I got a question about ADRF6850 & AD9265 interface circuit

hi i'm sarah

i'm looking for a making the Receiver

i select the chip of ADRF6850 & AD9265

so i need connection of ADRF6850(Receiver chip) & AD9265(ADC)

ADRF6850 have interner iq demodulator so it can offer the baseband signal , and it also got vcm voltage pin

the Common mode voltage range of the ADRF6850 VCM pin is 1.2~1.6 V

but VCM voltage range of AD9265 is maybe 0.85~1.1 V

1.2V VCM -> SNR is so bad

So, i wanna find how can i do

yesterday, i tested ADRF6850 evaluation board,

when i offer the 0.9V voltage at pin of ADRF6850 VCM pin (Actually VCM range in datasheet is 1.2~1.6V)

it's operating well

so i'm trying to interface the two chips

but i'm really worry about the problem that can occur

is it possible to operate well?

when i'm disign the circuit like that, what problem can i find?

and what's your recommendation ?

can you recommend the another ADC?

please reply quickly! thank you!

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  • Hi Sarah,

    The ADRF6850 will function with an output common mode level of 0.9V. But it is not supported by the datasheet and so will degrade some of the datasheet specifications such as P1dB. The typical output swing of the ADRF6850 is 2.5V p-p. With a 0.9V common-mode level, this will reduce to 1.4V p-p so the dynamic range of your ADC may be limited depending on your requirements. There is also a dc offset of +/-20mV which will impact the dynamic range as well which is why we recommend you use the part in AC-coupled mode and externally provide any dc offset that you require to the ADC. Also we recommend using a buffer between the ADRF6850 and the AD9265. I assume the AD9265 has an external buffer on the evaluation board. So I would recommend the following:

    1.     Use ac-coupled mode and use an external buffer such as one of those referenced in the AD9265 datasheet.

    2.     Identify another ADC which will support a dc bias of between 1.2V and 1.6V. This should be a buffered ADC.

    3.     Use the ADA4930-2 which is a dual differential op-amp which can do the required level shifting between the ADRF6850 and the AD9265 although this does not buy you anything as the ADRF6850 specs are still degraded.

    4.     Use your solution but it is not recommended as the ADRF6850 performance is degraded and I am not sure if it will drive the AD9265 sufficiently.

    Regards,

    Tim.

Reply
  • Hi Sarah,

    The ADRF6850 will function with an output common mode level of 0.9V. But it is not supported by the datasheet and so will degrade some of the datasheet specifications such as P1dB. The typical output swing of the ADRF6850 is 2.5V p-p. With a 0.9V common-mode level, this will reduce to 1.4V p-p so the dynamic range of your ADC may be limited depending on your requirements. There is also a dc offset of +/-20mV which will impact the dynamic range as well which is why we recommend you use the part in AC-coupled mode and externally provide any dc offset that you require to the ADC. Also we recommend using a buffer between the ADRF6850 and the AD9265. I assume the AD9265 has an external buffer on the evaluation board. So I would recommend the following:

    1.     Use ac-coupled mode and use an external buffer such as one of those referenced in the AD9265 datasheet.

    2.     Identify another ADC which will support a dc bias of between 1.2V and 1.6V. This should be a buffered ADC.

    3.     Use the ADA4930-2 which is a dual differential op-amp which can do the required level shifting between the ADRF6850 and the AD9265 although this does not buy you anything as the ADRF6850 specs are still degraded.

    4.     Use your solution but it is not recommended as the ADRF6850 performance is degraded and I am not sure if it will drive the AD9265 sufficiently.

    Regards,

    Tim.

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