REFERENCE DESIGN FOR AD9653BCPZ-125

Question

Hi, 
 We are trying to design the external reference for above mentioned ADC. This design is given by my expert. 
 The circuit of ADC as well as its external reference has been attached in this email and has been given by my expert but I am short of data in order to verify the correctness of the design. 
 I will request this intelligent community to please let me know how to verify this design and whether we are on right track or not. I have seen many a times that references do not behave properly [has ripple which causes conversion inaccuracy] based on the throughput of the requirement of the ADC and again adding more capacitance does not solve the issue as it provides instability to opamps at times. 
 View of experts here will matter a lot and will be thankful for it. 
 I have attached the schematic in pdf for ready reference. 
 Best Regards.

MohitH · Accepted Answer

Hi CCloris, 
 Thanks for the query. This ADC is sitting on a mezzannine card. On the main carrier card, we are using Altera FPGA where for LVDS ports, VCCIO port is recommended to be 2.5V. 
 Hence the VCCIO that we have used is 2.5V. Cyclone V Soc we are using. 
 Regards.)

ADIApproved · Answer

Hi Mohit, 
 Thank you for your interest in AD9653. 
 Regarding AD9653 PDWN pin being referred to AVDD, this is correct. The wording indicating PDWN being referred to DRVDD is not correct. I'll work on getting that changed in the next datasheet revision. I apologize for the error. 
 Regarding the capacitance on the AD9653 VREF pin, this is for stability when the AD9653 is using its internal VREF. In that case we like to see the capacitors to be as physically close to the pin as possible. You do not need to worry about this when using an external reference. 
 Also, the AD9653 can be configured for internal VREF = 1.3V, if that is of value. 
 Thanks, 
 Doug

ADIApproved · Answer

You should be able to remove the buffer and drive the ADC directly. The 6655 can drive 5mA so you could use a voltage divider with 500ohms of resistance. to be on the safe side I would use at least 1k ohms of resistance for margin.

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REFERENCE DESIGN FOR AD9653BCPZ-125