DAC39RF10EVM: 300ps short pulse application/design

Siyuan Zhang

Part Number: DAC39RF10EVM
Other Parts Discussed in Thread: LMG1020EVM-006, DAC39RF10, DAC39RF12, LMG1020

Hi,

Could I utilize the combination of LMG1020EVM-006 + DAC39RF10EVM to design something that can output minimum 300ps short pulse? the main reason of the DAC with high sample rates here is that I like to program the rise time slope. For example, 1ns short pulse is generated, and I like to change the rise time but keep the total of rise time, fall time and pulse width constant. Thank you.

Regards,

Simon

over 2 years ago

0 Matthew Kramer over 2 years ago

TI__Expert 3950 points

Hey Simon,

The DAC39RF10 can be configured to use all 16 lanes of the SERDES for a single DAC channel. In this mode you can use the DAC with 1X interpolation and have 10.24GSPS of real time bandwidth (12GSPS with the DAC39RF12). From a digital standpoint, this would mean pulse widths just under 200ps. (Imagine a simple square wave pattern with only 2 samples per period).

For something like a programmable slew rate, the pulse width will be longer as you need more samples per period.

The DAC has around 12GHz of 3dB bandwidth as well, though after 12GHz you still have some workable bandwidth.

Regards,

Matt

0 Siyuan Zhang over 2 years ago in reply to Matthew Kramer

Prodigy 10 points

Hi Matt.

Thank you very much for your reply.

What's the output level of the DAC? 0~reference voltage, which is 0~0.9V?

Regards,

Siyuan

0 Matthew Kramer over 2 years ago in reply to Siyuan Zhang

TI__Expert 3950 points

Hey Siyuan,

With 2X current mode you get about 7dBm into 50ohm and about 1dBm with 1x current mode (at low frequencies before any rolloff).

7dBm into 50ohm is ~500mV RMS.

1dBm into 50ohm is ~250mV RMS

For most applications we recommend ac coupling the output through a bias-T.

Regards,

Matt

0 Siyuan Zhang over 2 years ago in reply to Matthew Kramer

Prodigy 10 points

Hi Matt

Thanks for the clear explanation.

My next question would be: how I can convert the 500RMS voltage to to 5V peak voltage in such a fast speed? The low threshold voltage to trigger the LMG1020 is 1.1V.

Regards,

Siyuan.

0 Rob Reeder over 2 years ago in reply to Siyuan Zhang

TI__Mastermind 45670 points

Hi Siyuan,

We are reaching out to the groups internally on who can help support you with the LMG1020.

I will respond back tomorrow with an update.

Regards,

Rob

0 Rob Reeder over 2 years ago in reply to Rob Reeder

TI__Mastermind 45670 points

Hi Siyuan,

I am still working to find a contact for you.

I will respond back early next week.

Regards,

Rob

0 Rob Reeder over 2 years ago in reply to Rob Reeder

TI__Mastermind 45670 points

Forwarding this to the HVP-HPD team.

Thx,

Rob

0 Rubas Khalid over 2 years ago in reply to Siyuan Zhang

TI__Expert 8791 points

Hi Siyuan,

The LMG1020 device is a simple analog to analog driver, so as long as the input signals for the device meet the threshold and the VDD = 5, then the device will output. The input signal cannot be modified with our device or EVM, so it is dependent on if the input signal is being generated properly (whether it's from a function generator or the output of another device). If you would like to adjust rise time with affecting fall time and pulse width, then you can just adjust R1 on the LMG1020EVM (higher value = slower rise time) and just use a function generator for the input signal:

Thanks,
Rubas

0 Siyuan Zhang over 2 years ago in reply to Rubas Khalid

Prodigy 10 points

Hi Rubas

Thank you for your reply.

The limit of LMG1020 to my application is that it doesn't have the ability to output a stable pulse less than 1ns. Say 300ps.

Regards,

Siyuan

0 Rubas Khalid over 2 years ago in reply to Siyuan Zhang

TI__Expert 8791 points

Hi Siyuan,

Unfortunately, we do not have another device that can output a pulse that low. The LMG1020 is our fastest device with minimum input to output pulse of 1ns.

Best,
Rubas

0 Rubas Khalid over 2 years ago in reply to Rubas Khalid

TI__Expert 8791 points

Hi Siyuan,

I noticed that you rejected my last response. Is there anything else I can help with?

Thanks,
Rubas

0 Siyuan Zhang over 2 years ago in reply to Rubas Khalid

Prodigy 10 points

Hi Rubas

Yes If you have any ideas to help me to get 500ps pulse. Otherwise, no. Thanks for your reply.

Regards,

Siyuan

+1 Rubas Khalid over 2 years ago in reply to Siyuan Zhang

TI__Expert 8791 points

Hi Siyuan,

There may be a way to generate short pulses by inputting two pulses into the LMG1020, one on IN+ and another identical pulse with a 500ps delay (or whatever your desired pulse is) on IN-. Please refer to section 8.2.2.2 and figure 10 on the LMG1020 device datasheet to visualize this behavior (figure attached for your convenience):

EDIT: Please note that the datasheet only guarantees the LMG1020 operational performance to 1ns pulse. While this method may provide a shorter pulse, we do not guarantee the LMG1020's performance to be as if it were following the recommended guidelines in the datasheet.

Thanks,
Rubas

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DAC39RF10EVM: 300ps short pulse application/design