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Original question:

PCM5122: 2Vrms output THD+N only 88dB

PCM5122: Does reduced LPF bandwidth help with THD+N

Nicole Han
Intellectual 2230 points
Part Number: PCM5122


Hi, team, 

My customer is using PCM5122, but they couldn't pass their end customer's THD+N spec, which is 90dB at 2Vrms output. 

I wonder does decrease output LPF bandwidth ( increase output RC) can help. If yes, what's the rule to choose the RC value?

Thanks!

  • 0
    TI__Guru 54953 points

    Hi Nicole,

    We specify our THD specification with an external filter which should remove nearly all of the out-of-band noise from the PCM.  This device uses noise shaping which allows the out-of-band noise to start at around 180kHz, which is well outside of the audible range and the passband of the recommended external filter.

    Generally, if this device has compromised THD, it is due to either:

    1. the output is loaded beyond the 1kΩ max

    2. the THD test is using a 0dBFS signal instead of the -1dBFS signal that is specified in the datasheet.

    We use -1dBFS in the datasheet as THD can suffer on devices with a VDD/CPVDD voltage that is slightly low and where there is a load on the output.  Using the lower input signal gives the output a few extra mV of headroom, which improves performance.

    Thanks,

    Paul