noise when connecting an PCM1803 ADC to an ADAU1701 DSP - Page 1
Forum section: electronics / projects

author: jmibk
author_rank: Regular Contributor
timestamp: May 03, 2020, 12:20:22 am
content: I have designed a all in one board based on an ADAU1701 DSP from analog devices and want to expand the inputs to 4 inputs.To accieve that I use an PCM1803 ADC board and connected everything via I2S and routed all inputs to the outputs straigt through to test.Everything works basically, but there is a significant noise floor on the inputs of the PCM ADC. It sounds like white noise.First, before you have search for your crystal globe some schematics.Schematic of the DSP board I designed:http://www.audiowerk.at/downloads/temp/DSP/Schematic_ADAU1701_Solo_v1.pdfSchematic of the ADC board from mine:http://www.audiowerk.at/downloads/temp/DSP/adc_schematic.pdf- its basically the schematic from TI.Voltages of the ADC are connected to the 12V+- rails of the DSP board.I2S (LRClock, Bitclock and Data0) is connected to the MPxx ports of the DSP and configured proberly.Master clock for the ADC is coming from the crystal of the DSP and is 12.288Mhz. Bits are 24, sampling f = 48kHz.Here's an image of the two boards:The noise isn't noticeable while playing music but is there if there is no music playing. Its much more than the two DSP inputs have.It is gone if I short the ADC analog inputs to ground (shorting C12 and C13). It is there when shorting the opamp outputs to ground (shorting all pins on J2).At the ADC inputs there is an DC offset of 2.49V measured with an multimeter - thats because the allowable input range of the input signal, that it has to be lifted 2.5V.After some measurements I saw that there is much more noise at ALL power rails on the ADC board and also the DSP board.Here the measurement of the voltage on the 5V analog rail of the ADC (3,3V rail looks the same) when the master clock signal is DISCONNECTED:and when the master clock signal is CONNECTED:Zooming in it looks like:After that I powered the ADC board from two 9V cells instead of the power from the DSP board, but exactly the same problem.There is no noise from the DSP internal inputs in all cases. They set at the same gain as the external ADC inputs on the second board.Here are the waveforms from analog input to analog output through the whole DSP system:from INTERNAL DSP inputs:from EXTERNAL ADC inputs:There is some time delay noticeable and the fact that the tested sinewave is distorted. The lower ends are wider than the upper ends of the sine wave. I have no Idea where it comes from.On my last image you can see:yellow: input to the opamps of the ADC boardgreen(ish): analog input of the ADC chippurple: output of the DSP board (using internal DACs).Some might say that the error is the DSP but if I use the internal ADCs everything is fine (image above). Sad to say I have no other I2S source to check.Anyone any idea to test further to get rid of that extensive noise floor?There are some circuits in the internet where parallel to C12 and C13 some 220R resistors are soldered. I tried that but they pull the inputs to 0V and the negative wave is missing (because the missing 2.5V offset).

author: jmibk
author_rank: Regular Contributor
timestamp: May 03, 2020, 06:04:31 pm
content: Tried that same thing on an chinese DSP board featuring the same DSP and there it works without noise.What can cause noise in such a circuit and connections?

author: whollender
author_rank: Regular Contributor
timestamp: May 04, 2020, 05:54:01 pm
content: Your I2S lines are probably radiating quite a bit since there isn't anything limiting the rise/fall time.I'd try putting a series 30-50 ohm series resistor on all the I2S lines to try to prevent ringing and see if that helps.

author: diyaudio
author_rank: Frequent Contributor
timestamp: May 04, 2020, 08:04:05 pm
content: Quote from: jmibk on May 03, 2020, 06:04:31 pmLayout. show us where all the high speed paths are running on the board, 2 -Layer? 4-layer? Your flash and local regulator seems to be place far off from the processor, coupling ground noise maybe.Make an antenna loop like the one dave made in hes video, and scan for HF leakage.