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Audio DAC antialiasing filter?  (ADAU1701)

Question asked by joe42 on Dec 11, 2015
Latest reply on Dec 14, 2015 by DaveThib

I'd like to use a simple passive antialiasing filter for driving (arbitrary) external audio devices (powered speakers of user's choice).  I notice there are two "reference" designs, and am curious as to the reasoning behind them, and differences.

 

1.  ADAU1701 datasheet:  Figure 18 shows a simple LPF with a DC block, using a 47uF capacitor (probably a tantalum?), 560 ohms in series, and 5.6nF to ground.

2.  ADAU1701 EVAL schematic: They use a 100uF tantalum capacitor, 56.2 ohms in series, and then 56nF and 49.9kohms to ground.

 

They have the same open transfer function.  When loaded with 10k Option #1 has higher output impedance, and we lose about 0.5dB of level. With 10k loading, Option #2 loses about 0.7dB at 20kHz, which is acceptable.  (simulated)

 

The output current capacity of the ADAU isn't specified, but if the output is shorted #2 limits current to about 16mA, which I think will be OK.

 

I'm guessing that design #1 is designed for high-impedance (opamp) loads, while they expect people to plug random things into the EVB (#2). 

 

I may reduce the DC blocking cap for cost/space savings, perhaps to 22u, which seems to only effect frequencies lower than 10Hz, and even then only slightly.

 

Any other subtleties I should consider here?

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