I am interested in using the AD9208 as a wideband digitizer to subsample RF signals, and I like that the input bandwidth for this device is 9 GHz. However, I wish to have control over the sampling frequency (Fs). In the datasheets for AD9208, the tables list only three possibilities for Fs: 2.5 or 3 or 3.1 GS/s. But are these the only allowed values? What should I do if i wanted it to be lower value, for example?

Similarly, I am interested in AD9213 because of its wide RF input bandwidth (6.5 GHz), but the datasheets mention max Fs as 10 GS/s, without clearly explaining whether lower Fs values can be used and how?

Any help or suggestion would be appreciated.

(note: I am not necessarily stuck with these product. Any wideband sampler would be welcome, as long as it has good performance and number of bits (say 12 bits or over), and input BW higher than 4-5 GHz.)

Hi,

I believe the question has three parts. I will try to address them in the following three paragraphs.

Sample rate range for AD9208:The AD9208 datasheet specifies the sample rate as 2.5GHz to 3.1GHz. What does this really mean? This means that beyond 3.1GHz sample rate, the ADC basically stops converting. Consider this the red-line limit of your car engine. Below 2.5GHz, the ADC will function and convert analog to bits. However, we have not characterized the ADC at sample rates below 2.5GHz. Hence, we will not be able to guarantee any performance of the ADC to match what the datasheet says. Moreover, at sample rates <2.5GHz, the ADC will be burning more power and the performance will not be as good as say, the AD9689 running at 2.5GHz. The AD9689 is both power and performance optimized to run at 2.6GHz sample rate.

Sample rate range for AD9213:You may have noticed that for the AD9213, there are two different speed grades : 6GSPS and 10.25GSPS. You also may have noticed that the datasheet is still preliminary, meaning the AD9213 has not been fully characterized. Hence, the low limit for the ADC is not specified yet. What is the Fs range you wish to use for the AD9213?

Wish to have control over the sampling frequency (Fs):The latest generation of RF-ADCs are built with the JESD204B high speed serial interface. These chips also have pretty complicated digital calibration circuits running to adjust for the linearity and noise performance. Every time the clock rate is changed, both these circuits inside the chip need to be reset. Otherwise the ADC will not function properly. So you need to decide what amount of "down-time" you can handle with the change in sample rate. Gone are the "good ol' days" of LVDS converters where you expect some junk through the pipe and then get the newly sampled data.

Thanks

Umesh