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838 Views
Registered: ‎10-31-2019

How to DC couple ZCU111 DAC to RFIC

Hi,

I want to connect the ZCU111+xm500 board to an RFIC chip. I believe the DAC on the Xilinx is a current source DAC which has two settings: 20mA and 32mA (shown below). The input of the RFIC is a DC coupled differential signal with 0.125 V common mode voltage for each pair.

Can you please let me know what the common voltage of the Xilinx DAC is. And if it does not support the configuration that I intend to use, is there any way that I can modify the xm500 board to support this configuration. 

-Another note: my RFIC can also be configured in a "Current Sinking" configurtion with a common voltage of 1 Volt for each pair of the differential signal. I believe this is not something that DAC output circutry support. 

AirVine Maple Leaf #03.jpg

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Moderator
Moderator
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Registered: ‎04-18-2011

Hi hholisaz@airvine.com 

I will try to reply generally about DC coupling the DAC output first. 

As mentioned by you, A typical example is DC coupling of DAC output into an external IQ modulator or  other RFIC. For Zero IF applications RFSOC DAC may need to be DC coupled.
DC coupling can be done by passive (resistors) or active (using an amplifier) means.
Passive coupling is the most used approach and also seems to be what you are favouring. 

Most Modulators on the market are not common mode compliant with the RFSOC DAC.

This requires the DC coupling circuit to level shift the RFSOC DAC CM level to the target CM level of the Modulator.
The DAC CM levels need to be maintained to meet the DAC linearity specifications.

32mA mode – DAC VCM = 2.07 V
20mA mode – DAC VCM = 1.93 V

DAC_VCM = 2.5 – (IFS/2 + 0.08*IFS)*50 = 2.5-(20/2 +0.08*20)*50 = 1.93V


Normally an image rejection filter is applied between the DAC output and the Modulator input. This filter should be designed with a source and load impedance to match the level shifting network.

 

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Moderator
Moderator
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Registered: ‎04-18-2011

More specific to your case, you don't have a lot of flexibility on the xm500 card. there are broken out dac channels that don't make use of a balun but these are just SMA connectors to allow you to use your own balun for example. 

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Registered: ‎10-31-2019

Thank you for your replies which helped me to resolve the problem.

Just wonderin if the DAC on the Xilinx part is a current sourcing DAC or a current sinking DAC ?

 

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Moderator
Moderator
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Registered: ‎04-18-2011

It's a current steering DAC 

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Contributor
Contributor
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Registered: ‎04-28-2017

Hello hholisaz@airvine.com

Could you kindly share what your solution to this problem was? What circuit did you use between the RFSoC DAC and the RFIC chip? An pointers would be greatly appreciated. Thank you!

Aditya

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