I just finished my first DAC build, the reference design from the TI PCM2902 data sheet. I had some difficulties on the first PCB version; seems the oscillator being under the chip was a bad idea. Rev 2 is up and working with a REG103ua-a regulator outputting 4V through a diode to Vccci.
I noticed a loss in volume and what appeared to be a loss in dynamic range with my second version which included an external (to the chip) LPF with metal oxide 1k Dale Vishay resistors and Kemet 4n7F Polyester caps. This combo puts the -3db cutoff at 33khz or so. Removing them seems to have helped some, but I may just be hearing a difference in gain from the headphone jack splitter I'm used to. Does the -30deg phase shift at 10khz possibly have an effect?
Anyway, since the PCM2902 includes antialiasing filters and an analog LPF filter, are there any advantages to having an external one on the card? An external LPF is shown on the datasheet schematic but it explicitly states one is included on the chip.
Thanks.
I noticed a loss in volume and what appeared to be a loss in dynamic range with my second version which included an external (to the chip) LPF with metal oxide 1k Dale Vishay resistors and Kemet 4n7F Polyester caps. This combo puts the -3db cutoff at 33khz or so. Removing them seems to have helped some, but I may just be hearing a difference in gain from the headphone jack splitter I'm used to. Does the -30deg phase shift at 10khz possibly have an effect?
Anyway, since the PCM2902 includes antialiasing filters and an analog LPF filter, are there any advantages to having an external one on the card? An external LPF is shown on the datasheet schematic but it explicitly states one is included on the chip.
Thanks.