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# AD9288 - Common Mode Input Range

Hello All,

I am working with the AD9288 ADC. I have a DC coupled application, and single ended inputs. I am considering doing a single ended only implementation or a single ended to differential conversion.

1) On page 14 of the datasheet, it says "The nominal input range is 1.024 V p-p centered at VD × 0.3." If I use a VD (analog supply voltage) of 3.0V, that would correspond to a 900mV common mode bias point (correct?) If the input range is 1.024V, that would tell me that my upper range is 0.9 + 0.512 = 1.412V and my lower range is 0.9 - 0.512 = 0.388V. Are these calculations correct?

VD*0.3 center voltage seems a bit odd to me. I am used to single supply systems with the virtual ground being around 1/2 Vsupply. Can the AD9288 be modified to accept a common mode voltage range of (1/2)*3V = 1.5V? Will this significantly degrade performance?

2) If I choose to retain a fully single ended signal, should I layout a similar arrangement as the AD9281's datasheet on page 10 figure 22 (0V - 1V input)?

Thank you for your suggestions,

Alex

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• Chris,

Thanks for the quick reply. Now that I know that I must drive the 900mV common mode voltage, I need to figure out how that can be generated. Going back to the AD9288 datasheet, it appears that the Vref out is 1.25 V. Is this a buffered or un-buffered voltage reference? I was thinking of putting a voltage divider on it to generate 900mV, and then buffering that with an opamp to drive the common mode setting pins of the drive amplifiers. Is this a good approach? Alternatively, I suppose that I could use a DAC to set the common mode setting pins of the drive amplifiers.

Do you have a suggestion as to which method would be better? Is there another method that I could employ?

Thanks,

Alex

Reply
• Chris,

Thanks for the quick reply. Now that I know that I must drive the 900mV common mode voltage, I need to figure out how that can be generated. Going back to the AD9288 datasheet, it appears that the Vref out is 1.25 V. Is this a buffered or un-buffered voltage reference? I was thinking of putting a voltage divider on it to generate 900mV, and then buffering that with an opamp to drive the common mode setting pins of the drive amplifiers. Is this a good approach? Alternatively, I suppose that I could use a DAC to set the common mode setting pins of the drive amplifiers.

Do you have a suggestion as to which method would be better? Is there another method that I could employ?

Thanks,

Alex

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