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ADA4075-2 and SSM2167 Q's

Hi,

We are looking at using the ADA4075-2 instead of the OP275, as recommended by ADI, as a balanced line driver to the AD1974. However, the reference design uses trimpots whereas these are not shown for the OP275 reference design. As we are looking to have 12-16 inputs per PCB, it isn't feasible to tune each individual trimpot. Can these be left out or are there standard resistor values which are used which produce 'good enough' results.

Further to this, there are in some places specifications for the ADA4075-2 and SSM2167 in the datasheets as to what type of capacitors to use, however not always. I was wondering for a general idea of what types of capacitors are suitable for these devices. I have read conflicting information about how 'bad' ceramic capacitors are in audio etc and just want to get some device specific info.

Thanks

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  • Hi Daniel,

    The SSM2167 is a helpful device if you need the VCA or compression features that are included. If you need only gain with a Low-Pass Filter, I would use the ADA4075 to increase the voltage of your signal to reach the operating input level of the AD1974 (2 V RMS differential maximum). You could use a simple circuit as shown here on Page 9 of the User Guide:

    http://www.analog.com/static/imported-files/user_guides/UG-046.pdf

    Changing the value of the feedback resistor in the first stage will be the easiest way to adjust the gain of this circuit. I would change the feedback capacitor accordingly, to leave the first-order pole of this LPF around 100 kHz. Depending on your choice for a VREF (the voltage that appears at the non-inverting input of the op amps), you might not need to add AC coupling capacitors between this stage and the ADC input. And you will certainly need the inverting stage, as Harry has pointed out, in order to drive the ADC to full scale.

    Aluminum electrolytics are larger, but lighter, much less expensive and much more available in varying values and voltage ratings. Aluminum electrolytics dry out over time, where tantalum do not. I would not recommend tantalum because they do not go bad slowly over time, they either short (which can be dramatic if there is enough current available. BOOM!) or open. I would use Aluminum electrolyics in the audio path and if you need a large bulk reservoir for power supply decoupling, use tantalum. Remember to give yourself plenty of headroom with voltage ratings: double the DC voltage that is biasing the cap, plus any audio which is on top of the voltage.

    Regards,

    Coleman

Reply
  • Hi Daniel,

    The SSM2167 is a helpful device if you need the VCA or compression features that are included. If you need only gain with a Low-Pass Filter, I would use the ADA4075 to increase the voltage of your signal to reach the operating input level of the AD1974 (2 V RMS differential maximum). You could use a simple circuit as shown here on Page 9 of the User Guide:

    http://www.analog.com/static/imported-files/user_guides/UG-046.pdf

    Changing the value of the feedback resistor in the first stage will be the easiest way to adjust the gain of this circuit. I would change the feedback capacitor accordingly, to leave the first-order pole of this LPF around 100 kHz. Depending on your choice for a VREF (the voltage that appears at the non-inverting input of the op amps), you might not need to add AC coupling capacitors between this stage and the ADC input. And you will certainly need the inverting stage, as Harry has pointed out, in order to drive the ADC to full scale.

    Aluminum electrolytics are larger, but lighter, much less expensive and much more available in varying values and voltage ratings. Aluminum electrolytics dry out over time, where tantalum do not. I would not recommend tantalum because they do not go bad slowly over time, they either short (which can be dramatic if there is enough current available. BOOM!) or open. I would use Aluminum electrolyics in the audio path and if you need a large bulk reservoir for power supply decoupling, use tantalum. Remember to give yourself plenty of headroom with voltage ratings: double the DC voltage that is biasing the cap, plus any audio which is on top of the voltage.

    Regards,

    Coleman

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