AD8055(opamp) - AD9240 (ADC ) - Change in Common mode voltage at ADC input terminals

Question

Hi, 
 We are using AD8055 op amp for a DC coupled application, to drive the AD9240 ADC. 
 We have to set the common mode of the inputs as 2.5V. We have implemented a recommended circuit as per the AD9240 recommedation as per below image. 
 . 
 
 As per above circuit VINA should have a common mode of 2.5 V and VINB commn mode as 2.5 V. 
 In the above scheme, before Rs resistor we have RC low pass filter, with resistance of 221 ohm and Rs = 24 ohm. 
 But we are measuring a drop of 0.7V between 6 pin of Op amp and VINA pin of ADC. This decrease the common mode to a Value of 2.28 V. 
 We dont understand the drop between these output terminals,Kindly request you to provide us from which path does this DC current flows to make a drop in the series resistors. 
 Awaiting your respose.

PMH · Accepted Answer

Hello, 
 The series resistance of 221 is too high since the unbuffered ADC input also produced a signal independent current impulse (due to clock driver coupling across internal ADC sampling capacitor) that is fed back to the buffer (and VREF). 
 
 Suggest that you keep Rs at 24 ohms and place "C' portion of RC low pass filter at both VINA and VINAB with its value determined to give desired cut-off frequency with R=24 ohms.

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AD8055(opamp) - AD9240 (ADC ) - Change in Common mode voltage at ADC input terminals