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OPA365: Recommendation for Op-Amp with High-Impedance Output When Not Powered

WB
Intellectual 350 points
Part Number: OPA365
Other Parts Discussed in Thread: OPA727, OPA863A, OPA835, OPA837

Hello,

I’m looking for a precision op-amp recommendation that can be used as an error amplifier (supply = 12 V nominal) and whose output becomes high-impedance (Hi-Z) when the device is not powered.

In my application, the amplifier output connects to a control node that must remain isolated when this section of the circuit is unpowered. Therefore, I want to avoid any back-feeding or leakage paths through the op-amp output stage when VDD = 0 V or below UVLO.

Ideally, the device should:

  • Operate from a single 12 V supply (or ±6 V)

  • Have good DC accuracy and stability for use as an error amplifier in a feedback loop

  • Provide a defined Hi-Z output state during shutdown or when unpowered

  • Optionally include a shutdown or enable pin to control this behavior

Could you please recommend TI op-amps that meet these requirements or share design guidance to achieve a true high-impedance output condition when power is removed?

Thank you,

  • 0
    TI__Genius 17545 points

    Hi,

    There are many devices among our amplifier product lines that could work for your application. Is there any information on bandwidth and slew rate requirements for this amplifier? This information would help us narrow down potential devices. In terms of design guidance, I believe a true buffer circuit would be your best method to isolate these nodes when powered down. The concern you have with any other configuration with a feedback network (Rf and Rg) is that this would be the path any signal would take looking back into your amplifier even though the output is in Hi-z. Therefore, I believe a buffer circuit would be your best option for this circuit.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Intellectual 350 points

    Thank you for the quick response and helpful insight.

    The amplifier will operate as an error amplifier in a 12 V control loop, so I’m looking for a moderate bandwidth device—something in the 1 MHz to 10 MHz range with a slew rate around 1 V/µs or higher would be sufficient. The main priority is ensuring the output node remains isolated (Hi-Z) when the amplifier is unpowered, to prevent any interaction with the rest of the circuit.

    Using a true buffer configuration makes sense to avoid this issue. If there are any specific buffer you recommend that behave well under these power-down conditions, I’d really appreciate your suggestions.

    Thanks again for the guidance.
    BR,

  • 0 in reply to WB
    TI__Genius 17545 points

    Hi,

    Considering the bandwidth and slew rate requirement for your design I would consider opening a separate thread with say the OPA727, as this would direct your thread to the correct team and ask if they maybe had any options for this application. As a high-speed product line most of our parts would likely be overkill for what you need. However, as a consideration you could look at our OPA863A device that will work as a buffer. It is one of our lower bandwidth and slew rate devices with low offset voltage.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Intellectual 350 points

    Hi,

    OPA863A has ESD protection diodes (Clamping diodes) on it's output. The high-side diode can back feed current to the high rail. 

    Thank you.

  • 0 in reply to WB
    TI__Genius 17545 points

    Hi,

    If the output node reaches a voltage higher than your supply plus the forward voltage of the ESD diode, a current would flow from the output to the ESD cell. Most devices (if not all) will have some kind of ESD structure to protect the device.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    TI__Expert 4126 points

    https://www.ti.com/lit/ta/sszt937/sszt937.pdf?ts=1762751684728&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FOPA837

    the OPA83x family has special care to take the output into high impedance when disabled.

    While it may miss your 12V mark , is should meet all your specs.

    Please look at OPA835 , OPA837 for your application.

      can you check with design if they have output impedance vs frequency plot for opamp when put in power - down.