Reg: DAC5688 output harmonics...

Gauraw Srivastava90

Prodigy 185 points

Other Parts Discussed in Thread: DAC5688

Hi I am using DAC5688 in my design with following settings...

PLL off

DAC_CLK = 480MSPS

DACDATA_CLK = 120MSPS

Interpolation = 4,Invrse Sync Off,QMC gain/phase Off,QMC offset off,coarse_gain = 0xFF,mixer_gain off

NCO = 45 MHz,Sync = SIF_SYNC

At the analog output I am using the configuration of Fig47 in the data sheet.

I am trying to analyze analog output through DAC_SELF test mode using serial_dac_data register in DAC.I am configuring 0x7FFF as data.

At analog output second harmonic of 45 MHz is just 15 dB down the fundamental.

If I configure 0x5FFF it is 20dB down.

Why there is a presence of such high harmonic of IF frequency??

over 9 years ago

0 Neeraj Gill over 9 years ago

TI__Mastermind 20541 points

HI Gauraw,

Can you please send me schematic for the output part of the DAC, it will help us to find the root cause?

Regards,
Neeraj Gill

0 Gauraw Srivastava90 over 9 years ago in reply to Neeraj Gill

Prodigy 185 points

SCH-TI.rar

Dear Neeraj,

PFA the SCH,I am measuring IF at IF_TX_VGA_IN.

0 Neeraj Gill over 9 years ago

TI__Mastermind 20541 points

HI Gauraw,

The schematic for DACs output looks fine. I think the DAC might be running into digital saturation. With NCO gain enabled the input code need to be 3dB(0x50EF) down from full scale.

Regards,

Neeraj Gill

0 Gauraw Srivastava90 over 9 years ago in reply to Neeraj Gill

Prodigy 185 points

Hi Neeraj,

There was a mistake from our side,LPF output i.e. IF_TX_VGA_IN is getting fed to an VGA and 15dBc second harmonic of IFout measured at amplifier output.

But when i bypassed the amplifier and given 0x7FFF in serial_dac_data and measured at IF_TX_VGA_IN then second harmonic is 59dBc.But as per Fig 5 of data sheet it should be 70dBc.Where could be the problem??

In the current measurement setup NCO gain is OFF.If you are suggesting that DAC is getting saturated digitally then what should be the max code for 0dBFS output?What will be the power at IF_TX_VGA_IN in the 0dBFS case?

0 Neeraj Gill over 9 years ago in reply to Gauraw Srivastava90

TI__Mastermind 20541 points

Hi Gauraw,

Did your issue get resolved? I have mentioned the required value((0x50EF) in my previous post.

Regards,
Neeraj Gill

0 Gauraw Srivastava90 over 9 years ago in reply to Neeraj Gill

Prodigy 185 points

Dear Neeraj,
Sorry i couldn't follow it due to some reasons.

I made the setup as per the value suggested by you of 0x50EF.Following are the DAC settings--
DATACLK = 120MHz
DACCLK = 480MHz
Interpolation = 4
NCO = 10MHz
QMC =OFF
PLL = Off
SER_DAC_DATA = ON

Following are the observation on the Spectrum analyzer for the DAC output---
DAC Output@10MHz = -3dBm
HD2@20MHz = 57dBc
HD3@30MHz = 67dBc

Please suggest to get the performance near to the datasheet.

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Reg: DAC5688 output harmonics...