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TLV8802: output crossover distortion

Shinichi Inoue
Guru 19671 points
Part Number: TLV8802
Other Parts Discussed in Thread: TLV522, LM324, LM358

Dear Specislists,

My customer is considering TLV8802 and has questions.

Could you please advise?

---

(1) Generally, optput stage is AB class push-pull.

Can I think TLV8802 is the same?

(2)Is there a possibility that distortion will occur due to charge / discharge current to the capacitive load, equivalent to class B or class C operation?

(3)According to the datasheet, when capacitive load is heavy(>50pF), it is recommended to add Riso.

In this case, is there a possibility to increase output distortion?

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Mastermind 49896 points

    Hello Sinichi,

    Keep in mind the TLV8802 & TLV522 are nano power devices, and do not have a lot of output drive capability (<1mA for a linear response).

    The output stages are class A-B, as you suspected. However, to save on quiescent supply current, the output stage bias current is run very "lean". Low-distortion AC performance was not at the top of the wish-list for an 8kHz device, as it was targeted at DC applications and very low frequency applications. It is easy to run into slew rate limiting very quickly with these low bandwidth devices.

    These devices have a high output impedance, and are VERY sensitive to direct capacitance on the output. An isolation resistor is required for any capacitance drive.

    You could bias the output into class A with a load to V-, but that defeats the purpose of a nano power device.

    As with any amplifier, reducing the loading on the output will reduce the distortion. Because of the low gain-bandwidth, care must be taken for the amount of gain used in each stage. The more gain added to a stage, the less distortion-cancelling open loop gain is available (particularly at the higher frequencies - where it is more apparent).

    What range of frequencies will be involved? What is your load?

    The above applies to all the micro and nano-power amplifiers - so I will combine this with your similar TLV522 thread.

  • 0 in reply to Paul Grohe
    Guru 19671 points
    Hi Paul,

    Thank you for your reply.

    I agree two thread combine.

    I'll share your comment with the customer.
    Also I confirm input/output specs and load capacitance and then feedback to you.

    I appreciate your great help and cooperation.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    Guru 19671 points

    Hi Paul,

    I confirmed with the customer.

    Unfortunately I couldn't get capacitance and the other specs because of the confidential information.

    Their inquiry is whether TLV8802 and TLV522 are not cause crossover distortion, when output current is less than 1mA.  

    The circuit is the same as TLV8802 datasheet of P.1 Nanopower PIR Motion Sensor Amplifier.

    I would be grateful if you could advise.

    I appreciate your great help.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 49896 points

    Hello Sinichi,

    This is a very slow circuit - you should not see any crossover distortion.

    1mA is getting near the current limit for the 8802, but under 1mA should not have crossover distortion.

    You will see crossover distortion when the output is under stress, such as when under heavy load, or during slew limited output when the output is not under feedback control (non-linear operation).

    Do you know why they are so concerned about crossover distortion from such a slow circuit? Have they been using LM324 or LM358's in the past (which are known to have bad crossover distortion)?

  • 0 in reply to Paul Grohe
    Guru 19671 points

    Hi Paul,

    Thank you for your reply.

    I understand TLV8802 and TLV522 should be no problem for crossover distortion.

    I'll confirm the reason of concerning with the customer, and then feedback.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 19671 points
    Hi Paul,

    I sent your information to the customer, he could understand no problem <1mA

    And I could confirm the reason.

    The customer's product is sensitive, if crossover distortion happen, it can't be satisfied the spec of the product.

    Unfortunately the model of opeamp is not able to know.
    Because other engineer encountered this kind of problem and was well-known, but the parts number is not known.

    I appreciate your great help and cooperation.

    Best regards,
    Shinichi