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OPA1632: Drop in SINAD

vinay bhatt
Prodigy 220 points
Part Number: OPA1632
Other Parts Discussed in Thread: TINA-TI

My circuit consist of 3 OPA1632 Opamps, 1 for Bandpass filter and 2 opamps for gain. The output SINAD Of Audio Analyzer is 80 dB . But the after opamps the sinad is 55 dB, THD is less than 0.5%. Similar results are observed in breadboard impl and PCB implementation. Why there is drop in  SINAD? Pls help

  • 0
    TI__Expert 6050 points

    Hi Vinay,

    A few observations I have:

    1. When using a high speed part like the OPA1632, implementing the circuit on a breadboard isn't practical as the parasitic capacitance of the breadboard could very easily degrade the performance. To test the circuits, it is recommended to design a PCB board that follows all the recommended layout guidelines highlighted in the datasheet.
    2. You mention also implementing and testing your circuit on a PCB. Can you share the layout files of your design to confirm that all the recommend layout guidelines are met? There could be additional noise pickup from capacitance and inductance caused by poor layout techniques.
    3. For the gain setting resistors of the three op amp stages, you are using quite large resistors. Larger resistors will also introduce noise into your system which will, in turn, degrade the SINAD performance. Table 8-1 in the OPA1632 datasheet has recommended resistor values for different gain settings that use smaller resistor values.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 220 points

    Thanks Nick. How can I calculate current output of each stage in above circuit?

  • 0 in reply to vinay bhatt
    TI__Expert 6050 points

    Hi Vinay,

    There is an applications journal (found here) that walks through the analysis of a fully differential amplifier. You can use the equations to solve for Vout+ and Vout-. From there, depending on the output resistance and load of the amplifier, you can determine the current using Ohm's Law.

    You can also use TINA-TI and add current meters at the output pins of the OPA1632 to measure approximately what the expected output current will be. Please note that this will vary in the real world due to nonideal component tolerances, temperature variance, and other nonideal behavior.

    Thanks,

    Nick