AFE for 250MHz 2.5Vpp Signal for ADS9684

Question

Hi, 
 Preface : 
 My application requires the single input signal from a optical sensor which is in range of 2.5Vpp to be driven to an ADC through a FDA 
 The signals are in range of DC-250MHz 
 I have selected the ADC as AD9684( Only available ADC with Input range of 2.06Vpp Maximum) at 500MSPS 
 I wish to use the ADC for Both AC and DC coupling so i have to give the option as per requriement. 
 below is my signal nature 
 
 Q Channle is 2580mVpp and I channel is 2100mVpp 
 i need to condition this signal to take it to a ADC and make full use of resolution of ADC 
 1. Do i really require an FDA for the ADC Selected ? 
 2. If So, Kindly suggest a FDA for driving the ADC and AFE scheme for the input as above 
 3. The ADC input has range of FSR up to 2.06 but i am confused how my signal will be read by ADC when Vref is set to 1V 
 above 1V the ADC is saturated but the ADC says 2.06Vas FSR, kindly let me know how to understand this also 
 
 4. should i need to shift the DC of signals to make them fall in exact range of ADC FSR ?

Hooman · Accepted Answer

Hi shyam.sunder91 , 
 With regard to AD9684 specs, I'm going by this below from the datasheet: 
 But since I'm not an expert on ADC's, I'll try and move your post to the data acquisition forum for better support and in case I've misspoke. 
 Some Responses Below: 
 
 shyam.sunder91 said: what is the input equivalent value of digital 0x0000 ? 0V ? 
 Response : digital 0V will be a -1.03V differential input voltage centered at 2.05V CM. 
 shyam.sunder91 said: You meant to say that 2.06 is the Full-scale value which means an input signal of 2.06 would read all 1's in Digital output ? and Vref is nothing more than internal purposes and should be only 1.0V and not more than that. 
 Response : When Vin+ = 2.05V + 2.06/4 = 2.56, and Vin- = 2.05-2.06/4 = 1.54V, you'd register all 1's digital output. On the other hand, all 0's would correspond to Vin+ = 1.54V, and Vin- = 2.57V by looking at the specs above. 
 shyam.sunder91 said: How to read the negative values which are less than 0V ? how to use Vcm as a tool to read negative values 
 Response : All 0's = -1.03V diff input with 2.05V CM. All 1's = +1.03V diff input with 2.05V CM. 
 shyam.sunder91 said: you may see back my signal which is Q Channle is 2580mVpp and I channel is 2100mVpp 
 Response : I'd just reduce the LTC6419 driver gain lower for the Q channel than the gain needed for the I channel, if I've understood your question correctly? 
 shyam.sunder91 said: Attenuation would cause low voltage signals which are not shown which are arround 200-250mV to go un noticed, is there any other way arround ? 
 Response : As long as the input to the ADC is greater than 1 LSB, the ADC will digitize the analog input accordingly. I don't know what you mean by "it would go un-noticed"! That's another reason I should be moving your query to the ADC forum for better support in case I'm not fully understanding your problem. 
 Regards, 
 Hooman)

Hooman · Answer

Hi shyam.sunder91 , 
 FYI>> 
 I've received feedback from the Application Engineer that the answers I've posted in response to your questions about AD9684 ADC are accurate and you can use that information as-is. 
 I wanted to make sure that you get this so that you can have more confidence in my responses. 
 Regards, 
 Hooman

Hooman · Answer

Hi shyam.sunder91 , 
 
 shyam.sunder91 said: Can you please expalin with some picture how the ADC 9684 sees input and transaltes to digital equivalen with Vcm played arround, that would help me to do any signal conditioning requried 
 Response : Does this picture drawn with the help from Diff-amp Calculator program help with diff input visualization in seeing the all 0's and all 1's instances in time? 
 https://www.analog.com/en/design-center/interactive-design-tools/adi-diffampcalc.html 
 
 Regards, 
 Hooman

Hooman · Answer

Hi shyam.sunder91 , 
 You'll get the best performance if you drive AD9684 input(s) differentially. 
 For driving the AD9684 input(s), you could use a FDA like LTC6419, as shown below to attenuate your signal from a maximum of 2.6Vpp to make sure you don't exceed the ADC maximum FS voltage of 2.06V, as shown below, with a CM voltage of 2.05V per AD9684 input CM voltage specification: 
 
 Here is the LTspice simulation file if you'd like to work with it: 6036.LTC6419 attenuator FDA ADC Driver EZ 7_2_22.asc 
 The analog input requirements of AD9684 call for a CM voltage of 2.05V and a maximum FS input of 2.06V. 
 The configuration I've shown above achieves these goals. The 1.0V reference built into the AD9684 is used internally for other purposes. 
 Hope this helps. 
 Regards, 
 Hooman

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AFE for 250MHz 2.5Vpp Signal for ADS9684