ADAU1372 differential DAC filter

Greetings,

I've read the datasheet for ADAU1372 and cannot find an example for a differential filter for the DACs (optionally can be configured for differential output).

Would this filter ( AD1937 datasheet) be a good place to start?

My goal here is to achieve the lowest possible noise for driving a power amplifier.

Thanks for any assistance.

Parents
  • +1
    •  Analog Employees 
    on Dec 3, 2018 10:20 PM over 2 years ago

    Hello wctronic,

    Yes, this is a good filter to use. The OP275 requires higher voltage rails but since you are running differentially the output swing will be greater than the rails for the codec. What is good about this design is that it is a single stage. What is bad is that the CMRR will not be very good because the input impedance is not equal for both signals.

    If you look at Figure 10 in the ADAU1966 datasheet you will find a filter that has the same input impedance. The problem with that filter is that the output is still differential. So then you follow this with a simple unity gain differential input amp so it will be optimized for CMRR. It also buffers the filter output to prevent issues when driving heavy loads. The down side of this circuit is that it uses more parts.

    Dave T

Reply
  • +1
    •  Analog Employees 
    on Dec 3, 2018 10:20 PM over 2 years ago

    Hello wctronic,

    Yes, this is a good filter to use. The OP275 requires higher voltage rails but since you are running differentially the output swing will be greater than the rails for the codec. What is good about this design is that it is a single stage. What is bad is that the CMRR will not be very good because the input impedance is not equal for both signals.

    If you look at Figure 10 in the ADAU1966 datasheet you will find a filter that has the same input impedance. The problem with that filter is that the output is still differential. So then you follow this with a simple unity gain differential input amp so it will be optimized for CMRR. It also buffers the filter output to prevent issues when driving heavy loads. The down side of this circuit is that it uses more parts.

    Dave T

Children
  • Hi Dave, Thanks for the reply,

    I was concerned about the differing input impedance affecting CMRR with the attachment I posted previously.

    Attached an image of a MFB filter I've seen, will this one pose the same CMRR issues?

    The application may call for a gain of up to 3x after the DAC filter. Would it be best to stick with the filter you proposed and then add the gain in the differential input amplifier?

    Thanks for your help.

  • +1
    •  Analog Employees 
    on Dec 4, 2018 6:22 PM over 1 year ago in reply to wctronic

    Hello wctronic,

    This design looks good for CMRR. The resistors are all equal on each leg of the differential input.

    If you are running the signal a long way where you really need the best CMRR then having all the resistors on a differential amplifier matched to the same value is the best. So unity would be the best. However, I am guessing you are not running 100ft of cable so you really do not need the CMRR to be stellar. So you could get some gain out of the differential input amplifier I proposed. 3x is not huge so you should still be able to get a decent CMRR.

    Keep in mind that others may have different opinions. You can also ask these questions on our linear amplifiers part of the forum.

    Dave T