application schematic of AD9432

Question

Dear Sir/Madam, 
 We are designing with your ADC AD9432 for our product. 
 So I have a question. 
 As for your datahseet page.15 Fig-29 my enclosed, I can see the matching resister and capacitor between AD8132 amp and AD9432 AIN and AIN/. 
 1) 50ohm and 22pF, where can you bring these values? 
 2) And what are the input impedance of AINs?(3kohms on your datasheet analog input resistance , correct?) 
 I think that 50ohm and 22pF came from above 2)inout resistance. 
 If I connect AD8132 to AD9432 directly, what trouble will happen ? Oscillation? 
 Please let me know why are 2-50ohms and 22pF there? 
 Thanks, Kaos

UmeshJ · Accepted Answer

Hi, 
 Thank you for contacting EngineerZone regarding your question about AD9432. 
 The series R (50ohm) and the C (22pF) form a low pass filter that filters out the higher order harmonics going into the ADC. This is a generic low pass Anti-Aliasing Filter (AAF) design. In this particular case, the cut-off frequency is around 140-150MHz. But if you know your input frequency, you can design a filter with a roll-off that fits your application. 
 The AD8138 amplifier has a potential to "ring" when connected to capacitive loads like an ADC (AD9432 = 3kohm||4pF). Since this is a high impedance capacitive load the series R's might be necessary. You can however choose to not use the series R's but you could expect some ringing related performance in pulse situations. 
 If you are trying to use the AD9432 for a new design, I suggest you use the AD9628. This is a much lower power higher performance 12-bit part. 
 Thanks 
 Umesh)

UmeshJ · Answer

Hi Kaos, 
 The input impedance is 3kohm || 4pF. So for all practical purposes it is high impedance. 
 Thanks 
 Umesh

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application schematic of AD9432