Post Go back to editing

AD6676 PIN_0DBFS calculation

Category: Datasheet/Specs

Hi,

I am working on AD6676 chip,I have a doubt on PIN_0DBFS calculation, as per the equations(2) & (4) given in pg no - 28 and 29 in the datasheet of AD6676,will my PIN_0DBFS value change just by writing to the register 0x10A.

Also as per calculations based on both the equations I have got PIN_0dBFS = 10 × log10(1/2 × RIN × IDAC1FS2) as -33.1876 and if I change the IDAC1_FS value from 0x40 to 0x20 (PIN_0dBFS = -39.208) in the equation ,then I can see a 6dB difference between both as mentioned in pg no - 71 for register 0x10A (IDAC1FS Gain Scaling Register) .

But when I change the value from 0x40 to 0x20 in the register 0x10A, I am not able to observe any changes in fundamental amplitude in the output samples when plotted.

I need to configure  PIN_0dBFS to -2.7dBm as mentioned in one of the example in the datasheet ,so can u please provide with the configuration changes that needs to be done.

Hello,

When one adjusts the IOUTFS current level that defines the PIN_0dBFS, one is changing the input power level that would correspond to digital full-scale of the ADC.   So in your example above where -3.2 dBm represents digital full-scale for setting of 4 mA, an input CW level at IF center of -13.2 dBm would measure to be -10 dBFS on an FFT.   If one kept this input level constant at -13.2 dBm and then reduced IOUTFS to 2 mA (i.e. factor of 2) this will shift the PIN_0dBFS level  to -9.2 dBm (i.e. 6 dB) such that the measured CW tone on FFT will appear as a -4 dBFS on the FFT.  In the time domain, the signal will be 2x the pk-to-pk swing vs the case when IOUTFS is 4 mA.

Note that the AD6676 datasheet excludes the balun loss (which can easily be -0.5 to 1 dB) when stating the PIN_0dBFS level.   The datasheet characterization curves also show that the PIN0_dBFS level also is dependent to the IF setting due to 2nd order effects that are frequency/settings dependent (i.e. IF vs BW vs DAC sample rate).  Note sure what your exact settings are and whether the -3.2 dBm  PIN0_dBFS level
includes balun loss or not.   If one needed to shift the PIN_0dBFS level higher than what can be achieved with IOUTFS=4 mA.........than one can use the ATTEN function that allows one to shift upward in 1 dB increments.

Regards

  • Thanks for the reply ,In my case i am feeding an input frequency of 261 Mhz and IF frequency of 258 Mhz,Bandwidth is 56Mhz ,considering the  balun loss and cable loss to be  -0.76 dBm,so considering all the losses i am, feeding -2dbm to the ADC which equates to -2.7dBm to the input of the ADC,but after plotting the output samples , I am able to get a fundamental amplitude of -0.356 dBFS which is not excatly 0 dBFS

    Hence can I know what value needs to be set to the respective registers to obtain the exact PIN_0dBFS value