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Adau1701 dac output passive vs active.

Hi guys,

I'm not sure which type of adau1701 output I should use (Passive vs Active), what are the advantages of one over the other, not to mention the cost?

When using AD8608 or another operational amplifier...
   a) Is there any gain in quality?
   b) Are there any changes in the output voltage?

Also, about the OSC pin, should I leave the left float when using Active Oscillator? (4 Pins with vcc ), does this type of oscillator have enough power to be used with four external ADCs, without using a buffer?

Thanks in advance.

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  • Hello Rocha,

    This is a bit of a difficult set of questions because personal preference enters into this. 

    A passive filter will not be able to drive a lower impedance load or a very long cable with lots of capacitance. So your application enters into this decision. The passive filter is a single pole filter but the DAC is an oversampling DAC so the out of band energy is reasonable easy to filter out. An active filter is usually a two pole filter with a 12db/oct response. The Opamp will be able to drive a longer cable. 

    Your question about the AD8608 is best asked in the amplifier group part of this forum. This datasheet is rather old so I know there are many newer OpAmps that are available.  

    Amplifiers - EngineerZone (analog.com)

    Your question:

    Also, about the OSC pin, should I leave the left float when using Active Oscillator? (4 Pins with vcc ), does this type of oscillator have enough power to be used with four external ADCs, without using a buffer?

    Yes, leave it floating if you are using an active oscillator. Look at the datasheet of the active oscillator to see how much it can drive. I would expect it can easily drive four inputs and reasonably long traces. If the master clock will be going to other PCBs over cables and connectors then I would buffer it. But if all four ADCs are on the same PCB and the board is not huge you should be fine. However, do look at the datasheet and your capacitive load. 

    The OSC pin cannot be used as an output to go to other parts. You did not ask that but I want to make it clear to others. 

    Dave T 

Reply
  • Hello Rocha,

    This is a bit of a difficult set of questions because personal preference enters into this. 

    A passive filter will not be able to drive a lower impedance load or a very long cable with lots of capacitance. So your application enters into this decision. The passive filter is a single pole filter but the DAC is an oversampling DAC so the out of band energy is reasonable easy to filter out. An active filter is usually a two pole filter with a 12db/oct response. The Opamp will be able to drive a longer cable. 

    Your question about the AD8608 is best asked in the amplifier group part of this forum. This datasheet is rather old so I know there are many newer OpAmps that are available.  

    Amplifiers - EngineerZone (analog.com)

    Your question:

    Also, about the OSC pin, should I leave the left float when using Active Oscillator? (4 Pins with vcc ), does this type of oscillator have enough power to be used with four external ADCs, without using a buffer?

    Yes, leave it floating if you are using an active oscillator. Look at the datasheet of the active oscillator to see how much it can drive. I would expect it can easily drive four inputs and reasonably long traces. If the master clock will be going to other PCBs over cables and connectors then I would buffer it. But if all four ADCs are on the same PCB and the board is not huge you should be fine. However, do look at the datasheet and your capacitive load. 

    The OSC pin cannot be used as an output to go to other parts. You did not ask that but I want to make it clear to others. 

    Dave T 

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