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SSM2305 input configured in single ended mode.

We are using SSM2305 in our design. We require clarity about the input capacitor usage in SSM2305.

Input signal is single ended. 

The ultrasound signal ranges from 20 HZ to 500Hz, Do we need to have C15 values at the input section as it act as a high pass filter.

100 Hz = 45 nF (C15), does it pass frequency above 100Hz?

Kindly review the circuit as per the requirement.

  • Hello saravanaprabhu,

    I don't handle class d amplifier products but I had some experience on such devices, the main purpose of the coupling capacitor is to remove any dc offset voltage present on the source and only allow the AC signal to pass through. The coupling capacitor ensures that the input signal will swing to mid supply of SSM2305 regardless of the dc offset given by the source so to prevent clipping of the output signal. In your circuit I am seeing that the signal coming out of the AD8544 has a DC offset of 2.5v which is the mid supply of SSM2305, if you are in differential input configuration you can remove the coupling capacitor on both input, but since you are operating in single ended configuration you still need both capacitor in the input to ensure that the dc offset on both input are the same.

    With regards on the input capacitor acting as a HPF, this is correct but since your input impedance in in the range of several thousand ohms, this will set the cutoff of your HPF close to DC (less than 1Hz), hence your signal BW of 20Hz to 500Hz will pass through the capacitor with minimal to no attenuation.

    I allow the class d products experts to correct or add more information in this thread to further help clarify your concern.



  • Thanks for your inputs. Your timely response is greatly appreciated.

  • Hi Joven, 

    We are finding 2 problems associated with SSM2305

    a) When speaker is connected we are hearing a humming noise/sound. 

    When the inputs are isolated even then we are hearing a humming noise. The product is designated for medical applications and the humming noise is not accepted as it kill the accuracy of Ultra sound signal. 

    b) At the output of SSM we have passive filter circuit (Inductor and Capacitor). As per our requirement the audio signal frequency range used in our application is 100Hz to 7KHz. 

    The cut off frequency is set at 40KHz considering the size of the inductor. we are requesting to review the schematics and provide your valuable feedback. 

    Requesting to connect to audio team also for better visibility.