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Hi Guoping,

This table is from the Operating Ratings in the datasheet.  In order to understand these ratings more, it is helpful to look at Note 15:

For d.c.-coupled applications, the input voltage which may be safely applied to the part are listed in the above ratings.  The 'life-time duty-cycle' refers to the percent time which the part may have a particular voltage applied, as a percentage of its total life time.  For example, for a scope application, the signal will be d.c. coupled to the ADC.  If the probe were left on a 2.8V source over the weekend (2 days), this would be allowable, as long as it were left off for 18 days, in the sense that it would not reduce the life time of the part.  To be explicit, 2 days / 20 days is the 10% duty-cycle.  The issue which this operating rating addresses is electromigration.

Let us know if you have further questions.

Kind regards,

Marjorie

Marjorie

thanks for your answer.

As the datasheet said the analog differential input full scale range of adc12d500rf is 1000mv.but this range is too small for my project. can we use the resistance to attenuate the signal which is input to the ADC.for adc12d500rf,the figure is below.

for SMA interface ,the differential input full scale range is 2000mv?Am i right? 

Hi Guoping,

Yes, you are correct.  The differential input termination resistance of the ADC at VIN_I+/- is 100 ohm.  This, in parallel with R2 and R4 (series 100 ohm) will attenuate the voltage which appears at the ADC inputs by 2x.  What is the frequency range of your input signal? 

Let us know if you have further questions.

Kind regards,

Marjorie

Marjorie

thank you for your patient answer.

The frequency of input signal is about 70K.  the trace to each VinI+ and VinI- should always be 50Ω single-ended.does that mean R1 and R3 should always be 50Ω？and if I want to enlarge the differential input full scale range to 3000mv.then R1=R3=50Ω,R2=R4=25Ω？

Hi Guoping,

You are most welcome. 

What is the source impedance?  Please specify single-ended or differential.

Kind regards,
Marjorie

Hi Marjorie,

The input impedance for each I- and Q-input is 100Ω differential (or 50Ω single-ended),so the trace to each VinI+,VinI-,VinQ+,VinQ- should always be 50Ω single-ended.there is a 100Ω differential source is driving the ADC in my project.so the value of R1 R2 R3 R4 should always  be 50Ω?or if I want to enlarge the analog differential input full scale range to 3000mv.How to decide the value of R1 R2 R3 R4?

-guoping

Hi Guoping,

Yes, that is correct - the Vin+/- input impedance is 100 ohm differential.  What is the impedance of the source with which you are driving the network?

Kind regards,

Marjorie

Hi Marjorie

there is a 100Ω differential source is driving the ADC in my project.

Hi Guoping,

A better way to achieve some attenuation of the input signal would be to use an in-line attenuator.  For example, from your schematic, remove R2 and R4.  In place of R1 and R3, use an attenuator.  For an example part, see this one:

http://search.digikey.com/us/en/products/EXB-24AT5AR3X/P11107TR-ND/307333

For your application, the attenuation should be 10*log(3000/1000) = 4.77dB, which is close to 5dB. 

Let me know if you have any further questions.

Kind regards,

Marjorie

Hi Marjorie

Thank you so much for the reply.

Do you mean the connecting will be like this in AC coupled mode ?

Guoping

Hi Marjorie,

thanks a lot.

Guoping