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AD6676 IDAC_FS calculation and its impact on setting PIN_0DBFS

Category: Datasheet/Specs


I am working on AD6676 chip ,and I am referring to Wideband IF Receiver Subsystem AD6676 Datasheet.

I have to change the default PIN_0DBFS value to -2.7dBm as per the specification provided for my project.So in order to change that I used the equation (2) & (4) given in pg no - 28 and 29 in the datasheet ,

PIN_0dBFS = 10 × log10(1/2 × RIN × IDAC1FS^2)   ---> (2),

IDAC1FS = 4 mA × (IDAC1_FS/64)            ----> (4),

I have set the following parameters as follows :

FIF  => 258 Mhz,

BW  => 56Mhz,

FADC => 3200Mhz,

Lext => 10nH,

IDAC1FS => 0x40 (0x10A register)

RIN => 60 ohm

Therefore considering all these above parameters ,as per the above equations ,I got PIN_0dBFS value as -3.18759 dBm theoretically.

But when we overwrite the IDAC1FS register(0x10A) ,we are not able to see any changes in the PIN_0dBFS value practically ,Is there anything that I have to take care so that I can see changes in the PIN_0dBFS.

Please do let me what changes needs to be done to obtain my PIN_0dBFS value as -2.7dBm

Also refer to this link as well because it also points to the same issue :  AD6676 PIN_0DBFS calculation  

  • Hello,

    Since you PIN_0dBFS for 4 mA measures to be 0.5 below your target of -2.7 dBm,  one can set the RxPGA attenuator to 1 dB (0x181=0x01) thus increasing your PIN_0dBFS from -3.2 dBm to -2.2 dBm and then reduce your IOUTFS by a small amount (set to IOUTFS  register 0x10A=0x3C) to reduce your PIN_0dBFS so that it is -0.5 dB lower thus giving you -2.7 dBm.  

    Once can increment or decrement this register by 1 to see if it gets you closer to target -2.7 dBm. 


  • Thanks for the reply.

    But as per the datasheet pg-14 .figure -20 corresponds to -2.7dBm wihout any attenuator changes,it is kept 0 in the configurations.

    So is there any other way to increase the PIN_0dBFS value to -2.7dBm without using the attenuator register as suggested above??