1701 input noise
Whenever I connect an audio source to the input of the 1701 (this is not the evaluation card), I get crackling and white noise. This happens with any audio source, but for testing purposes – I am using a battery operated CD player [so it is isolated from ground]. The 1701 circuit is also battery powered.
When there is nothing connected it is silent. I have been looking at the circuit for a while trying to narrow the problem down and I believe the noise is current related. The 1701 ADC is also current driven. So I was wondering if you could check my input resistor selections and give me some advice. Maybe my resistor settings are too low? …And more prone to noise?
My input is 1Vrms [I decided to try to keep it at 1Vrms to get the full dynamic range of the ADC]. And my sampling rate is 192kHz.
I calculated that my ADC_RES = 3K and the Rin=500 Ohm.
Can anyone help?
Thanks,
Bruce
Audio Signal Processors
noise
SigmaDSP Audio Processors
adau1701
The 1701 evaluation card which is also running at 192KHz does not have this noise using my test setup and seems to work fine with a ADC_RES=18K and ADC input resistors = 8K.
And...the parts list (Rev 0, EVAL-ADAU1701MINIZ) shows the input resistors R15, R20 as 8 Ohm, but I measured them on the evaluation card and they are 8K.
I just changed my resistors to match the 1701 Evaluation card (ADC Rin=8.2K 5% and ADC_RES=18K 5%, these are the closest values I had on hand) and the noise is gone - regardless of whether the audio source is connected.
The volume of the input went down, so I am going to adjust the gain in SigmaStudio.
I would still like to know how to correctly set the ADC input resistors and ADC_RES for optimal dynamic range at 192KHz without the noise. Can anyone help?
Thanks,
Bruce
Hi bruce. Thanks for the updated information.
We're attempting to confirm this with the ADAU1701's analog designers. Once we have an answer we'll post it here.
I received a somewhat vague response from the designers so I'm trying to follow up and get a more concrete answer. So far I have not gotten the exact information I was looking for. I will update you once I do.
The deatasheet's description of the sample rate bits is as follows:
SR [1:0] Sample Rate
These bits set the number of DSP instructions for every sample and the sample rate at which the ADAU1701 operates. At the default setting of 1×, there are 1024 instructions per audio sample. This setting should be used with sample rates such as 48 kHz and 44.1 kHz.
At the 2× setting, the number of instructions per frame is halved to 512 and the ADCs and DACs nominally run at a 96 kHz sample rate.
At the 4× setting, there are 256 instructions per cycle and the converters run at a 192 kHz sample rate.
However, I have a suspicion that this may be incorrect. I believe these bits are only changing the rate of the DSP, and that the audio converters may not be scaling accordingly. In that case you should be doing the calculation for the ADC resistors based on the "base" sample rate of 48 kHz.
Like I said, I'm not certain about this. I will post here again once I receive confirmation.
Any news yet?
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Thank you,
EZ Admin
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