldn't seem to be anytime relatively soon. Depending on your gain requirements, we also have single channel integrated TIA such as the LMH32401 which has integrated selectable gains, 2kOhm and a 20kOhm, with the addition of ambient light cancellation.

    Thank you,

    Sima