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AD9251 OEB

Category: Hardware
Product Number: AD9251

Hi,

In DS of AD9251 on page 27, there is the section about the Digital Output enable function. There is the sentence:

This OEB function is not intended for rapid access to the data bus.

I couldn't find a timing definition what this sentence especially means. How fast the Outputs can change from driving to high-z state and back?

My problem: an already done pcb-design implements two AD9251. But instead of routing each output pin of the ADCs to a separate fpga-gpio, both adcs are connected together. So only 2x14 lines go to the fpga. Each bit of each channel is connected to both ADCs.

To salvage the hardware-design (we only have pdf-schematics and gerbers), we want to retrieve the sampling data from both adc-chips in a time-multiplexed manner. Only one chip can be driving the same net on the same time. So we have to time the OEB pins of the two chips.

I'm interested in the following three times, that are illustrated in the picture above. (i have done a little error while drawing the picture: the levels of the OEB-Lines are no hardware-values, but to be understand as logical values. "High" means "driving" and "low" means "high-z", the voltage-levels on the ADCs are inverted)

  1. "t_blanking": time to wait after the falling edge of OEB1 (OEB pin of chip1) before OEB2 can be enabled.
  2. "t_data_valid": time to wait after rising edge of OEB2 before correct data can be captured from the data-lines
  3. "t_min_stable": is there a minimum time-period, the output-enable should be stable?

My problem is, that neither the OEB pins nor the spi-interface is routed to the FPGA. Would it be better to have the spi-interface and is there a possibility that one chip uses the Channel-A interface for interlaced ouput of its both channels and the other chip uses Channel-B for its interlaced output? Then all samples can be collected on both chips...

The DCO outputs are 4 separate nets into the FPGA.

Thank you!

greetings Daniel



changed from "software" to "hardware"
[edited by: schrolli at 3:52 PM (GMT -4) on 12 May 2022]
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  • Hi Daniel,

    Unfortunately, we don't have the information that you are looking for. The OEB pin for AD9251 is intended to be used as a static control pin. As the datasheet says, "This OEB function is not intended for rapid access to the data bus", means that timing was not characterized on this pin. 

    Regards,

    Meriam

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  • Hi Daniel,

    Unfortunately, we don't have the information that you are looking for. The OEB pin for AD9251 is intended to be used as a static control pin. As the datasheet says, "This OEB function is not intended for rapid access to the data bus", means that timing was not characterized on this pin. 

    Regards,

    Meriam

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