AD9364 Tx attenuation steps error

Question

Hi, 
 We are developing a hardware from the FMCOMMS4EBZ. We copied the hardware except the AD7291 and defined the pin assignment as the same as FMCOMMS4 to connect with Zedboard. Finally we compared with our board and FMCOMMS4 and found a strange thing. 
 When we set TX attenuation by FMCOMMS4EBZ, the max power of FMCOMMS4EBZ measured at attenuation 0. This is correct and makes sense. 
 But when set TX attenuation by our board, the max power of our board happend at attenuation 20. This is very strange. When I changed the attenuation from 20 to 0, the power reduced 20 dB. Or I changed attenuation from 20 to 0 and the power reduced 20 dB , too. The attenuation should be related to SPI control only and I have no idea why this happend? 
 
 best regards

srimoyi · Accepted Answer

You need to put a balun at the TX output for converting to single ended from differential, and measure the power at the single ended output. Also, you need to give a DC bias to TX output differential port. 
 Refer to the "Tx signal path interface" section in UG for more details.)

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AD9364 Tx attenuation steps error