DAC39J84EVM: Question about choice of analog output transformers

Zzzzzzzzzz

Part Number: DAC39J84EVM
Other Parts Discussed in Thread: DAC39J84,

Hello,

I'm hoping to use the DAC39J84 in a design was wondering what the design reasoning for using the JTX-2-10A transformer followed by a MABA-007159 transformer was.

The JTX-2-10A is a Mini-circuits part with 75 ohm impedance, 50-1000 MHz bandwidth, impedance ratio of 2.

The MABA-007159 is a Macom part with 50 ohm impedance, 5-3000 MHz bandwidth, impedance ratio of 1.

The dac39j84.pdf datasheet suggest a single 1:1 or 4:1 transformer on page 59.

Thanks for the help!

over 6 years ago

0 Jim Brinkhurst1 over 6 years ago

TI__Mastermind 44680 points

Hi Zzzzzzzzzz
One of our DAC experts will provide a more detailed response soon.
Best regards,
Jim B

0 jim s over 6 years ago in reply to Jim Brinkhurst1

TI__Guru* 91947 points

The JTX-2-10TA+ is specified as a 75-Ω transformer, which simply means that the datasheet characterization was done at 75 Ω. The part will still work at 50 Ω, but the performance may be different. We found that this combination of transformers gave us good flatness across the desired output band of the DAC. We try to design the EVM to provide the best performance as possible over the largest bandwidth possible. Depending on your application, this setup may need to be changed to optimize your performance.

Regards,

Jim

0 Zzzzzzzzzz over 6 years ago in reply to jim s

Prodigy 50 points

Jim,

Thanks! I am hoping to use the DAC with reasonable flatness from low frequency to 800 MHz. I have a DAC39J84EVM now and will check the output power across that range.

0 Weiyu Leng over 6 years ago in reply to jim s

Intellectual 360 points

Hi Jim,

What should I do, if I want to bypass the two transformers?

Regards,

Weiyu

0 jim s over 6 years ago in reply to Weiyu Leng

TI__Guru* 91947 points

Weiyu,

For channel A, remove T1, T2, and R12. Install R14 and J4. Connect pin 3 to 4 and pin 1 to 6 using the pads of T1 with 0 Ohm resistors. Connect pin 1 to 6 and 3 to 4 using the pads of T2 with 0 Ohm resistors. Do the same for the other 3 channels if needed.

Regards,

Jim

6761.DAC3XJ8XEVM-SCH_D.pdf

0 Weiyu Leng over 6 years ago in reply to jim s

Intellectual 360 points

Hi Jim,

Thank you! Those are what I plan to do.
So as you implied, DAC3XJ8X does convert low-pass signals from digital to analog, doesn't it?
Is there a lower limit on that; i.e. can it provide, say, a DC 0.5V differential voltage?

Weiyu

0 jim s over 6 years ago in reply to Weiyu Leng

TI__Guru* 91947 points

Weiyu,

By increasing the value of R135, the IOUT current of the DAC will decrease, which will allow you to reduce the differential voltage. See sections 7.3.27, 7.3.28 and 7.3.29 of the data sheet for more information regarding this.

Regards,

Jim

0 Weiyu Leng over 6 years ago in reply to jim s

Intellectual 360 points

Hi Jim,

Sorry, I was asking if DAC3XJ8X can really produce a constant output voltage, after I bypass the balun.

Thanks

Weiyu.

0 jim s over 6 years ago in reply to Weiyu Leng

TI__Guru* 91947 points

Weiyu,

Yes, if you terminate the output with a resistive load and give either the DAC a constant digital input value or place it in constant input mode with a register setting. Since this is current source DAC, you will need to terminate it to a resistive load to get a voltage.

Regards,

Jim

0 Weiyu Leng over 6 years ago in reply to jim s

Intellectual 360 points

Hi Jim,

Thank you so much for your help!

Weiyu

0 Star_Li over 6 years ago in reply to jim s

Prodigy 165 points

Hi Jim,

If I terminate the output with a resistive load and bypass the transformer and inductors, how can I change the output common mode voltage and output swing?

Thanks,

Star

0 jim s over 6 years ago in reply to Star_Li

TI__Guru* 91947 points

Star,

There is no common mode adjustment, only current and voltage adjustments. For adjusting the output current, see section 7.3.27 of the data sheet. For adjusting the voltage, see section 7.3.28. After making any changes, just make sure the voltage on the IOUT pins is in compliance with the data sheet. This max value is 0.6V and is shown in section 6.5.

Regards,

Jim

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DAC39J84EVM: Question about choice of analog output transformers