• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 51 subscribers
  • Views 669 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

OPA209: THD+N error

Shu-Cheng LIN
Guru 11785 points
Part Number: OPA209

Hi Sirs,

Sorry to bother you.

OPA209 have some THD+N error 

Description:

In the [QC Test Item_11] AI Dynamic test, when imputing the signal of 20KHz / 6.3Vrms(17.825Vpp), the THD shows that there are some abnormal phenomena of Channel AI which is more than -78dB. 

This problem can be solved by changing relative OPA209(U10/U15/U20/U25).

We use APx515AI to measure the THD signal of OPA209 directly to avoid the impact of other trace, measuring point as shown in Figure 1.

Measuring 3 pcs of defective OPA209 discovered that the output signal THD of OPA209 is less than 78dB when imputing signal of 20KHz. The difference between the normal OPA209 THD is more than 10dB, as shown in Figure 2, Figure 3.

Actually we have do A-B-A swap analysis, looks this phenomenon is follow IC. The fail rate around 18% (9/50)

We think THD is a typical specification in the TI datasheet and FA is not able to determine a physical cause.

So, could you help us to check this?? 

Any suggest is welcome, thanks!!

  • 0
    TI__Guru* 93105 points
    Hello Shu-Cheng,

    Unfortunately, the THD(dB) number tells us little about what within the frequency spectrum is responsible for the lower THD performance for the "Fail" units. Since you are testing the OPA209A devices on an APx515 you should be able to collect an FFT for them. If you can provide us FFTs for the "Good" and "Fail" devices that should provide us more useful information about their distortion characteristics.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0
    Guru 140200 points

    Hi Shu,

    sometimes added protection diodes at the inputs of OPAmp can cause distortion. As the diode's capacitance varies with reverse voltage, the AC signal can modulate the diode's capacitance and by this introduce distortion. The same is valid for the built-in protection diodes. These do also show a variation of diode's capacitance with reverse voltage and can also be modulated by the AC signal. The impact on signal distortion can be minimized by lowering the feedback resistances of OPAmp. This is especially true for higher signal frequencies.

    I would test the performance without protection diodes. In a second step I would decrease the feedback resistors by a factor of 10.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points
    Hi Shu,

    Indeed, the feedback and input resistors are too high and are likely contributing more noise than the OPA209 op amp to the circuit. I agree with Kai, I would reduce them by a factor of 10x. That will help reduce the noise portion of the THD + noise and improve that number a bit. However, that probably isn't the dominant factor in the lowered THD performance of the "fail" devices. We really need to see the FFTs.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    TI__Genius 11135 points
    Shu-Cheng

    We haven't heard back from you so we assume this answered your question. If not, post another reply below.

    Thanks
    Dennis