Other Parts Discussed in Thread: PCM1808, TMS320VC5401,
Dear TI FAE,
I'm working on a project using PCM1808 and PCM1754. Partial schematics and layout below. The goal is to replace an existing codec with these ADC and DAC, on a DSP daughter board doing audio processing. The DSP is TMS320VC5401 using an OTP EPROM with no possibility to debug. I plan to connect another microcontroller to drive the DAC to do more tests later today.
The problem is excessive audio band noise, flat across the whole audio spectrum. It seems the noise goes away if I reduce the supply voltage to the DAC. It would seem that increasing Fs to 48kHz also reduces the noise, but I'm not 100% sure about this yet, as the test firmware has some issues.
To change the supply voltage, I removed FB2 and connected an Agilent E3632A supply to +5V_DAC net. I was decoding the I2S data to the DAC, and measuring the noise with a computer soundcard (RME ADI-2), lacking a better audio analyzer for the time being.
LRCK = 24 kHz
BCK = 1.536 MHz (64 x fs)
SCKI = 6.144 MHz (256 x fs)
+5V to DAC VCC
Soundcard measured noise -59 dBA
I2S Data out: Standard deviation 55000 (decimal)
+3.3V to DAC VCC:
Soundcard measured noise -79 dBA
I2S Data out: Standard deviation 55000 (decimal)
I did the test with Fs = 48kHz and the noise difference between the two different supply voltages was only 3dB, which is expected. However, I'm not confident that the 48kHz test firmware works properly.
The datasheet doesn't specify the dynamic range for 24kHz operation. Is this behavior expected? Also what's going on with the DAC voltage dependency? Is it safe to operate the device at +3.3V if that solves the issue and provides the audio performance needed for my application? Running the system at 48 kHz is not possible for the final application.
I'm happy to provide more measurements if needed. I thought I'd post this question now before doing more measurements, if you already know these results are expected.
Thank you for your help!
