Audio ADC 2V RMS??
Hi,
we just powered our A-Sample design with the 1702 using the resistor values stated in Table 12 (Audio ADCs, Page 20) of the data sheet.
We've tried to raise the input signal to 2V RMS but the signal gets distorted above ~1V RMS (thats not a DAC or DSP clipping!!).
I'm wondering how a ADAU1702 should be able to accept a 2V RMS input signal if the analog supply voltage of the chip is only 3,3V?
The necessary voltage swing would be ~5,6V but we only have something below 3,3V.
Is there an error in the datasheet or what is the magic behind that?
Regards
Christian
Audio Signal Processors
adau1702
adc
SigmaDSP Audio Processors
Hi Bernd,
yes, we use decoupling capacitors and 18k/18k resistors.
Our sampling rate is 48kHz.
Regards
Christian
Hi Bernd,
the schematics are according to the data sheet.
The distortion is measured on the output of the DAC (but it's definitely not a DAC clipping).
It starts around 1V RMS input signal and is asymmetric i.e. positive half-wave is clipping first.
I inverted a second DAC output and here the negative half-wave is clipping first so I can exclude a DAC/analog output issue.
I use DC-Blocking blocks as first processing blocks in the chain and I just route the signals thru.
Currently we work on measuring the ADC signal on the digital serial outputs of the 1702 directly.
I also plan to replace the 8k resistores on the evalboard with 18k and do a measurement if that works.
Regards
Christian
Hi,
meanwhile we've found the error - the reason was on our side.
Our HW engineer had an op-amp on the second ADC input signal chain between the capacitor and the resistor for testing purpose (gain ranging). Putting the capacitor behind the op-amp solved the problem.
Regards
Christian
Hi Christian,
as I understand the audio inputs of the ADAU1702 are current inputs. So with the input resistors you perform a voltage to current conversion. That means by increasing the serial input resistors you can expand your input voltage range without problems to values larger than ADAU1702 supply voltage.
Did you use coupling capacitors? Because the 1702 inputs are biased and DC-coupling will seriously affect performance.
What sample rate do you use? Samplerate also affects calculation of the input resistors.
I hope I helped somehow.
Regards
Bernd
Hi Christian,
at which point do you measure distortion? Maybe it is not the 1702's input causing the problem ... Can you show an excerpt of your schematics?
Regards
Bernd
Thanks for the update, Christian!
And thanks for helping out, Bernd!
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