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Isolated Amplifier acting as audio isolation

Shawn_Wang
Expert 3811 points
Other Parts Discussed in Thread: DRV134, DRV135, INA163, INA134

Hi team

My customer met a problem regarding audio output noise:

The 3.5mm analog audio output of the device was found to be noisy and current noise was suspected, after adding an "Audio isolator" between 3.5mm analog audio output and the audio input at receiver side, the problem is solved.

The customer found "Audio isolator" is actually a low frequency transformer and hence they want to know if TI has similar device to solve the problem.


It seems that isolated amplifier could be used here, but I am not sure whether it is available. Could you help give your comments on this case? Let me know if you have any questions.

  • 0
    Guru 140200 points

    Hi Shawn,

    hum in audio systems is usually caused by ground loops. An audio isolation transformer can break the ground loop and suppress the associated hum and noise. Almost the same benefit can be drawn from differential signalling which is widely used in the professional audio industry and recording studios.

    Kai

  • +1 in reply to kai klaas69
    Guru 140200 points

    Hi Shawn,

    true differential signalling can be performed by the help of balanced line drivers DRV134, DRV135.

    Even simple pseudo differential signalling can help to eliminate hum and noise in many cases, as shown in the simplified schematic below:

    shawn_ina163.TSC

    The driving audio circuit on the left is balanced by the introduce of R2 = R1. The hum superimposed to the signal ground, simulated by VG1, is suppressed by the common mode rejection of instrumentation amplifier INA163. Many other instrumentation amplifiers can be used at this place, even simpler differential amplifiers.

    The suppression of ground loop hum is not as ideal as with a special, expensive and heavy audio transformer, but sufficient for the most cases. And the "electronic" solution is way less expensive. 

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 3811 points

    Hi Kai

    Thanks for your detailed explanation.

    In addition to a DRV134/5 at TX side, it seems that a differential-to-single device like INA134/7 is needed at RX side, but the question here is that the customer only design the TX side and hence DRV134/5 is not suitable for their design.

    Could you help check if my understanding is correct?

  • 0 in reply to Shawn_Wang
    Guru 140200 points

    Hi Shawn,

    yes, you are correct. For differential signalling a balanced (or at least pseudo balanced) driver and a balanced receiver are necessary.

    Kai

  • 0 in reply to Shawn_Wang
    Guru 140200 points

    Hi Shawn,

    can you tell more about your application? Have you an idea where the noise is coming from? How is the circuit powered? Is it a class I or class II circuit? Or by other words, is signal ground connected to protective earth?

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 3811 points

    Hi Kai

    Thanks for your strong support on this case, and I learn a lot from your solution.

    Actually I know little about this application, and it is a sudden request from my customer.

    Now the customer has decided to add a transformer in their board.

    Thank you again.

  • 0 in reply to Shawn_Wang
    Guru 140200 points
    Shawn_Wang said:
    Now the customer has decided to add a transformer in their board.

    This is a good idea Relaxed

    Kai

  • 0 in reply to kai klaas69
    TI__Guru** 109185 points

    Hi Shawn, It seems you got it covered by Kai, let us know if you have any further questions.

    Hi Kai, Thanks for your inputs!

    Best regards,
    -Ivan Salazar
    Applications Engineer