Analog Audio Inputs of the 1701: What is the voltage range and can I use a variable resistor to level them?

Hello Ararien,

Thanks for your inquiry.

We will check this internally and get back to you soon.

Regards,

Harish

Hi Harish,

is there any update yet? As I said, I have an imput amplitude of 10V +/- 10V to the DSP. I have chosen an 91k input resistor for the DSP and wanted to know if I will run into problems or if the DSP can handle this.

Thanks,

Christian

Hello Christian,

The datasheet section for the ADC does go into this with a fair amount of detail.

It uses the example of an 18k input resistor for a full -scale voltage of 2.0Vrms. The 18k plus the internal 2k gives it 20k total input resistance. So doing the math, 2V / 20k = 100ua which is the full scale current for the ADC.

So with that in mind. Your input value is listed as +10V to -10V so I read that as a peak-to-peak value of 20V correct?

So converting that to an RMS value I get 14.142Vrms.

You have a 91k resistor so the total input resistance is 93k.

To calculate the full scale current:....

14.142vrms / 93k = 152ua which is way greater than the max input current of 100ua.

If I take 14.142Vrms and divide that by 100ua I get 141.42 Kohms. So I will round down to 141K then subtract the internal 2K I get 139k for the input resistor.

So give that a go and I think it will work.

Also, are you running at 48kHz fs?

Thanks,

Dave T

Hi Dave,

thank you for your detailled explanation. My 91k came from a simulation where I did not know how to replicate the input of the 1701. I have adjusted it to 130k, my swing is a bit lower than 20V.

Other things that bothered me however (and were the reason for posting) were:

Are the inputs of the 1701 protected with diodes going to 3,3V and GND? Will they care about a voltage swing of 20V?

Apparently not, because the resistor converts the voltage swing into current before it can hit the limiting diodes.

Why is everybody calculating with Vrms when the current limit is 100µA? What about the peak voltage?

This was an error on my side. These 100µA are indeed 100µA_rms. There is even an information 238µA_pkpk the datasheet. I just read over these details.

I have alread tried my configuration and instead of smoking the Level-Block showed an appropriate level.

Also, I am running at 48kHz and have left ADC_RES at 18k.

kind regards,

Christian