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OPA855: OPA855 TIA Noise

Oriol Sanchez
Intellectual 360 points
Part Number: OPA855

Hello,

A customer is having problems with a custom board using OPA855, with the EVM they see much less noise than with the custom board, with exactly the same schematic and parts. This is probably a layout issue that we would like to check with you.

Can somebody let me know his/her email and discuss about this privately? 

Thank you very much,

Oriol.

  • 0
    TI__Mastermind 22574 points

    Hello Oriol,

    I would be happy to assist you both.  I can look into our EVM and be prepared to help examine the customer's board files.  Please search for my name in the TI directory to find my email, I should be rather quick to find.

    Best,

    Alec Saebeler

  • 0 in reply to Alec Saebeler
    TI__Mastermind 22574 points

    Hello Oriol,

    I am pasting part of my response to your regarding power down (PD) functionality here on the forum, in case anyone else has a similar question.  You may continue to contact me via email or this forum regarding any updates on your issue.

    Email Excerpt: Discussion on Power Down for OPA855

     I can help you out with your PD question to start.  The PD selection functionality is dependent on the power rails of your circuit.  Power down is an active logic low function, meaning a logical high will enable the amplifier and a logical low will disable the amplifier.  Those thresholds are explored below.  Examining the following excerpt from the OPA855 datasheet (page 19, section 9.4.2):

     “9.4.2 Power-Down Mode

    The OPA855 features a power-down mode to reduce the quiescent current to conserve power. Figure 23 and Figure 24 show the transient response of the OPA855 as the PD pin toggles between the disabled and enabled states. The PD disable and enable threshold voltages are with reference to the negative supply. If the amplifier is configured with the positive supply at 3.3 V and the negative supply at ground, then the disable and enable threshold voltages are 0.65 V and 1.8 V, respectively. If the amplifier is configured with ±1.65-V supplies, then the disable and enable threshold voltages are at –1 V and 0.15 V, respectively. If the amplifier is configured with ±2.5-V supplies, then the threshold voltages are at –1.85 V and –0.7 V.”

     For the OPA855, the amplifier will turn off at voltages less than 0.65V from the negative power rail.  Similarly, the amplifier will turn on/be enabled at voltages 1.8V or less from the positive power rail.  In the examples above, driving the PD pin below the disable threshold will disable the op-amp, while driving the PD pin close to the positive supply will enable the amplifier.

     By driving PD towards negative supply or floating the PD pin, you are either forcing power down or introducing ambiguity (respectively).  For normal amplifier operation, it is recommended to connect the PD pin to the positive supply rail.  As to whether noise performance should differ between power down state and a floated PD pin, there is not a clear answer here.  Placing the amplifier into power down state puts the output into high impedance.  The quiescent PD current is at maximum 140uA, with a typical value of 70uA.  The noise issue you are encountering may involve how you are operating/driving the PD pin.  I suggest running a test with the amplifier in full operation, PD = positive supply rail.  Follow up this with a test having PD = negative supply rail to check if the noise source persists.  Unless there is a strong reason for operating the PD pin in a floated/open state, I would suggest driving the PD pin at all times.  Depending on the IC you are working with, you may be able to test and justify floating the PD pin, should design constraints make it viable/needed.

    Best,

    Alec Saebeler