Input Pin Resistors and Capacitors

Question

Hello, 
 
 My team and I are designing a PCB for the AD1938 and we have a question about what the datasheet for the AD1938 is saying 
 In the AD1938 Datasheet it states that for the ADCs "each input pin should be isolated by using a series connected, external, 100 &Omega; resistor together with a 1 nF capacitor connected from each input to ground. This capacitor must be of high quality, for example, ceramic NPO or polypropylene film." 
 Is this something that is required for the ADC to work? If so, so what exactly is it saying? Is it saying that we should connect the resistor and capacitor in series from the input pin on the AD1938 to the ground and then connect our input signal in parrallel with that resistor and capacitor (Connect our input signal and the resistor/capacitor directly to the input pin of the AD1938)?

DaveThib · Accepted Answer

Hello thaines3, 
 
 This is to keep RF out of the input as an anti-aliasing filter. We also use a 100pf cap as a filter. It is just a starting point but the components should be good quality. The input pins are also floating up at a common mode voltage so you usually cap couple the entire circuit. We do have an example circuit shown in the datasheet and also you can look at the schematic in the user guide of the evaluation board. 
 Here is that is in the datasheet: 
 
 You can see that we used a resistor that is larger than 100 ohms. 
 I also suggest you refer to the document in this post regarding the PCB layout. 
 PCB Layout Best Practices for AD193x codecs - Q&A - Audio - EngineerZone (analog.com) 
 
 Dave T)

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Input Pin Resistors and Capacitors