High Speed DAC Buffer?

Question

Hello 
 I need an urgently answer. 
 I want to place a ultra low distortion op amp to buffer the output of a high speed 14Bit-DAC as below: 
 
 The output frequency is DC- 50MHz . 
 if I want to have the maximum CMRR I should minimize any mismatch between the Rg and Rf. 
 on the other hand i want to have the maximum SFDR. 
 can I use a resistor network instead of Rf and Rg? 
 I found some kinds of such networks from Vishay and other companies. 
 
 the network mismatch is only 0.01%. the resistor inside the Network are completely isolated. 
 Best Regards

landsman · Accepted Answer

Hi, 
 Which op-amp are you using? 
 The following is directly from the AD9744 datasheet: 
 
 The AD8047 is a 250MHz, general purpose op-amp. Given the specs you provided it should work well. 
 Regarding the resistive network, it is ultimately your call. Personally I cannot see any issue with using a packaged resistor array, although it would make the layout more complicated, depending on the pinout of the op amp you are planning to use. 
 My first choice would be to use 0.01% (or 0.005%) resistors, placing them as close to the op-amp as possible, which would allow easily adding small filtering caps along side.I also doubt there would be any significant common-mode swing, if your op-amp is co-located with the DAC in the same layout. Power supply noise and other noise sources would be likely of greater consequence. 
 If a true concern, I would suggest to use a diff-amp - a well-known variant of op-amps which targets excellent CMRR. We have ones from ADI that cover 6GHz - surely there are ones that cover 50-100MHz with good specs otherwise. 
 I hope this helps. And please checkout our options on analog.com! 
 Best Regards, 
 Arik)

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High Speed DAC Buffer?