PCM1865: About filter settings

Shuji Ishiwata

Genius 5930 points

Part Number: PCM1865

Hi All,

I have aquestion about PCM1865.

Figure 63 in the datasheet shows 47Ω and 0.01uF, but the customer's design shows 330Ω and 1500pF.

(1) Please tell me the reason for 47Ω and 0.01uF for the filter in Figure 63 of the data sheet.

(2) The cutoff frequency of the customer's filter (330Ω and 1500pF) is set to be the same as 47Ω and 0.01uF.
　　Please let me know if there are any recommended resistance values or restrictions to set.

(3) The customer checks the input level (dBu) of the PCM1865 to make a pass judgment.
　 Please let me know if there are any points to note about this evaluation.

Best Regards,
Ishiwata

over 1 year ago

0 Daveon Douglas over 1 year ago

TI__Genius 11360 points

Hi,

Apologies for the delay, please wait at least 24 hours for our ADC expert to respond to your question.

Thank you for your patience.

0 Sanjay Dixit over 1 year ago in reply to Daveon Douglas

TI__Mastermind 22080 points

The differential Input impedance of the chip is of the order of 50k and in my opinion does not pose any issue when used with 300 ohm series resistor.

The Only issue with 47 oh series resistors i see is that of possible loading of input source at high frequencies.

At high frequencies the c is a short circuit. Now Isource pk =Vdif_pk/100. If we have 1v p signals then a peak current of 2/100 or 20ma shall be drawn from the source at high frequencies. This possibly might be an issue. Perhaps customer has increased resistors to 330 ohm

0 Shuji Ishiwata over 1 year ago in reply to Sanjay Dixit

Genius 5930 points

Hi Sanjay,

Thanks for your answer.
I understand.

What is the recommended range of resistor values?

Also, I have a question about the differential input impedance value.
Your answer says 50kΩ, but the single in the datasheet is 10kΩ.
Is the differential input impedance 50kΩ correct?

Best Regards,
Ishiwata

0 Sanjay Dixit over 1 year ago in reply to Shuji Ishiwata

TI__Mastermind 22080 points

i am sorry i made a typing error. Yes the figures you indicate are correct

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PCM1865: About filter settings