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OPA1662-Q1: Output Clock (763.4KHz)Noise without input Mic signal

beyondeva
Intellectual 340 points
Part Number: OPA1662-Q1
Other Parts Discussed in Thread: OPA1662

Dear All

             Sorry, I want to ask  strange problem about OPA1662-Q1 in Mic circuit.

             Please see annex Mic Line Out circuit.png

            I found there is a phenomenon that when the Micbias has 8V and there is no microphone signal at the input(MIC1_SIGNAL_IN_FLT), its output will have a clock noise in TP20015, the same as TP20018.

           When MIC1_SIGNAL_IN_FLT connect to GND then the Clock noise(764.4KHz) is disappear at same time.

             Although it does not affect the  hearing of the output audio sound(Micphone audio), but we want to know why?

            could you explain it if you know why.

          Thanks  Best regards

Clarion- hejun

          The clock(763.4KHz) noise like this waveform...

   

           

  • 0
    TI__Genius 10850 points

    Hi,

    Thank you for your post. I can help you look into this and get back to you.

    -Tamara

  • 0 in reply to Tamara M Alani
    Intellectual 340 points

    Hi Tamara

                 How about this use case issue?

                 Could you please confirm it continue?

     Thank you.

    Clarion-hejun

  • 0
    TI__Genius 10850 points

    Hi User,

    The reason why you are seeing a signal even when there is no mic input is because pin 3 of the OPA1662 does not have a bias current return path to ground. This is one of the most common problems when there is no dc return path for bias current in ac-coupled circuits.

    Without a path to ground, the input bias current will flow through the coupling capacitor, and charge it either toward the positive supply or the negative supply. The simplest solution is to connect a resistor between the input and ground to provide a path for the input bias current. 

    Also, I noticed that pin 5 has a 120k-ohm resistor connected to it, but pin 4 doesn't. I don't know if this was intentional.

    -Tamara