DAC38J84EVM: Configure DAC38J84EVM tohave DC coupled/differential outputs

Hello Kelly,

A few suggestions:

1. the DAC38J84 can certainly be DC coupled to drive to amplifier. Now, the amplifier may have different common mode requirement than the DAC output compliance. You may need to do some interface to translate the compliance + DC offset accordingly. Please refer to the app note below for details and strategies that may work for you. 

http://www.ti.com/lit/ug/slua647a/slua647a.pdf

2. Mini-circuits typically may be willing to do certain custom baluns to fit your need. You can probably contact their sales to see if they can do so.

3. The DAC38j84 compliance voltage is similar to the DAC3484 (highlighted in the app note). It cannot drive 10Vpp as required in your application. The schematic for the modification for differential output configuration, assuming you have a circuit in mind, can be found in the following directory:

www.ti.com/.../slac646

Hi Kang,

Thank you for your prompt reply. 

I had looked at one of your reference design: 

http://www.ti.com/tool/tida-00684

and was wandering if I can add opamps to amplify the dac outputs. I understand that the slew rate of opams might be a problem in the design, but since the fundamental frequencies I plan to operate is below 50MHz, say 30-40MHz, 320MHz opams like THS3491 or similar products might be sufficient. I've noticed that my DAC38J84 EVM has spare sma port for differential outputs, and I was wandering if there is a way to bypass the two transformers before the smas to directly give me differential outputs at the smas so that later I can directly connect to some opamp EVMs with differential input smas. The file you sent me seems include the transformers (which are rated between 50MHz and 1GHz), which means I may not have good frequency response at frequencies below 50MHz. Would it be possible to remove them to get better low frequency response?  

I also have  another question hopefully you can help me understand:

Why the DAC38J84 EVM uses a 75ohm (ratio of 2) transformer followed by a 1:1 transformer between dac outputs and the 50ohm sma (I think the impedances before the transformers are either 50 or 25, did you use the default 75ohm to just improve high frequency response to 1G)? I cannot find baluns or transformers with similar specs and same footprint. Can I use transformers, like ADT2-1T+, it is a 50ohm transformer with same pins.

Many thanks for your help.

Best wishes,

Kelly

Hi Kelly,

to answer your questions:

SHUAI YANG said:

Would it be possible to remove them to get better low frequency response?  

yes, it is possible. You may need to use good quality 50ohm wires to connect from the DAC output with the available pads on the EVM to connect to your amplifier EVM. This is doable if you think you can do the soldering or contact external board shop to do the soldering for you.

You may also need to use some zero ohm resistors to short out the pins of transformer to complete the connection.

SHUAI YANG said:

Why the DAC38J84 EVM uses a 75ohm (ratio of 2) transformer followed by a 1:1 transformer between dac outputs and the 50ohm sma (I think the impedances before the transformers are either 50 or 25, did you use the default 75ohm to just improve high frequency response to 1G)? I cannot find baluns or transformers with similar specs and same footprint. Can I use transformers, like ADT2-1T+, it is a 50ohm transformer with same pins.

Mini-circuits actually characterized the part for 50ohm applications. Please see below for characterization result:

JTX-2-10TA+_AP121094_RF_(5)_VM.PDF

Hi Kang,

If JTX-2-10T was used as 50ohm transformer does it mean that I can use any 50ohm transformer with similar pins and impedance ratio, like the one I mentioned ADT2-1T+?

Best wishes,

kelly

Kang,

I have removed the transformers and the low frequency response have been improved. I have a few other questions hopefully I can get answers from you:

1.  I'm using automation'dll in matlab. I've found that I have to first program the DAC38J84EVM using the GUI and then running the matlab script to load settings. Is there an easy way to program the DAC38J84EVM using MATLAB (assume I just need to update the registers)? 

2. Is there a way to calibrate the DAC output? Although I've improved the low frequency response the output seems lowered by 10s mV.

3. What is the maximum sample rate the EVM can achieve for a single port and how should I configure the EVM (currently with the onboard clock I can only get optimal output with number of serdes lanes 4, interpolation of 1 @ 614MSPS )? 

many thanks for the help.

kelly

Hi Kelly

Kelly said:

1.  I'm using automation'dll in matlab. I've found that I have to first program the DAC38J84EVM using the GUI and then running the matlab script to load settings. Is there an easy way to program the DAC38J84EVM using MATLAB (assume I just need to update the registers)? 

Check out the Labview based DLL that you can try to use. You may need to create within Matlab an interface to the Labview to work with this DLL

Kelly said:

2. Is there a way to calibrate the DAC output? Although I've improved the low frequency response the output seems lowered by 10s mV.

Not sure your definition for calibration. please elaborate. if i understood correctly you are asking frequency response calibration. No, the DAC does not have frequency response calibration. We have group delay compensation to compensate the delay for each channel

Kelly said:

3. What is the maximum sample rate the EVM can achieve for a single port and how should I configure the EVM (currently with the onboard clock I can only get optimal output with number of serdes lanes 4, interpolation of 1 @ 614MSPS )? 

The maximum sample rate depends on the DAC in the family. The DAC39j84 can support 2.6GSPS maximum. Refer to datasheet for detail

-Kang

Labview Based DLL link:

https://e2e.ti.com/support/data-converters/f/73/p/432855/1631029?tisearch=e2e-sitesearch&keymatch=dac3484%20dll#1631029