1701 INPUT OUTPUT HALF

Sojourney,

It sounds like you are clipping the signal, because the input of 1 V RMS might be slightly above -6 dBFS. By applying +6 dB of gain inside the DSP, you'll raise the signal level to 0 dBFS or slightly above, which will create clipping.

What is the signal source in your application, and what is its signal level? You can choose the proper ADC resistor to match the signal source. That way, there will be no need for gain inside the DSP.

Hi,

Are you using one of our evaluation boards? Can you please be more specific about the quality of the noise? Or is it distortion? If you remove the tone, is there any noise? if you reduce the source voltage by half or more, does the noise go away? If you lower the gain in the DSP, does the noise go away?

Best regards,

Coleman

By the way, with an 18 kOhm resistor on the ADC input, the full-scale input level of the ADC is 2 V RMS.

The DAC full-scale level is 0.9 V RMS.

This means that if you input an analog signal and you set the DSP to pass-through, then the output level will be 0.9/2 = 45% of the input level. So, the analog-to-analog gain of the chip is 0.45.

If you want the analog-to-analog gain to be 1 (or 0 dB), then change the ADC resistor to 7 kOhm, as shown in the table above.

Hi Coleman

yes, i have a 1701 MinZ board. and i also did a board with a MCU control DSP as my project.

already confirm is do some input ground isolation, the total noise will reduce lot, but still some little noise there.

ok, i will test more details one MinZ board and my board.

thanks

Sojourney

Hi Brett,

thanks for your quick response,

ok, today i will go to shop to buy 7 and 8 kOhm, and have a test for this resistors,

the signal for testing is 1kHz sine tone, level is 1V rms to the ADC 18k, and the ADC_RES is 18k, ref. from the minZ board.

so if set to ADC_RES 18k and ADC0/1 7k , this will have input/output to 1:1 ?

if the audio device output , i mean some device is 2.0 V output, some is below 2.0V output, which parameter of those resistors set will be better ?

Sojourney wrote:

so if set to ADC_RES 18k and ADC0/1 7k , this will have input/output to 1:1 ?

Yes, if you change those resistors, then the input full-scale level will be 0.9 V RMS and the output full-scale level will be 0.9 V RMS, meaning that the analog-to-analog gain will be 1:1. However, this means you can't exceed 0.9 V RMS (2.5 V p-p) on the ADC input, or else it will clip.

Sojourney wrote:

if the audio device output , i mean some device is 2.0 V output, some is below 2.0V output, which parameter of those resistors set will be better ?

Well, you can only set the input level of the ADC. There's no way to configure the DAC output level. It will always be a maximum of 0.9 V RMS. If you want to increase the signal level coming from the DAC, then you must do it in the analog domain using an op amp gain circuit.

Hi Brett,

i just can buy the resistors 7k5 and 8k2, and have testing today. yes, it can follow the datasheet.

yes, if set to 18k , DAC output need the op amp, next version board will be add to it for 1:1.

sorry, what i mean is audio source device(like CD player ...) , the output maybe is 2.0V, maybe is below 2.0V.

and when those signal connect to my board RCA ports, if i set resistors to 7k or 8k, it will be clip right ?

so, maybe it is better set to 18k , and then add the op amp to have 1:1 when DSP inside is passthrou.

and the about the noise, i still checking the ground and the power, today i buy 2 pcs of BA3121 to isolate the input ground, add the magnetic beads(330 ohm) to isolate the 3.3V power, and then the noise reduce a little, but still have some when ear close to speaker.

Sojourney

The SNR and THD+N performance of the ADCs and DACs in the ADAU1701 are "ok" but certainly not state of the art. In one of my early applications that required a system with unity input/output gain I ran into that same problem with the noise floor. For demanding projects I have switched to the ADAU1446 with external ADCs and DACs that have significantly better SNRs. This has solved any problems with residual hiss and other noises, even with high gain amplification stages.