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AFE for 250MHz 2.5Vpp Signal for ADS9684

Category: Software


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 ?

ADC Doubts
[edited by: shyam.sunder91 at 2:20 PM (GMT -4) on 1 Jul 2022]

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  • Hi ,

    The Vocm is to be maintained 2.05 with good accuracy, is it ok if i choose a LDO with good accuracy with output 2.05 V ± 100 mV ? or there will be any special temperature independent Vcm regulators like we use for Vref in case of ADCs.


    I'm not an expert on the Vcm regulators which are temperature independent. However from AD9684 ADC datasheet page 25 you see:

    "Therefore, in dc-coupled applications, set the common-mode voltage to 2.05 V ± 100 mV to ensure proper ADC operation."

    So, up to +/-100mV max variation can be tolerated.



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