Does the Input and output filter required for the codec

Question

Hi Support Team, 
 We are currently designing a new audio board utilizing the AD1938 codec, and we plan to use only one channel for input (ADC1LP, ADC1LN) and output (OL1, OR1). Given our application's low-end requirements, we do not necessitate high audio precision or quality. 
 After reviewing the user guides UG-045 and UG-87, we noticed that both schematics incorporate input and output filters. We understand that these filters serve two primary purposes: 1) converting single-ended inputs to differential-ended signals, and 2) shifting the signal reference from ground to 1.5V. 
 We would appreciate guidance on the following queries:

Is it possible to eliminate the use of input and output filters, considering our goal to minimize the bill of materials (BOM) cost and reduce the board's size?

If filters are unavoidable, can we utilize +12V/0V for the op-amp(ADA4075-2ARZ-R7) supply, given that +12V and +3.3V (for the codec) will be readily available?

We appreciate your support and look forward to your input on this matter. 
 Best regards, Avnendra

DaveThib · Answer

Hello Avnendra, 
 I will start with the ADC side. 
 To obtain the best possible performance you have to remove any out of band energy (frequencies) from getting into the ADC. The better the filter the better the performance. These converters are oversampling so that does remove some of the strict requirements of the filter but you still need to filter to get the best specs. Then you are correct in that the other purpose is to make the single ended signal differential to drive the ADC. You certainly can simply cap couple one of the inputs to ground and only drive the other ADC input. You will not be able to drive the ADC to full scale 0dBFS. The best you will get is just below -6dBFS. 
 Then you can use a simple RC filter for the input and that will get rid of any RF that might cause problems. This will work and you will get decent signal into the ADC but do not expect datasheet specs. It does not seem like you are expecting perfect performance. 
 
 For the DAC side it is similar. On the DAC side the problem is that the DAC output is oversampled so there is a lot of out-of-band energy and that is the purpose of the filter. On the ADAU1962A datasheet I showed a passive filter on the evaluation board. Here is the schematic: 
 
 Again, it is not as good as an active filter but does a decent job. This schematic is setup single ended so you will have to stuff R40, R41 and C30 and remove R119 and you will have differential outputs. If you need single ended then use it like it is. Keep in mind the max output will be 6dB less. 
 
 Dave T

Electronic Design Services - PartnerZone

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Does the Input and output filter required for the codec